BOMBARDED BRITAIN A Search for British Impact Structures
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Richard S t r a t f o r d INSPEC, UK
BOMBARDED BRITAIN A Search for British I m p a c t
-iffi
Structures
Imperial College Press
Published by Imperial College Press 57 Shelton Street Covent Garden London WC2H 9HE Distributed by World Scientific Publishing Co. Pte. Ltd. 5 Toh Tuck Link, Singapore 596224 USA office: Suite 202, 1060 Main Street, River Edge, NJ 07661 UK office: 57 Shelton Street, Covent Garden, London WC2H 9HE
British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library.
Maps reproduced from Ordnance Survey mapping on behalf of The Controller of Her Majesty's Stationery Office © Crown Copyright. Licence Number MC 100038893.
BOMBARDED BRITAIN: A SEARCH FOR BRITISH IMPACT STRUCTURES Copyright © 2004 by Imperial College Press All rights reserved. This book, or parts thereof, may not be reproduced in any form or by any means, electronic or mechanical, including photocopying, recording or any information storage and retrieval system now known or to be invented, without written permission from the Publisher.
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ISBN 1-86094-356-X
Typeset by Stallion Press
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This book is dedicated to the memory of my beloved wife Sylvia, whose enthusiasm and encouragement were the main-spring of the work
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Contents Foreword Acknowledgements
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Part I. Impacts and Geology 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
A Curious Omission Of Calculations and Craters The Search for Impact Structures The Shetland Craters Midlands Geology The Ashby Inlier Charnwood Forest The Midlands Basin — A Cometary Impact Structure? The Herefordshire Domes The Rochford Basin — A Digression into Essex Fuller's Earth and Bagshot Sands — A Surrey Crater? Gabbro, Granite, and Grampians Other Circular Structures
3 21 31 39 45 53 60 65 82 96 106 112 124
Part II. Impacts in History 14. 15. 16. 17.
Small Craters, Airbursts, and Tsunami Dozmary Pool and Other Craterlets Levin-Bolt and Blast British Atlantis?
139 145 153 162
Epilogue: The Silverpit Crater
175
Appendix 1 Appendix 2 Bibliography Index
178 193 195 203
Foreword The last 40 years have seen a revolution in planetary science. Unmanned and manned missions to the Moon, studies of impact craters on Mars, Venus and Mercury and on the satellites of the outer planets, and the discovery of a large population of nearEarth asteroids have shown that impact cratering is an important process, and for many bodies the dominant surface process, throughout the solar system. The recognition of large terrestrial impact structures and the realisation that the Cretaceous-Tertiary mass extinction was almost certainly caused by the impact of an asteroid have shown that impact processes are important for the geological and biological history of the Earth. About 200 terrestrial impact structures are now known, and these structures have been discovered on every continent except Antarctica. However, no impact structures have yet been identified in Great Britain or Ireland. I have set out to remedy this omission for Great Britain by searching for circular landforms and reexamining their geology with explicit consideration of the impact hypothesis. This research has sometimes required a re-assessment of British geological history and of the actual formation of impact structures. In particular, atmospheric break-up of asteroids and comets before they hit the ground may radically alter the morphology of the resulting crater. Observations by satellites and from the Earth's surface have shown that large meteoric fireballs often explode in the atmosphere, and that these explosions can cause damage on the Earth's surface. I have analysed the frequency of such events, and suggest that damaging explosions may occur over the British Isles on a time-scale of decades. There are a number of historical records that may describe fireball explosions, some of which have killed people, but many of these have been previously identified as thunderstorms, tornadoes, and even earthquakes.
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Finally, I draw attention to the largely overlooked danger of tsunami created by impacts in the oceans. In particular, if the Carolina Bays of the south-eastern United States were produced by a cometary impact in late glacial times, this impact would have caused a tsunami tens of metres high on the western coasts of Europe and the British Isles. The date of this event corresponds to the date of the destruction of the legendary island of Atlantis.
Acknowledgements My thanks and acknowledgements are due, first to Mr. Nic Howes, who sent me the maps, diagrams and photographs of the Woolhope and Hope Mansell domes that appear as Figure 6 and Figures 8 to 12. Thanks also to Bedfordshire Libraries for providing me with information about the Stevington meteorite fall and for drawing my attention to the Chilterns fireball of 1887. Also to Darlington Library for their assistance in obtaining information about the Hell's Kettles craters, to Gloucester Library for information about the Coleford meteorite fall of 1946, to the Archive Service of the Natural History Museum for information about the Tetbury meteorite fall of 1929, to Sidmouth Library and the Sid Vale Heritage Centre for information about the fireball of 1970, and to Wells Library and the Somerset Studies Library (Taunton) for details of the Wells fireball of 1596. I should like to thank the anonymous referees for comments that improved the organisation and layout of the book. Finally, I cannot sufficiently express my gratitude to my wife Sylvia for her encouragement and for her handling of correspondence and of the business side of getting the book into print.
Impacts and Geology
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CHAPTER
A Curious Omission 'I have not got housemaid's knee. Why I have not got housemaid's knee, I cannot tell you, but the fact remains that I have not got it. Jerome K. Jerome, Three Men in a Boat The curious incident of the dog in the night-time.' The dog did nothing in the night-time' Sir Arthur Conan Doyle, Silver Blaze In t h e nearly 6 0 y e a r s since t h e end of t h e Second World War, there h a s b e e n a revolution in b o t h a s t r o n o m y a n d t h e E a r t h sciences, a s t h e importance of i m p a c t cratering a s a p l a n e t a r y process h a s come to be recognised. In 1945 m a n y scientists believed t h a t the craters of the Moon were of volcanic origin, a n d the few terrestrial meteorite craters t h a t h a d been identified were regarded a s local curiosities rather t h a n a s being geologically significant. There w a s some justification for this attitude: Hey's (1966) catalogue of meteorite craters included 18 craters t h a t were regarded a s authentic (the largest being Deep Bay, Saskatchewan, with a diameter of 12 km), b u t only 12 of these 18 craters h a d been recognised before 1945. These 12 craters were Aouelloul (Mauritania), Boxhole, Dalgaranga a n d H e n b u r y (Australia), C a m p o del Cielo (Argentina), Haviland
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(Kansas), Kaalijarv (Estonia), Meteor Crater (Arizona), Mount Darwin (Tasmania), Odessa (Texas), Tunguska (Siberia), and Wabar (Saudi Arabia). The largest of these craters was Meteor Crater (now re-named Barringer Crater), with a diameter of 1.2km. However, even as early as the end of the 19th century and the beginning of the 20th, a few scientists were willing to accept a role for impact cratering in both astronomy and geology. In particular the American geologist Grove Karl Gilbert (1893), the Estonian astronomer Ernst Opik (1916), and the German meteorologist Alfred Wegener (1920) all advocated an impact hypothesis for the origin of the craters of the Moon, even though their arguments attracted little attention at the time. The first terrestrial impact structure to be identified as such was, of course, Meteor Crater (35°02'N, 111°01'W), where A.E. Foote (1891) discovered large numbers of iron meteorites that were clearly associated with a deep, circular and non-volcanic depression. The impact interpretation for this crater was confirmed beyond dispute by the undaunted efforts of Daniel Moreau Barringer and E.M. Shoemaker (1928-97). However, the first application of the impact hypothesis to a really large terrestrial structure was the suggestion by Werner (1904) that the Ries Kessel 1 (48°53'N, 10°37'E), a circular depression in southern Germany with a diameter of 24 km, was a meteorite crater. In 1910 A. Hogbom compared Lake Mien and Lake Dellen, 2 in Sweden, to Meteor Crater, and suggested that they were also impact craters (von Engelhardt, 1972). In 1921, P. Eskola described supposedly volcanic rocks from Lake Janisjarvi, 3 and pointed out that these rocks were very similar to those of Lake Mien and Lake Dellen, and to those of Lake Lappajarvi (63°10'N, 23°40'E), in Finland. Later, M. MacLaren (1931) suggested that Lake Bosumtwi (6°32'N, 1°24'W), in 1
This structure is now called the Nordlinger Ries, or simply the Ries. The name Ries Kessel means Giant Kettle. ^ h e impact melt rock 'dellenite' was regarded by Tyrrell (1950) as the type for the volcanic rock rhyodacite. This shows how difficult it can be to distinguish impact melts from volcanic rocks. 3 Janisjarvi (61°58'N, 30C55'E) is now in Russian Karelia; however, during the 1920s it was in eastern Finland. The word jarvi is the Finnish for 'lake'.
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Ghana, was an impact crater; and in 1936 F.E. Suess and O. Stutzer separately suggested that Kofels Hollow (47°13'N, 10°58'E), a 4-km wide circular basin in the Otztal of the Austrian Tirol, was a meteorite crater of late Pleistocene or even Holocene age. I should also mention a mysterious person named J. Kalkun [alias J. Kaljuvee), apparently an Estonian, who in a 1933 work entitled 'Die Grossprobleme der Geologic' ['The Main Problems of Geology') suggested that the great Hungarian Plain was a meteorite crater. According to Baldwin (1963), Kalkun compared the Kaalijarv craters in Estonia to Meteor Crater as early as 1922. During the same period the Englishman William Comyns Beaumont (1873-1956) argued that most if not all natural disasters were due to meteorite impacts, and pointed to the Sedgemoor basin in Somerset and the glacial lochs of the Hebrides, the Orkneys and the Shetlands as examples of meteorite craters. During the same period, a number of anomalous terrestrial structures had been found that showed brecciation and faulting without any obvious geological cause. The prototype of these structures was the Steinheim Basin (48°42'N, 10°04'E), near Heidenheim in south Germany; this basin was described by Branca and Fraas (1905) and was called a 'crypto-volcanic structure.' W.H. Bucher (1933) described six similar structures in the United States, namely Serpent Mound (Ohio), Jeptha Knob (Kentucky), Upheaval Dome (Utah), Decaturville (Missouri), Wells Creek (Tennessee), and Kentland (Indiana). Bucher (1936) later added Hicks Dome (Illinois) and Crooked Creek (Missouri) to this list. These crypto-volcanic structures were characterised by a circular central uplift, a few kilometres in diameter, where concealed sedimentary rocks had been tilted and uplifted by a few hundred metres to be exposed at the surface, and had suffered faulting and brecciation. The oldest rocks exposed in the uplift were in the centre, and they were surrounded by successively younger rocks, which dipped steeply outwards. (In some of the cryptovolcanic structures the sedimentary rocks had actually been overturned and therefore dipped inwards; however, these structures could be distinguished from ordinary synclines by the fact
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that older rocks were exposed nearer to the centre.) Radial and concentric faults were also present. In well-exposed structures, the central uplift could be seen to be surrounded by a ring syncline (called s j , which was in turn surrounded by a ring anticline (called aY). In the Wells Creek Basin (36°23'N, 87°40'W), in Tennessee, the ring anticline was encircled by an outer ring syncline (s2), which was itself encircled by an outer ring anticline (02), with a diameter of about 8.5 miles (13.6km). On geological maps these crypto-volcanic structures appeared as circular inliers of older rocks surrounded by concentric circular outcrops of successively younger rocks; this concentric pattern was, however, disturbed, and often disguised, by intense faulting. When they were first discovered, these crypto-volcanic structures were thought to have been produced by explosive outbursts of hot, high-pressure volcanic gases. However, in 1933 Rohleder suggested that the Steinheim Basin, the prototype of these structures, was actually an impact crater. A few years later Boon and Albritton (1938, 1942), in America, suggested that the crypto-volcanic structures of Bucher (1933, 1936) were actually the roots of eroded meteorite craters. Boon and Albritton argued that the rocks under the crater would respond as a fluid to the shock wave produced by the impact and explosion of a giant meteorite, and that the geological structure created by the shock would consist of a central uplift formed by the rebound of the rock, encircled by concentric ring synclines and anticlines. These features were exactly those observed in the 'crypto-volcanic structures.' Boon and Albritton had, in fact, identified the 'crypto-volcanic structures' as complex impact structures, with flat floors and central peaks, rather than simple bowl-shaped craters like Meteor Crater. However, the distinction between the two types of crater was not to be recognised for many more years. European study of lunar and terrestrial impact craters was checked by the man-made catastrophe of the Second World War, and interest in the subject lapsed for nearly 20 years. Very few papers were published about the Nordlinger Ries and the Steinheim Basin during the 1940s and 1950s. The Swedish and Finnish impact structures appear to have been almost forgotten, and they were rediscovered only during the 1960s. Kofels Hollow was likewise forgotten, in spite of its location in a popular tourist area, and has received little serious attention since the war.
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In America, however, there was an increased interest in meteorites and impact craters. The impact hypothesis for the craters of the Moon was at last put on a firm footing by R.B. Baldwin (1949) in his book The Face of the Moon. Meanwhile, on Earth, R.S. Dietz showed that the striated conical rock fractures called shatter cones were indicators of high-pressure shocks that could be produced only by meteorite impacts. These shatter cones had been first discovered in the Steinheim Basin by Branca and Fraas (1905), and Dietz and other geologists now proceeded to find them in several of Bucher's crypto-volcanic structures. Dietz himself found large shatter cones in the Kentland (Indiana) structure in 1945; and shatter cones were later found in Wells Creek, Flynn Creek, Decaturville, Crooked Creek, Serpent Mound, and Sierra Madera (Texas). Outside the United States, shatter cones were also found in 1961 in the huge Vredefort crypto-volcanic structure (D= 140 km) in South Africa, confirming a suggestion by Daly (1947) that it was an impact structure. In the light of these discoveries, and to avoid the suggestion that they were volcanic, the 'crypto-volcanic structures' were renamed 'crypto-explosion structures.' Another demonstration of the reality and the importance of impact cratering came on February 12, 1947, when a large iron meteorite broke up in the atmosphere over the Sikhote Alin Mountains, north of Vladivostok, and fell as a hail of iron masses, some of which weighed several tons. The largest intact piece of this meteorite had a mass of 1.75 tons; larger pieces broke up when they hit the ground. This meteorite produced a field of 106 craters, the largest of which was 26.5 metres in diameter. The systematic search by C.S. Beals and his co-workers for impact structures in Canada must also be mentioned. This search yielded a large number of candidates, notably the Brent and Holleford craters (Ontario), Deep Bay (Saskatchewan), Clearwater Lakes and Lac Couture (Quebec), and West Hawk Lake (Manitoba). It is partly as a result of this work that Canada can boast 26 authenticated impact structures (Grieve 1991, 1996). In 1953 a new mineral called coesite, a dense, high-pressure form of silica (SiOa), was created in the laboratory, and was quickly recognised as a criterion for the identification of meteorite
Part I: Impacts and Geology craters. It was first found in nature in Meteor Crater (Chao et ah, 1960) and in the Ries (Shoemaker and Chao, 1961). Also in 1961, an even denser high-pressure form of Si0 2 , called stishovite was produced; and this mineral was also found in both Meteor Crater and the Ries. Later in the 1960s a new indicator of impact shock was recognised in the rocks of crypto-explosion structures. These were the so-called planar deformation lamellae, microscopic fractures in crystals of quartz and feldspar. According to Grieve (1987), these fractures correspond to glide planes filled by solid-state glass. These lamellae occur along specific crystallographic orientations, and they form at shock pressures between 5 and 35GPa (50 to 350kbar). At higher shock pressures (30-45 GPa) the crystal structure of minerals is destroyed, although the crystal habit is retained, and the mineral is converted to diaplectic or thetomorphic glass. At still higher pressures (>45GPa) the rock is actually melted and forms sheets of impact melt, which has often been mistaken for volcanic rock. The identification of these mineralogical stigmata of impact made it possible to identify impact structures with certainty, and to measure the shock pressures reached in them. Recognition of impact cratering as an important geological process was also advanced by space missions to other planets. The discovery by Mariner 4 in 1965 of craters on Mars took most scientists by surprise; and, for my own part, I was astonished by the close resemblance of the surface of Mercury (as observed by Mariner 10) to the lunar surface. Missions farther afield, to the satellites of the giant planets, showed that cratered planetary surfaces were the rule. It became clear that in this respect the Moon was typical of the solid bodies of the solar system and that it was the almost uncratered surface of the Earth that was exceptional. As if photographs of cratered planetary surfaces were not enough, the close approaches of the asteroid 1566 Icarus during 1968 and of many other asteroids since then provided immediate reminders that impact cratering of planetary surfaces remains an active process. The advent of piloted space missions and of Earth surveillance satellites led to the discovery of many new terrestrial impact structures, which could be identified by their circular shape.
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Some of these structures also showed a characteristic 'bull's-eye' appearance due to the concentric outcrops of rock in and around the central uplift. Examples of such structures were the Araguainha Dome and Serra da Cangalha in Brazil, Aorounga in Chad, and Gosses Bluff in Australia. These new developments, the identification of circular structures on satellite images and the application of the mineralogical criteria of impact shock, led to a rapid increase in the number of known impact structures, which have now been identified in every continent except Antarctica. Grieve (1987) listed 116 authenticated impact structures; the number had increased to 131 in Grieve (1991); and Grieve (1996) stated that 149 such structures were known in 1995. The most recent list, compiled by Fortes (2000), lists no fewer than 224 impact structures. When one includes possible but not yet authenticated impact structures the number is increased to more than 250. Classen (1977) already listed 230 terrestrial impact structures, although many of these have not yet been authenticated by detailed study. Three particularly significant recent discoveries are Chicxulub, in the Yucatan Peninsula of Mexico (D~ 180 km, age = 65Myr), which was probably responsible for the Cretaceous-Tertiary mass extinction; Chesapeake Bay (D~85km, age ~34Myr), which is probably the source of the North American tektite strewn field4 (Poag et ah, 1994); and Lake Tonle Sap, in Cambodia (about 100km x 35km, age ~0.78Myr), which may be the source crater of the Australasian tektites (Hartung & Koeberl, 1994). That three such large craters, all <100Myr old and one forming the site of a national capital, should be discovered in the last 20 years, indicates both that there are many more such structures waiting to be discovered and that impact rates calculated in the 1980s are likely to be underestimates. The Earth's impact structures range in size from mere fragmentation pits like Dalgaranga (Australia), Haviland (Kansas) and Tannas (Sweden), which could almost fit into a suburban back garden, to vast structures like Chicxulub, Sudbury and Vredefort, which are larger than Wales. They range in age from 4
It has been suggested that the Everglades in Florida mark the site of an even larger impact structure of the same age, with D~ 120-130 km.
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the Sikhote-Alin crater field, formed on 1947 February 12, 5 to the Vredefort structure of South Africa, which is ~2000Myr old. They range in latitude from the Haughton structure in Canada, at 75°22'N, to Mount Darwin, in Tasmania, at 42°15'S. Some of these craters are in the Earth's most inaccessible places, for example Aorounga in Chad and Lake Elgygytgyn in NE Asia. Other structures are in densely populated regions: the Des Plaines structure, in the suburbs of Chicago, has had an airport built on top of it; and the south German town of Nordlingen has been built inside the Ries out of the fused and shattered rocks of the impact structure. (In the light of European history, it is not surprising that two battles have been fought inside the Ries, in 1634 and 1645.) There are even submarine impact structures: Montagnais (42°53'N, 64°13'W; D=45km) on the Nova Scotian continental shelf; Toms Canyon (39°08'N, 72°51'W; D= 10-15km) off New Jersey (Poag et al, 1992); Mj0lnir6 (73°48'N, 29°40'E; D=40km) in the Barents Sea; the Neugrund structure off the north-west coast of Estonia; the Fohn structure (13°15'S, 128°39'E) off north-western Australia; and the large Eltanin structure (57°47'S, 90°47'W) in the south-east Pacific Ocean (Fortes, 2000). It is notable, though, that impact structures tend to prefer continental interiors and avoid offshore islands; no impact structures have been identified even on such large islands as Greenland, New Guinea, Borneo, Madagascar, and Baffin Island, or on the densely populated islands of J a p a n and New Zealand. Some impact structures, such as Serra da Cangalha in Brazil, Manicouagan in Canada, Gosses Bluff in Australia, and the mysterious Richat structure in Mauritania, are spectacular landforms, which provide some of the most impressive images in satellite photographs. Others, such as Wells Creek and Popigai, are barely discernible in aerial and satellite photographs; still others, such as Steen River in Alberta, have been buried by younger rocks, and can be studied only by drilling and by geophysical sounding. 5
Several unconfirmed cratering impacts have been reported since 1947, among them the formation of a 50-metre crater in Honduras in November 1996. 6 Named after the hammer of the Norse god Thor.
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Studies of these structures have led to a better understanding of both simple meteorite craters and of complex impact structures, and to identification of the characteristic morphological features of both classes. Recognition of these characteristic features may make it possible to discover new craters and impact structures. Simple meteorite craters, with diameters < 2 - 4 km, form bowlshaped depressions with steep inner slopes and raised outer rims. The floors of these craters are underlain by lenses of impact breccia and impact melt; the breccia shows the characteristic effects of impact shock. Some of these depressions (such as the Barringer Crater) are dry; but in temperate regions these craters often form circular lakes, such as New Quebec Crater in Canada and Kaalijarv in Estonia. In glaciated regions such as the Canadian Shield, these circular lakes are easily recognised by their contrast with the elongated lakes formed by glacial erosion; they are thus both readily identifiable as meteorite craters and useful landmarks for aerial navigators. Simple meteorite craters are generally short-lived, but a few, such as the Brent and Holleford craters in Ontario, and probably Lake Hummeln in Sweden, have survived for geologically long periods as a result of burial and later exhumation. Complex impact structures show different features. Structures with diameters between 2-4 km and ~25km possess central uplifts, which are analogous to the central peaks of lunar craters. These central uplifts have a diameter of 0.22 ± 0.03 times the diameter of the crater. In larger structures, with D > 2 5 k m , the central peak is replaced by a ring of peaks with a diameter 0.50 times the diameter of the crater. The Ries is regarded as an example of a terrestrial peak-ring crater of this sort (Melosh, 1989). On the Moon the transition from central-peak craters to peakring craters takes place at a crater diameter of D~140km, rather than D~25km as on Earth. This difference is a result of the weaker surface gravity of the Moon. In consequence lunar peakring craters are rather rare; and there are no good examples on the Earth-turned side of the Moon. In regions of crystalline (igneous and metamorphic) rocks, complex impact structures form either circular lakes, with or without central islands (e.g. Lake Mistastin, Lappajarvi, Deep Bay,
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Lake Mien), or arcuate chains of lakes surrounding a central uplift (e.g. Siljan and Lake Manicouagan). The most beautiful of these structures is the Clearwater Lakes, in Quebec; the western member of this pair of lakes contains a concentric ring of islands that forms the inner peak ring of the crater. In regions of sedimentary rocks, complex impact structures are characterised by a circular central uplift, consisting of steeply tilted or even overturned rocks brought up from beneath the floor of the crater. This central uplift is severely faulted, and its rocks are often brecciated and mixed together. In uneroded complex impact structures, such as Haughton Dome in Canada, the brecciated central uplift may be covered by a layer of allochthonous breccia that originally covered the floor of the crater. The central uplift is often covered by an annular peripheral depression, which is itself surrounded by a faulted outer rim; these components are particularly well developed in Wells Creek (Tennessee), Gosses Bluff (Australia), and Richat (Mauritania). Geologically, complex impact structures generally appear as circular faulted and brecciated inliers corresponding to the central uplift of the structure, surrounded by concentric circular outcrops of successively younger rocks towards the rim. In three dimensions these impact structures appear as circular domes, with the outward dips of the rocks increasing towards the centre; strictly, in view of this centripetal increase in dip, they would be better described as cusps. Some impact structures have, at first, been mistaken for tectonic domes or for alkaline igneous complexes, which often uplift the country rocks. Such misinterpretations are more likely in deeply eroded impact structures, since the signs of shock metamorphism inevitably die out as the shock pressure decreases with increasing depth beneath the original floor of the crater. The deformation of the rock may thus become almost indistinguishable from ordinary tectonic folding and faulting. The only remaining evidence for impact is then the circular shape of the uplift, and, in geologically stable regions, the isolated nature of the disturbance. One can turn this argument round to predict the existence of a class of eroded impact structures that are exposed at a level below the reach of megabrecciation and shock metamorphism, but that still show the characteristic circular central uplift and radial and concentric faulting. Such structures might be called
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'hypo-astroblemes' or 'infra-impact structures.' Studies of complex impact structures suggest that the effects of shock metamorphism are observable to a depth (dgh) below the crater floor of about a tenth of the final diameter of the crater (i.e. d^ ~ 0.1 x Df), whereas appreciable structural deformation occurs to depths ddej -0.2-0.3 xD f . Thus there should be 1-2 times as many 'hypoastroblemes', without shock metamorphism, as there are classical impact structures with shock metamorphism, shatter cones, and dense polymorphs of silica. The Richat and Semsiyat domes in Mauritania, which have almost the ideal morphology of impact structures but which lack evidence of shock, are probably examples of such 'hypo-astroblemes'. Although very large iron meteorites, with masses of thousands of tons, reach the Earth's surface intact and form single large craters with D~ 1km, smaller irons, with masses of a few hundred tons, generally break up in the atmosphere and fall as an 'iron hailstorm' which produces a group of craters. The largest craters of such groups have diameters between about 25 and 300 metres. Several such groups of craters are known, for example the Henbury, Odessa, Campo del Cielo, and Sikhote-Alin groups. This morphology is so characteristic that it can be used by itself as evidence for impact. For example, near Quillagua (21°30'S, 69°20'W), in northern Chile, there is a chain of five groups of craters and isolated craters, with diameters ranging up to 300 metres, strung out over a distance of about 97km from north-east to south-west. 7 These crater fields were probably formed by a disintegrating iron meteorite. Eight large hexahedrite meteorites have been found in the same area of northern Chile, and these may be fragments from the same fall. In the diameter range between these crater fields and single bowl-shaped craters like Barringer Crater is a transitional impact regime associated with iron meteorites with diameters of about 10-100 metres, where the effects of aerodynamic pressure are extremely important. This transitional regime is poorly understood, although it appears that the outcome of such impacts depends on both the 7
Or from south-west to north-east; it is not clear which direction the meteorite came from.
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initial size of the meteorite and the speed at which it enters the atmosphere. 8 However, there are differences of opinion over the details. Hills and Goda (1993) calculate that iron meteoroids with initial velocities v0 > 2 0 k m / s and radii between 1 and 20 metres will break up explosively in the atmosphere and yield meteorites with maximum masses of only a few kilograms. Of course such meteorites will not form craters, and the explosion will be essentially identical to a Tunguska-type airburst. On the other hand, Melosh (1989) argues that large iron meteoroids ( r > 2 0 metres) with u 0 > 2 5 k m / s will be crushed to fragments by aerodynamic pressure, and that the resulting 'meteorite swarm' will be compressed and flattened into the shape of a discus or a fat pancake. The impact of such a flattened swarm of meteorites may produce a 'shotgun blast' crater, very shallow and with a nearly flat floor and a very low narrow outer rim; such a crater may be regarded as a crater field in which all the individual craters overlap. There is indirect evidence for the reality of this transitional impact regime in the fact that there are few known meteorite craters with diameters (D) between 300 and 1000 metres, and none of them are of Holocene age, whereas there are many single craters and groups of craters with D< 300 metres, and a large proportion of these craters are Holocene, or at least post-glacial [t< 12kyr). The rarity of craters with D between 300 and 1000 metres might be explained on the supposition that the shallow, flat-floored craters proposed by Melosh have simply not been recognised as meteoritic owing to their unusual morphology; moreover, such craters will be destroyed by erosion more quickly than deep bowl-shaped craters (formed by impacts with u 0 < 2 0 k m / s ) like Barringer Crater and New Quebec Crater. This reasoning implies that understanding atmospheric interactions and surface impact processes in this transitional regime may be important for estimates of impact fluxes and cratering rates. There may be a similar transition from groups and clusters of craters through shallow, low-rimmed structures to the classical large complex impact structures among the impact structures
8
Hills and Goda (1993) suggest that even the height of the crater above sea level, which governs the surface atmospheric pressure, may be important. They point out that Barringer Crater, which is ~2 km above sea level, may be anomalous in this respect.
A Curious Omission
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formed by stony asteroids and even perhaps by cometary nuclei. Empirical evidence suggests that craters in groups formed by the impact of stony meteorites or asteroids have maximum diameters of about 2-4 km (e.g. Rio Cuarto, in Argentina) (Schultz and Lianza, 1992). The transitional regime for stones corresponds to asteroids with diameters between about 200 and 600 metres and impact structures with diameters of about 4-15 km. Melosh (1989) identifies Flynn Creek and Decaturville as shallow impact structures belonging to this transitional regime. It appears that disintegrating cometary nuclei with D < 500 m cause damage at the Earth's surface through their associated air blast, rather than producing crater fields by direct surface impact. The transitional impact regime corresponds to a cometary diameter of about 1 to 3 km, and to craters with diameters of roughly 20 to 80 km. Thus similar morphological transitions occur with increasing size in the craters and impact structures formed by all three of the main types of impactors. Examples of such transitional impact morphologies produced by all these types of impactors will be cited in this book. The importance of these aerodynamic effects has also been demonstrated by the discovery that complex impact structures, like simple meteorite craters, sometimes occur in chains. These chains are thought to have been formed by asteroids or comets that broke up shortly before colliding with the Earth. For example, the Aorounga structure in Chad has been found to be a member of a chain of three or four craters, the largest of which is 17 km in diameter (Ocampo and Pope, 1996). Aorounga itself is 12.6km in diameter. Again, a study by Rampino and Volk (1996) has identified a chain of no fewer than eight impact structures strung out along a distance of at least 600 km in the Midwestern United States, from Kansas to Illinois. These eight structures are the notorious members of the '38th parallel lineament' identified by Snyder and Gerdemann (1965). They are, from west to east, Rose Dome (Kansas), the Weaubleau area, Decaturville, Hazel Green, Crooked Creek, Furnace Creek and the Avon diatremes in Missouri, and Hicks Dome (Illinois). Crooked Creek and Decaturville were identified as impact structures in the 1950s and 1960s, but the other six structures were thought to be of igneous origin.
IIMIIIIWtBI
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Part I: Impacts and Geology
The Steinheim and Ries craters in Germany appear to belong to a complex chain, which has been strangely neglected by scientists. Near the spa town of Bad Urach (48°29'N, 9°25'E), in the Swabian Alps of south-west Germany, is a field of breccia pipes or tuffisite pipes of supposedly volcanic origin. It may be significant that this volcanic area is directly in line with the Ries crater and Steinheim Basin, about 58 km west-south-west of Steinheim. East of the Ries, in central Bavaria, seven craters or groups of craters have been identified, ranging in diameter from 0.85 km to 2.5 km. These craters lie exactly between the Ries and a field of tektites in Bohemia (the Czech Republic); moreover, the craters diminish in size from west to east, that is, the largest craters are nearest to the Ries. North-east of the Ries is another possible impact structure, called the Stopfenheim Kuppel (or Stopfenheim Dome); this structure is 8 km in diameter (Classen, 1977). This structure lies between the Ries and another field of tektites in the region of Lausitz (or Lusatia), near Dresden, about 300 km from the Ries (Storr and Lange, 1992). The full length of this chain of possible impact structures and tektite strewn fields, from Bad Urach to yet another tektite field in Moravia, is about 500 km. However, the 38th parallel lineament and the Ries chain pale before the groups and chains of impact structures in southern Africa. The vast Bushveld Igneous Complex (about 25°S, 29°E) of Transvaal appears to consist of at least three Precambrian impact structures (Rhodes, 1975), which were probably formed at the same time as the Vredefort impact structure south of Johannesburg. The largest of the Bushveld impact structures, centred near 25°S, 291/2°E, is probably 200-250 km in diameter, and the whole complex is about 500 km from east to west. This enormous impact complex is similar in size to Mare Crisium on the Moon. Even the Bushveld Complex does not exhaust the possibilities of southern Africa. Recently a huge Upper Jurassic impact structure, at least 70 km in diameter, has been identified at Morokweng (26°08'S, 23°45'E) in northern Cape Province. Other geologists have suggested that the Bangweulu Basin (11°1'S, 29°45'E) and the Lukanga Swamps (14°26'S, 27°45'E) in Zambia
A Curious Omission
MM
are also impact structures, with diameters of about 150 km. Examination of a map shows that the Okavango Delta (19°27'S, 22°54'E) in Botswana is aligned with the Bangweulu Basin and the Lukanga Swamps. Moreover, the large complex depression of the Makgadikgadi Saltpan (20°41'S, 25°26'E), south-east of Okavango, is located essentially midway between Lukanga and Morokweng. These two basins are about 100-130 km in diameter. If these depressions form a chain of impact structures, the length of this chain is about 1800 km, approximately the distance from Liverpool to Naples. It may be noticed that both this chain and the Ries chain split in two along their length. The large number of known impact structures implies that they are quite thickly distributed over the Earth's surface. The total land surface area of the Earth is about 150 x 10 6 km 2 . If there are about 200 known terrestrial impact structures, the mean surface density is 1 impact structure per 750,000 km 2 . This figure is slightly more than three times the area of the United Kingdom (244,030km 2 ), and 2.4 times the total area of the British Isles (315,173 km 2 ). However, it would be an error to suppose that this density of impact structures is typical of any area of the Earth's land surface. Most impact structures have been identified on the stable continental cratons, particularly the Midwestern United States, the Canadian Shield, Europe north of the Alps, and the desert regions of Australia. The Earth's mobile belts (for example, the Alpine Himalayan belt and the circum-Pacific belt) have hardly any impact structures, owing to their rapid destruction by tectonic deformation and by erosion. 9 Likewise, there are no known impact structures in active sedimentary basins, such as Bangladesh, the Netherlands, or the Gulf States of America, where they are quickly covered by younger rocks. In addition, the continents of Asia, Africa, and South America, which have been less thoroughly explored than North America, Europe, and Australia, have so far yielded fewer impact structures. Grieve (1991) lists only twelve craters and impact structures in Africa (30.335 x 10 6 km2) and five in South America ^ h e r e is one exception to this rule: Lake Kara-Kul (D = 45 km), in the Pamir Mountains of Tajikistan, is identified as an impact structure by Grieve (1991).
HUPS |Hal
Part I: Impacts and Geology
(17.611 x l 0 6 k m 2 ) , against eight in Quebec (1.541 x 10 6 km 2 } and five in Ontario (1.069 x 10 6 km 2 ). The implied density of impact structures in Quebec and Ontario is thus about one per 2 x 10 5 km 2 . The stable European craton tells a similar story to the United States and the Canadian Shield; most of the countries of northern Europe have impact structures. The Ukraine has seven such structures 1 0 ; Belarus has one (Logoisk); European Russia has eight; and the Baltic States (Estonia, Latvia, and Lithuania) have four impact structures between them, as well as two groups of small Holocene craters (Ilumetsy and Kaalijarv). Poland has a group of seven small Holocene craters at Morasko (52°29'N, 16°54'E), north-west of Poznan, and a single crater, 100 metres in diameter, at Frombork (54°20'N, 19°41'E). No large impact structures have been discovered in Poland, probably because more than half the area of the country is covered by Quaternary sediments. Germany has the Ries and Steinheim Basin, and the smaller craters that may have been formed by the same impact. In addition, Gallant (1964) points to the 1-km Randecker Maar as a possible meteorite crater. However, the main search for European impact structures has focused on the countries of Fennoscandia, with spectacular results. A list compiled by A.D. Fortes (2000) includes four impact structures in Norway, where the terrain and recent glacial erosion must hamper searches, 12 in Finland, and no fewer than 42 in Sweden. Four of these structures (all of them of Proterozoic age) have D> 100 km, the largest being the vast Uppland structure, with its centre near Uppsala, with a diameter of about 320 km. It should be remembered that Norway, Sweden and Finland combined have an area less than four times that of the British Isles. Denmark and the Netherlands also have no large impact structures, again because they consist predominantly of Quaternary sediments. Belgium and Luxembourg also lack impact structures, probably because both are small countries and because Belgium consists largely of Tertiary rocks.
'By an odd quirk of nomenclature, the Odessa craters are not in the Ukraine.
A Curious Omission
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Elsewhere in Western Europe, France has one large impact structure, with a diameter of 23 km, at Rochechouart (45°50'N, 00°50'E), west of Limoges. It is of historical interest that the town of Chalus, where King Richard the Lionheart was killed, is near to the Rochechouart structure. It may be of more geological significance that the village of Montmorillon (46°26'N, 00°50'E), the original source of the clay mineral montmorillonite, is about 60 km from Rochechouart. In addition to this large structure, France has a group of six or seven small craters, which are described by Baldwin (1963), at 43°32'N, 03°08'E, near Cabrerolles and Faugeres in the departement of Herault. These craters have diameters between 45 and 220 metres, and are thought to be about 10,000 years old. Graham et al. (1985) regard these craters as discredited. However, as I have explained previously, meteorite craters with diameters of about 200 metres generally occur in groups, and the conformity of the Herault craters to this morphological pattern is evidence that they are indeed meteorite craters. A number of small lakes at Sucy-en-Brie and Alentours, near Paris, have also been mentioned as possible meteorite craters. Farther west still, Spain has the 30-km Azuara structure (41°10'N, 00°55'W), south of Zaragoza. Even the Alpine nations of Switzerland and Austria have possible meteorite craters. A 400-metre crater at St.-Imier (47°10'N, 07°00'E), in the Swiss J u r a Mountains, is said to have shatter cones and small iron particles (Graham et al, 1985); shatter cones have also been found at Lago di Tremorgio (d= 1.36 km), in Ticino canton. An impact origin was suggested for Kofels Hollow (47°13'N, 10°58'E; D=4km) in the Austrian Tirol in 1936; and this suggestion has been a matter of debate ever since. According to von Engelhardt (1972), Storzer et al. (1971) identified diaplectic glass and planar features in quartz in a glassy dyke from Kofels; however, Officer and Carter (1991) argue that the hollow is the trace of a giant landslide. The formation of the hollow has been dated at about 8150 B.C. (von Engelhardt, 1972), or 8700 BP (Officer and Carter, 1991); if it is an impact structure it is a very young one and probably of Holocene age. However, the largest suggested impact structure in Europe outside Sweden is in the heart of the continent, in the Bohemian
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Part I: Impacts and Geology
Massif of the Czech Republic. According to Rajlich (1992), the entire massif constitutes an impact structure 260 km in diameter, associated with breccias and shock-metamorphosed glass. The structure is probably of Late Proterozoic age (about 600-1000 Myr). Since impact structures are so abundant in other parts of Europe, it seems strange that there are none known in the United Kingdom or Ireland. The British Isles are, after all, part of the stable European craton; they have outcrops of rock belonging to every geological period since the Late Proterozoic; and they have been very thoroughly explored. It is difficult to believe that any large impact structure, or even a reasonably well-preserved meteorite crater larger than a few hundred metres in diameter, could have escaped the notice of the Ordnance Survey or the British Geological Survey. It must also be remembered that the British Isles are surrounded by the shallow seas of the northwest European continental shelf, and that this shelf covers a larger area than the islands themselves. It is therefore strange that exploration for oil and gas on the continental shelf has not yet led to the discovery of impact structures there. This anomaly is strengthened by the recent discoveries of the Montagnais, Toms Canyon, Mj0lnir and Neugrund structures. Thus the curious omission that forms the title of this chapter is the fact that there is not a single established impact structure or cryptoexplosion structure anywhere in the United Kingdom or Ireland, or on the surrounding continental shelf; there are not even any small groups of craters comparable to Morasko or Kaalijarv. The absence of British meteorite craters and impact structures is a geological anomaly; and this book will discuss the reasons for the absence of such structures, as well as identifying and describing some circular landforms that may in fact be the products of impacts. The first step in the search for British impact structures is to study the statistics of impacts, that is, to measure the surface densities of impact structures in other countries, and to obtain estimates of cratering rates. Such statistical analyses can give indications of the probable number and sizes of British impact structures, and even of their age distribution. These calculations will be the subject of the next chapter.
CHAPTER
Of Calculations and Craters Who hath laid the measures thereof, if though knowest? Or who hath stretched the line upon it? Job, 38, 5. The first chapter of this book argued that impact structures could no longer be regarded as merely local curiosities, and that instead they were common features of the Earth's surface a potentially of geological importance. However, it might have been felt that the suggestion that there are impact structures in Britain might have been based purely on nationalistic sentiment, a feeling that if Sweden, Finland, Germany and France have impact structures it is only fair that the United Kingdom should be similarly favoured. It is therefore necessary to provide quantitative arguments in favour of the thesis. These arguments, besides their bearing on the impact question, have interesting geological implications in their own right. It was stated in Chapter 1 that there are at least 250 authentic and possible terrestrial impact structures, implying a surface density on land of 1 impact structure per 6 x 10 5 km 2 . However, the fact that impact structures in the Earth's mobile belts are quickly destroyed by tectonic processes and by erosion means
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Part I: Impacts and Geology
that we must turn to the stable cratons to obtain an accurate estimate of the true surface density. Estimates of one impact structure per 2 x 10 5 km 2 have already been obtained for Quebec and Ontario. However, of the nine craters in Quebec, the two Clearwater lakes form a pair, and should be counted as a single impact; and the New Quebec Crater, with a diameter of only 3.44km, is likely to be short-lived, even as a crypto-explosion structure. Of the five craters in Ontario, the Sudbury structure is Precambrian (Lower Proterozoic), and should not be used in calculations of the areal density of Phanerozoic craters. These adjustments reduce the densities of impact structures in Quebec and Ontario to 1 per 2 . 2 x l 0 5 k m 2 and 1 per 2 . 7 x l 0 5 k m 2 respectively. Larger areal densities of impact structures are found on the continental platform of the eastern United States, west of the Appalachian Mountains. In analysing these data, it should be remembered that there are several probable impact structures in the eastern USA that are not on the lists of Grieve (1987 and 1991), or even of Fortes (2000), and that inclusion of these structures increases their areal density. For example, Grieve (1991) lists two impact structures (Wells Creek and Flynn Creek (36°16'N, 85°37'W)) in Tennessee (109,411 km 2 ). In addition to these two, Officer and Carter (1991) regard Howell (also mentioned by Baldwin (1963)) as an authentic impact structure; and Hey (1966) mentions the Dycus structure (36°22'N, 85°45'W), in Jackson County, as having upwardly directed shatter cones. The Howell structure is admittedly very small, only 1.6 or 2.4km in diameter; and Dycus is so close to Flynn Creek (only 16 km to the north-west) that they may be the result of a double impact. Thus there may be either two, three or four independent impact structures in Tennessee, yielding an areal density of between 1 per 55,000 km 2 and 1 per 27,000 km 2 . Likewise, Grieve (1991) mentions only one impact structure, Middlesboro (D = 6km), in Kentucky (104,623km 2 ). Officer and Carter, however, regard both Jeptha Knob (38°06'N, 85°06'W) and the Versailles structure (38°02'N, 84°42'W) (D = 1.5km) as authentic impact structures. If these are accepted as genuine, they yield an areal density of impact structures of 1 per 35,000 km 2 .
Of Calculations and Craters
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Baldwin (1963) describes Jeptha Knob as 'a small cryptovolcanic structure' with a total diameter of about 2^ miles (3.8 km). Other authorities have given larger diameters, up to 24 km (Seeger, 1985), and the morphology, with a central uplift and surrounding ring anticline, implies a diameter of at least 8-10 km. The proximity of Versailles and Jeptha Knob (40 km apart) suggests at first sight that they were formed by a double impact. However, the ages are different; Jeptha Knob is believed to be of Silurian or Ordovician age (about 410-490Ma), but the age of Versailles is given as <400Ma. Grieve lists two impact structures (Glasford and Des Plaines) in Illinois (146,075km 2 ), yielding an areal density of 1 per 73,000km 2 . However, Hicks Dome, the largest of Bucher's original crypto-volcanic structures (D = 16km), is also in Illinois. Although Hicks Dome has been regarded as being of igneous origin, its association with the 38°N chain (Rampino and Volk, 1996) and its resemblance to established impact structures suggest that it is also of impact origin. If so, the areal density of impact structures in Illinois is increased to 1 per 49,000 km 2 . Other states yield similar results. Missouri, with the Crooked Creek and Decaturville structures, has 1 impact structure per 90,000 km 2 , although if Avon, Furnace Creek, Hazel Green Creek and Weaubleau are added the density increases to 1 impact structure per 30,000 km 2 . Kansas (Rose Dome and Haviland) has 1 per 106,500km 2 ; Ohio (Serpent Mound) has 1 per 106,000km 2 ; Michigan (Calvin structure) has 1 per 151,000 km 2 ; Wisconsin (Glover Bluff) has 1 per 145,000 km 2 ; Indiana (Kentland) has 1 per 94,000 km 2 ; Iowa, with the great Manson structure, has 1 per 146,000km 2 ; North Dakota (Newporte and Red Wing) 1 per 91,500 km 2 ; Oklahoma (the Ames structure) 1 per 181,000 km 2 ; Mississippi (Kilmichael structure) 1 per 124,000km 2 ; and Alabama (Wetumpka) has 1 per 134,000 km 2 . Texas, as befits its size has no fewer than five impact structures and craters (Sierra Madera, Marquez Dome, Bee Bluff, Hico, and Odessa); the crater density is 1 per 138,500 km 2 . The block of Midwestern states from Wisconsin and Michigan to Texas has a total area of 2,527,164 km 2 and about 29 meteorite craters and impact structures; the average areal density of these structures is therefore 1 per 87,000km 2 .
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Part I: Impacts and Geology
Michigan and Wisconsin may have other impact structures. Hey (1966) and Graham et at. (1985) mention the Limestone Mountain disturbance in Houghton County (Michigan), which was apparently first identified by Bucher; however, Classen (1977) regards this structure as discredited. Officer and Carter (1991) mention the Rock Elm disturbance 1 in Wisconsin without giving any more details. Not enough is known about either of these disturbances for them to be regarded even as probable impact structures. The same crater counts can be performed for the nations of Europe. There are 12 known Finnish impact structures, four of them Cambrian or later, four Proterozoic, and four of unknown age. The areal density of Phanerozoic craters is therefore between 1 per 42,000km 2 and 1 per 84,500km 2 . Of the 42 Swedish impact structures, 12 are of Cambrian age or later, and seven are definitely Precambrian. However, the Tannas crater, (62°24'N, 12°42'E), near the Norwegian border, hardly counts, since it is only 20 metres in diameter. 2 It may therefore be taken that there are between 11 and 34 significant Swedish craters of Phanerozoic age, implying an areal density between 1 per 41,000 km 2 and 1 per 13,000 km 2 . The Baltic States (Estonia, Latvia and Lithuania) have seven impact structures (or eight if one includes the Neugrund structure off the north coast of Estonia). The total area of these three states is 174,000 km 2 , yielding an areal density of impact structures of about 1 per 25,000 km 2 . The Ukraine has seven impact structures in 603,700km 2 , and a crater density of 1 per 86,200km 2 . European Russia, with an area of about 3.7 x 10 6 km 2 , has eight known impact structures, and thus a crater density of 1 per 460,000 km 2 . This is a much smaller crater density than the Ukraine's, and there are probably more Russian impact structures to be discovered. Thus the largest crater densities are in the range of 1 impact structure per 25,000 to 100,000 km 2 ; such areal densities imply :
Rock Elm is in Pierce county in western Wisconsin, but further information is lacking. 2 A crater this small can hardly have survived the Pleistocene glaciations, and it must therefore be of Holocene age (< 11 kyr).
Of Calculations and Craters
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that the United Kingdom should have between 2 and 10 Phanerozoic impact structures, and that the Republic of Ireland should have 1 to 3 such structures. Another method of analysing the distribution of impact structures is to calculate the percentage of the area of a territory that is occupied by such structures. This method has the advantage over simply counting craters and impact structures that it weights the measurements towards larger and longer-lived structures, and is less affected by random variations in the number of small craters. For example, the percentage areal coverage by impact structures of 13 of the Midwestern United States (Texas, Oklahoma, Mississippi, Tennessee, Kentucky, Ohio, Missouri, Kansas, Illinois, Indiana, Wisconsin, Michigan and Iowa) ranges from 0.03% for Michigan to 0.26% for Missouri (with Weaubleau, Furnace Creek and Avon being counted as impact structures) and 0.7% for Iowa. (The high value for Iowa is due to the presence of the large Manson impact structure.) The average areal coverage for these 13 states is 0.11%. If the Florida Everglades genuinely mark the site of an Eocene-Oligocene impact structure, the areal coverage for the whole of the United States is about 0.26%. Similar results are obtained from the Canadian provinces. 0.07% of the area of Ontario is covered by Phanerozoic impact structures (excluding Sudbury, which is Precambrian). So are 0.09% of Alberta, 0.15% of Newfoundland and Labrador, 0.2% each of Manitoba and Saskatchewan, and 0.75% of Quebec, which contains Manicouagan, Charlevoix and Clearwater Lakes within its border. The average over all six provinces is 0.3%. The total area of Canada, the United States and Mexico that is covered by impact structures is about 67,000 km 2 , equivalent to nearly 0.3% of the area of the continent. Mauritania, in north-west Africa, whose meteorite craters and impact structures were described in my previous book Richat A Mauritanian Impact Structure, has 0.14% of its area covered by impact structures. This figure is dominated by the Richat structure, which by itself covers >0.13% of the area of Mauritania. In Africa as a whole, the great chain of possible and established impact structures from Bangweulu to Morokweng alone covers 0.2% of the area of the continent.
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Part I: Impacts and Geology
Among the European nations, Sweden has 0.68% of its area covered by its eleven Phanerozoic impact structures. The 34 structures of uncertain age add another 0.29% of the area of the country to this figure. The other European nations mostly fall short of this figure, perhaps because they have been less thoroughly explored or because their impact structures have been concealed by later sediments or destroyed by erosion and tectonic activity. The four Finnish craters that are certainly Phanerozoic cover 0.08% of the area of Finland; Logoisk covers 0.11% of Belarus; the Ries and Steinheim together cover 0.13% of Germany; Rochechouart covers 0.08% of France; and the Azuara structure covers 0.14% of Spain. The Baltic States have 0.06% of their area covered by impact structures, Ukraine has 0.12%, and European Russia has 0.16%; this last figure, however, is dominated by the vast Puchezh-Katunki structure (D =80km). On a larger scale, the Bohemian Massif structure, with D = 260 km, covers 0.5% of the total area of Europe by itself. There are eleven large impact structures in Asiatic Russia, which together cover 0.11% of the country. Although no definite impact structures have yet been identified in China, the large Duolun structure (D~ 170 km) in Inner Mongolia and Taihu Lake (D~70km) in Kiangsu province have been named as possible impact structures. If so, about 45,000 km 2 of Asia are covered by known impact structures, or 0.1% of the area of the continent. The state of affairs in Australia is unusual. The Acraman and Woodleigh structures, with D = 160 km and D = 120 km respectively, cover 0.41% of the area of the continent by themselves. The remaining Australian impact structures (excluding the Teague structure 3 , which is Lower Proterozoic) add only another 0.02% to bring the total up to 0.43%. It appears from this analysis that in well explored stable continental shields or platforms, on average 0.1-0.2% of the territory is covered by medium-sized Phanerozoic impact structures. In most of the territories studied the largest of these impact structures occupies more than half (often >80%) of the total area •^he Teague structure has now been renamed the Shoemaker structure, after the famous astronomer and geologist E.M. Shoemaker (1928-97).
Of Calculations and Craters occupied by impact structures, and this largest structure is often <200 Myr old, that is of Jurassic, Cretaceous or Cainozoic age. However, on the scale of the great continental landmasses, the areal coverage by impact structures tends to about 0.3%, with the largest individual impact structure being > 100 km in diameter (e.g. Bohemia, Acraman, Morokweng, Chicxulub and Popigai). It appears then that over a large enough area of the Earth's surface a 'saturation percentage areal coverage' of 0.1-0.2% is reached in <200Myr of meteorite bombardment, and that after this time destruction of medium-sized impact structures by erosion keeps pace with production of new ones. However, impact structures larger than the normal scale of geological exploration may also exist, structures that can be identified only from satellite photographs and small-scale maps; these structures may well cover an area larger than all the medium-sized structures put together. Given that the area of the United Kingdom is 244,030 km 2 , the predicted area covered by impact structures is between 244 and 488 km 2 , equivalent to the diameter for a single impact structure between 18 and 25 km. Even if the percentage areal coverage is only 0.08% (as in France), this amounts to an area of 195km 2 , equivalent to a diameter of 16km. A similar calculation reveals that between about 70 and 140 km 2 of the Republic of Ireland should be covered by impact structures; these areas are equivalent to diameters for a single structure of 9 and 13km. The same sort of analysis can be applied to the continental shelf of north-west Europe. The area of the North Sea is 575,300 km 2 (2.6 times the area of Great Britain); the area of the Irish Sea is 88,550km 2 (6,000km 2 larger than the island of Ireland), and the area of the English Channel is 89,900 km 2 (more than two-thirds of the area of England). The total area of these three seas is 753,750 km 2 , or 2.4 times the area of the British Isles. If the percentage are covered by impact structures is the same as that found elsewhere in Europe, then the total area of the continental shelf seas occupied by such structures is between about 750 and 1500 km 2 . These areas are equivalent to a diameter for a single impact structure between 31 and 44 km.
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Part I: Impacts and Geology
Yet another approach to the solution of this problem is to use the frequency-magnitude relations (or N-D relations) for impact and cratering rates. This approach must be employed with care for several reasons. First, different authorities give different frequency-magnitude relations, and they also use different measures of the magnitude of an impact, for example the radius or the mass of the impacting asteroid, the kinetic energy of the impactor, or the diameter of the crater formed by the impact; to convert these different measures to a common scale one must adopt some typical density and speed for the impacting object. Second, the relation between the kinetic energy of the impactor and the diameter of the resulting crater is still uncertain, and one must adopt one of the varied relations given in the literature. I have calculated frequency-crater diameter relations by assuming that the craters are produced by stony asteroids with a mean density (p) of 3,650kgm" 3 , a speed (v) of 20.8kms~ 1 , and an energy-crater diameter [E-D) relation log E = 16.02 + 3.18 xlogD, with D in kilometres and E in joules (Hughes, 1998). Third, it must be remembered that both iron and stony meteorites with initial radii between about 1 and 100 metres usually break up in the atmosphere (Hills and Goda, 1993), and that as a result single craters with diameters between about 0.1 and 3 km, which would be formed by such meteorites, are rather rare. For example, 10-20 Mton (4-8 x 10 16 J) fireball explosions similar to the Tunguska event of 1908 probably occur about once a century, and every one of these Tunguskid' meteorites could produce a crater of about the size of Arizona's Barringer crater if it reached the ground. Similarly, the Sikhote-Alin iron meteorite fall of February 1947 would have made a crater with D~ 120 metres if it had struck the ground intact; in fact it produced a field of craters, the largest of them with D = 2 7 metres. As a result of this aerodynamic fragmentation, the cratering rate for D < 3-4 km is less than one would expect from a simple downward extrapolation of the N-D relation for larger craters. Nevertheless, if allowance is made for these sources of error it is possible to obtain provisional estimates of the cratering rates for the United Kingdom and the British Isles. For example, it may be estimated that a crater the size of the New Quebec crater (D =3.44km) is formed in the British Isles about once per 0.3 to I I Myr. The true rate is probably towards the high end of this
Of Calculations and Craters
nn
large range, i.e. about one New Quebec crater in the British Isles per 2Myr. (This rate is in fair agreement with that found from counts of lunar craters, which imply that a 5-km crater should be formed in the British Isles about once per lOMyr on average.) Even the lowest estimated rate implies that about 50 craters as large as New Quebec have been formed in the British Isles during Phanerozoic time, and that six of these craters were formed since the extinction of the dinosaurs at the end of the Cretaceous period. The higher rates suggest that there may have been as many as 30 post-Cretaceous craters as large as New Quebec, and 250 Phanerozoic craters of this size or larger. Similar methods imply that a 10-km crater should be formed in the British Isles about once per 10 to 80Myr, that is there should be between seven and 50 such craters of Phanerozoic age. A 20-km crater should be formed about once per 80 to 300 Myr, and a 40-km crater about once per 260 to 1,400 Myr. According to these relations the largest Phanerozoic crater in the British Isles should have a diameter between about 27 and 65 km. The smaller diameter is in good agreement with the estimated diameter obtained from studies of craters in North America, Scandinavia and Australia, but, as will appear in later chapters, the larger diameter may be nearer to the truth. The impact rates that I have calculated imply that between about 3,000 and 11,000 terrestrial impact structures with d > 2 0 k m have been formed in the 540 Myr of Phanerozoic time, and that between about 1,900 and 3,200 of these craters are on land. 4 However, the list of Grieve (1991) includes only 27 such impact structures identified by the presence of shock metamorphism, and the list of Fortes (2000) increase this number only to between 34 and 37, 5 or < 4 % of the expected number. Thus the known large impact structures constitute much less than the proverbial tip of the iceberg. 6 ^ h e implied density of craters with D > 2 0 k m is between 1/170,000km" 2 and 1/50,000 k m ' 2 . 5 The ages of Highbury (Zimbabwe), Smola (Norway), and Vara (Sweden) are not known. 6 It might be more appropriate to compare the geological record of impact cratering to the fossil record of evolution. Neither record is anything like complete, but valid conclusions can be drawn from both of them.
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Many of the missing impact structures have simply escaped discovery, and others have been buried by later sedimentary or volcanic rocks; but most have probably been destroyed by erosion. However, erosion is not an all-or-nothing process. As was argued in Chapter 1, between half and two-thirds (or, say, between 50% and 70%) of an erosion-dominated population of impact structures will be 'hypo-astroblemes' or 'infra-impact structures', in which all traces of impact shock metamorphism have been removed but which still show the radially symmetric uplift and deformation characteristic of impact structures. A well-explored country like the United Kingdom should be ideal for the discovery of such eroded impact structures. Moreover, exploration of such 'hypo-astroblemes' may be of value for identifying the characteristic features of deeply eroded impact structures, and for understanding the nature of the roots of impact structures in general. The impact cratering rates used above imply that between two and seven impact structures with D > 2 0 k m should have been formed in the British Isles during the last 600 Myr, and that the largest of these structures should have a diameter between about 27 and 65 km. One would expect that some of these structures would have escaped erosion or burial, and that they would have attracted the notice of geologists. However, no such structures have yet been certainly identified. The next chapter is therefore dedicated to a search for anomalous geological features in Britain whose form suggests that they are the missing impact structures.
CHAPTER
The Search for Impact Structures They sought it with thimbles, they sought it with care; They pursued it with forks and hope; They threatened its life with a railway share; They charmed it with smiles and soap. Lewis Carroll, The Hunting of the Snark Obviously in any search for British impact structures one must know what one is looking for. Are we to search for clusters of craters, like Henbury, Kaalijarv, or Morasko; for small circular lakes like Merewether (Labrador); for large circular lakes like Deep Bay, Lake Mien, or Lappajarvi; for classical crypto-volcanic structures like Serpent Mound or the Steinheim Basin; or even for huge circular depressions like the Nordlinger Ries or Richat? One might expect that small, simple meteorite craters would be commoner and therefore easier to find. However, the recent geological history of the British Isles presents obstacles to a search for such craters. Specifically, Scotland and Ireland and, to a lesser extent North Wales, have a multitude of small lochs, loughs and llyns created by glacial erosion; and identifying a meteorite crater among these glacial lakes would be like the proverbial task of finding a needle in a haystack. Moreover, since
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the ice sheets of Ireland and of the highland areas of Great Britain melted only about 12,000 years ago, there has been little time for these areas to collect any new craters. In England there are fewer glacial lakes to confuse the search for craters. Indeed, the contrast between the abundance of lakes in the glaciated regions of Scotland, Wales and Cumbria and their scarcity in the unglaciated regions of southern England illustrates very forcibly the pre-eminence of glaciation in the creation of lakes. However, in England the problem is not the excessive number of lakes, but the destruction by human agency of those that do exist. Many of the natural lakes and ponds of England have been drained or filled in for agricultural purposes, or during the building of towns; and at the same time there have been many artificial excavations. These excavations include village ponds, quarries, opencast mines, chalk and gravel pits, lime kilns, peat diggings (such as the Norfolk Broads), and, in inland towns, medieval fishponds to provide fish for people to eat on Fridays. There are also genuine explosion craters, namely bomb craters formed during World War II, and shell holes from artillery practice. The presence of these artificial excavations, together with the destruction of natural lakes and ponds, makes the identification of small meteorite craters very difficult. There are two facts that tend the other way, and that would make the identification of craters easier. The first is that meteorite craters with diameters <200 metres almost always occur in groups. Thus any isolated group or cluster of circular depressions in this range of diameters could be impact craters. Such craters, however, would be known only in their immediate neighbourhood. A 200-metre crater would not appear on small-scale maps; and it would be only 4 m m across on the Ordnance Survey 1:50,000 series. This is a field where local knowledge and a study of old maps would be of great value. The second fact is that the sheer scarcity of natural lakes in the Midlands and Southern England means that almost any regularly shaped lake that is large enough to appear on a typical road atlas deserves attention. One such lake, in fact almost the only natural lake that I have found in an unglaciated part of England, is Dozmary Pool, on Bodmin Moor; this lake basin will be discussed in detail in a later chapter. The difficulties facing searchers for British meteorite craters are illustrated by the fact that there have been only two meteorite
The Search for Impact Structures
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finds in the British Isles (as opposed to 24 observed falls), and that these two finds were both made since 1985. In contrast, meteorite finds in Canada, the USA and Australia far outnumber observed falls; and most of the countries of Europe have some finds. The absence of British and Irish meteorite finds testifies to the effect of erosion and of human modification of the landscape, both of which are effective in destroying the traces of small meteorite craters. Thus there appears to be little hope of finding small meteorite craters in the British Isles, although such craters may exist in quiet rural areas, known only to people who live near them. When we move to larger craters, however, the search becomes more interesting, because two British structures have already been compared to the circular impact-created lakes of the Canadian and Baltic Shields. Flinn (1970) and Sharp (1971) suggested that St. Magnus Bay (60°25'N, 01°34'W), a semi-circular bay (D = 11 km) in the west of Mainland, in the Shetland Islands, and The Firth (60°30'N, 00°58'W), a deep marine basin between the Shetland islands of Whalsay and Yell, were impact craters. St. Magnus Bay, indeed has the distinction of being the first entry in Classen's (1977) 'Catalogue of 230 Meteorite Craters' by virtue of being the westernmost proposed impact structure in Europe. It is given a 'C classification, and is regarded as a possible impact structure. On satellite photographs, St. Magnus Bay looks very similar to Deep Bay (Saskatchewan), which is only slightly larger (D = 13km). However, neither St. Magnus Bay nor The Firth shows the geological signatures of impact, and most geologists regard the crater-like form of the basins as the result of glacial overdeepening during the Pleistocene ice ages (Mykura, 1976). It is only fair to point out that that both basins are under the sea and are therefore largely inaccessible to geological exploration. The two basins are discussed in more detail in the next chapter. Besides St. Magnus Bay, there are no large circular or arcuate lakes or marine bays in the British Isles that can be plausibly identified as impact structures, and we must therefore ask whether there are any geological features that are the counterparts of the crypto-explosion structures of the Midwestern United States. There are certainly no classical crypto-explosion structures in either the United Kingdom or Ireland; the central breccia nests
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and shatter cones of such structures could hardly have escaped discovery in two such well explored countries. However, as was said in Chapter 1, deeply eroded impact structures may be exposed at depths beyond the reach of shock metamorphism, and may be mistaken for ordinary tectonic domes. Thus the best way of finding such impact structures is to search geological maps for circular inliers or structural domes, which may represent their deeply eroded central uplifts. An examination of the 10-mile geological maps of Great Britain published by the Ordnance Survey has revealed a number of such circular inliers and domes. The most obvious of these is the Ashby Anticline, named after Ashby-de-la-Zouch, in the English Midlands. This anticline brings Coal Measures and Lower Triassic rocks up through the Triassic rocks of central Staffordshire and western Leicestershire to form the Leicestershire and South Derbyshire coalfields. On the geological map the Lower Triassic and Carboniferous rocks form a roughly circular inlier about 16 km in diameter; the centre of this inlier is at about SK335190 (52°46.5'N, 01°30.2'W). Because of the presence of coal the structure has been studied in great detail, and it is generally regarded as a much faulted anticline. South-east of the Ashby Anticline is the classical Precambrian area of Charnwood Forest, in Leicestershire; this area forms a sector of a circular topographic and structural uplift. There are two other Precambrian inliers in the same area, the South Leicestershire diorites of the Enderby-Narborough-Croft district, SSE of Charnwood, and the Caldecote Volcanics near Nuneaton. The Triassic and Palaeozoic cover must be very thin in this part of the Midlands, so that the Precambrian basement is near to the surface. The third and fourth structures are in the Welsh Borders. The larger is the Woolhope Dome (52°02'N, 02°34'W), between Hereford and Ledbury, which forms an inlier of Silurian rocks in the Devonian (Lower Old Red Sandstone) cover of Herefordshire. The smaller structure is the Hope Mansell Dome (SO635205; 51°53'N, 02°32'W), with a diameter of 3-4km. This structure consists of a dome of Old Red Sandstone which is enclosed to the east, south and west by the Carboniferous rocks of the Forest of Dean. The dome is on the Herefordshire-Gloucestershire border,
The Search for Impact Structures
IE9
about 8 km south-east of Ross-on-Wye, and 17 km in the direction S10°E from the Woolhope Dome. The fifth structure demonstrates the need for detailed local knowledge. It is a nearly circular topographic basin, 6.5 ± 0.5km in diameter, centred near Rochford (TQ873904) in south-east Essex, and therefore called the Rochford Basin. The interesting feature of this basin is a very small outcrop of Chalk located near TQ890903 (51°34.7'N, 00°43.7'E), 1.7km east of Rochford on the lower River Roach. This Chalk inlier may represent the central uplift of a small impact structure; it does not appear on geological maps, and I became aware of its existence only through my good fortune in having been born inside the Rochford Basin and therefore knowing it intimately. These circular uplifts are not the only British landforms that can be tentatively identified as impact structures. Specifically, the Palaeozoic inliers that form the Warwickshire and South Staffordshire coalfields appear to be the central peak ring of a huge circular structure, with D = 80-90 km, that occupies most of the Midlands. This circle actually contains the Ashby Anticline and the Charnwood uplift, which may therefore be subsidiary components of the Midlands structure rather than independent impact structures or tectonic uplifts. Even this circular landform is dwarfed by the East Grampian Structure in northern Scotland. This structure, or the part of it that is on land, consists of sectors of three concentric mountain rings east of the Cairngorms, that partly encircle the lower-lying Buchan area and the outcrops of the Younger Gabbros of Aberdeenshire. Another possible English impact structure has been identified not from its central uplift but from what may be the remaining traces of its ejecta blanket and from the sediments deposited millions of years after its formation in a crater lake. This putative crater is located in north-west Surrey, with its centre at 51°23' ± 3'N, 00°40' ± 5'W; its diameter is 20-30 km. None of these landforms can be considered as anything more than a possible impact structure. However, all of them are interesting enough to deserve detailed geological study with the impact hypothesis in mind.
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It will be noticed that seven of these eight structures are in England, and that all these seven are in the 'Midland Belt' between the Pennines in the north and the Tertiary uplifts of the Weald Dome and Salisbury Plain in the south. Moreover, and contrary to expectation, only one of the eight structures, the Surrey basin, is definitely of Mesozoic age. The Rochford Basin is Tertiary and probably post-Eocene; the East Grampian Structure is Cambro-Ordovician; and, as will be shown later, the Midlands Structure is probably Permian. The age of the Woolhope Basin can only be fixed as post-Silurian, and the Shetland craters and the Hope Mansell Dome are post-Devonian. The absence of circular geological inliers from Scotland, Wales and northern England is the result of uplift of the British Isles (particularly the highland areas of the north and west) during the Tertiary period. It is this uplift that has made Scotland and Wales the most mountainous parts of the British Isles, and has enabled them to maintain themselves as separate countries, independent of England. This uplift was largely a consequence of the opening of the North Atlantic Ocean between the British Isles and Greenland. At the beginning of the Tertiary period most of Great Britain (and possibly Ireland as well) was blanketed by Jurassic and Cretaceous sediments. Where these rocks have been preserved they often amount to a total thickness of between 2,000 and 3,000 metres, and this may be taken as a maximum value for the original thickness of the rocks in other parts of the country where they have been destroyed by Tertiary erosion. Remnants of this Mesozoic blanket have been found around the margins of the highland blocks, for example in Cardigan Bay (with 2,150 metres of Jurassic rocks), at Helmsdale in Sutherland, and in the Inner Hebrides (where the Jurassic is 1,170 metres thick). The Tertiary uplift led to the erosion of the Jurassic and Cretaceous sedimentary cover over most of Great Britain, and to the destruction of any Mesozoic impact structures. In the highland areas of Scotland, Wales and northern England erosion bit deeper, removing any Upper Palaeozoic cover and exposing the folded, faulted and metamorphosed rocks of the Caledonian and Hercynian mobile belts, where Lower Palaeozoic or older impact structures had already been destroyed by tectonic activity.
The Search for Impact Structures
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However, in the lower -lying country of the English Midlands the removal of the Mesozoic cover exposed the underlying Palaeozoic sedimentary rocks, with their possible impact structures. Even southern England did not entirely escape the effects of tectonic disturbance. There were at least two important episodes of uplift, the first during the Middle Eocene epoch (~45Ma), which led to the formation of the Weald-Artois Anticline and to the unroofing of the Dartmoor Granite, and the second during the Miocene epoch. The thickness of the Upper Eocene and Oligocene rocks of the Hampshire Basin and the Isle of Wight testifies to the effectiveness of erosion after the Middle Eocene episode, and the great Miocene unconformity from about 30 Ma to 5 Ma bears silent witness to the severity of the second episode of uplift and erosion; all the pages of the geological record for Late Oligocene and Miocene times have been torn out and destroyed. During these episodes of uplift more than 500 metres of Mesozoic rocks were removed from the Weald Arch by erosion, and it is likely that a similar thickness of rock was destroyed in other parts of Britain. Thus Tertiary uplift and erosion were effective in destroying impact structures; and the glacial periods of the Quaternary (the Pleistocene) added to their effects. Most of Great Britain north of the Thames-Severn line suffered some glacial erosion, which was particularly severe in Western Scotland, the Lake District, and North Wales. Moreover, large areas of East Anglia, the Midlands, Lincolnshire, north-east England, Lancashire and Cheshire are covered by a thick blanket of glacial deposits. These deposits are up to 47 metres thick in parts of Leicestershire, 90 metres at Cromer (Norfolk), 98 metres at Crewe, and as much as 140 metres in the valley of the River Stour in Suffolk. This thick blanket of drift conceals the pre-glacial topography, and much of the underlying geological structure, over large parts of the country. However, in eastern Essex the ice sheets did not reach as far south as the River Thames; instead their advance was blocked by the line of hills to the north of the River Crouch. Thus the Rochford Basin lies between the eroded dome of the Weald Anticline to the south and the drift-blanketed region of central Essex and East Anglia to the north. It is therefore in an area that has escaped the worst of both Tertiary erosion and burial by
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Fig. 1. Topographic map of St. Magnus Bay and The Firth (Shetland Islands). Scale ~1:300,000. Reproduced by permission of the Ordnance Survey.
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Pleistocene glacial sediments. If the basin is in fact an impact structure, its location, rather than implying that it was formed by a chance impact that just happened to find an area where the structure would be preserved from both erosion and from concealment by later sediments, suggests that there were many Tertiary impact structures in Britain, of which only the Rochford Basin has been preserved. This preservation bias applies to all meteorite craters and impact structures, and probably dominates their observed geographical and age distribution. This discussion of the destruction and concealment of impact structures applies specifically to the island of Great Britain. The situation in Ireland may be different. Uplift and erosion have probably destroyed any impact structures in the mountainous west and north of the island and in the south; but one might expect that the stable Carboniferous Limestone platform of central Ireland would preserve some impact structures. However, this part of the country may be too heavily blanketed by glacial deposits for impact structures to be recognised. Furthermore, I know little of the detailed geology of Ireland; and it would be presumptuous of me to try to identify impact structures there, when there are Irish geologists who are in a better position to study the matter. Thus it is understandable that most of the possible impact structures of the British Isles are in the English Midlands. However, the next chapter will be devoted to a detailed discussion of the two Shetland craters. The following chapter will return to England and describe the geology of the Midlands, in order to introduce the examination of the origin of the Ashby and Charnwood structures and of the large circular Midlands Basin.
The Shetland Craters Yet at first sight the crew were not pleased with the view, Which consisted of chasms and crags. Lewis Carroll, The Hunting of the Snark. The first structures in the British Isles that were mentioned as possible impact structures were St. Magnus Bay and The Firth, in the Shetland Islands (Flinn, 1970; Sharp. 1971). St. Magnus Bay forms a nearly semi-circular indentation on the west side of the island of Mainland; its position is given (Graham et ah, 1985) as 60°25'N, 01°34'W (Grid Reference HU238702). The diameter of the bay is about 11 km, and its greatest depth is 160 metres (Mykura, 1976); at depths of >100 metres the bay is nearly a completely enclosed depression. The Firth forms a deep marine basin, with a maximum depth of about 145 metres (Mykura 1976), between the islands of Fetlar and Yell to the north and north-west, the peninsula of Lunna Ness (Mainland) to the west, and the islands of Whalsay and Out Skerries to the south. The centre of the basin is at 60°29'N, 00°54'W (approximately HU610785), and its diameter is about 14 km. The two basins are about 38 km apart. According to Hughes (1996) the craters may be of Late Tertiary age (i.e. <25Myr), although Sharp (1971) prefers an age of 200-300 Myr, that is, Permian or Triassic. The suggestions that these two basins are impact craters do not appear to have been followed up in detail, and the evidence
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for this hypothesis is inconclusive; according to Mykura (1976) both basins may have been formed by glacial overdeepening during the Pleistocene ice ages. On satellite photographs St. Magnus Bay, with a diameter of 11km, looks very similar to Deep Bay (Saskatchewan), which, with a diameter of 13 km, is only slightly larger. Moreover, St. Magnus Bay is similar in depth to Deep Bay; Baldwin gives the average and maximum depths of Deep Bay as 500' (150 m) and 720' (220 metres). This similarity may not be significant, since both bays, whatever their origin, have been much modified by glacial erosion and deposition. Neither St. Magnus Bay nor The Firth shows the geological signatures of impact. First, there is no central topographic or structural uplift under either basin, although it is only fair to point out that both basins are under the sea and are therefore largely inaccessible to geologists. Second, the rock outcrops on land do not show any concentric pattern, such as is seen in Wells Creek, Gosses Bluff and Richat. If these two basins are indeed impact craters their morphology and structure appear to be similar to those of such craters as Deep Bay, Clearwater Lake East or Pilot Lake, rather than to those of the typical crypto-explosion structures of the Midwestern United States, with their central uplifts and encircling ring synclines. This morphology is consistent with the geology of the Shetland Islands, which consist largely of metamorphic and igneous terrains belonging to the Caledonian orogenic belt. The northern and eastern margins of St. Magnus Bay consist predominantly of plutonic igneous rocks (the Northmaven Complex, the Ronas Hill Granite, and the Muckle Roe granophyre), with smaller areas of metamorphic rocks. The southern margin of the bay consists of metamorphic rocks, with, to the south, the Devonian sandstones of the Walls Peninsula. In the far west, the promontory of Esha Ness and the island of Papa Stour are made chiefly of Devonian volcanic rocks. The margins of The Firth again consist mostly of Caledonian metamorphic rocks. Most of the island of Yell is formed of garnetiferous mica-plagioclase gneiss, and other gneisses and migmatites form the western part of Fetlar. Most of eastern Fetlar is formed of the serpentines and metagabbros that also compose
The Shetland Craters
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most of the island of Unst. Whalsay and Lunna Ness consist mostly of pelitic and semipelitic gneiss, much of it almost transformed to granite. The Out Skerries contain a great variety of types of rock, including crystalline limestone, migmatite, and granite. There are no plutonic or volcanic rocks to speak of around the margins of the Firth, although there are some unusual volcanic breccias and vents farther south in Bressay and South Mainland. These outcrops are 40 km and more from the centre of the Firth. There is no evidence of igneous activity associated with the formation of these two basins. The plutonic and volcanic rocks of West Mainland, around St. Magnus Bay, are of Caledonian age; and the bay itself was formed much later. Essentially these two bays are basins of superficial excavation; whether the excavating agent was ice or a giant meteorite remains to be determined. As was mentioned above, there are some volcanic breccias and vents in South Mainland and Bressay; and these may have some bearing on the origin of The Firth. So far as they go, they may be evidence that The Firth is of terrestrial rather than impact origin. However, the rocks in question are of unusual type, and some of them have been identified in established and probable impact structures. These volcanic rocks are found in association with two 'steep belts' (monoclines) of steeply inclined and locally inverted strata that cross western Bressay and eastern Bressay and Noss from north to south. Most of Bressay consists of gently inclined Middle and Upper Old Red Sandstone. These steep belts are associated with sedimentary breccias, or rather megabreccias (with blocks ranging in size up to tens of metres or even 200 metres), with finely comminuted sediments (sand and silt grade), and with sedimentary intrusive tuffisites and crypto-vents of intrusive breccia. There is also a dyke of what may be altered aegirine-trachyte 1 in north-western Noss. The Muckle Hell vent (HU525399), in eastern Bressay, contains vent breccias and dykes of flowbanded carbonate rock (ankerite-calcite) with clasts of sediment. Barytes mineralization is also present. The carbonates may have
^egirine is a sodic pyroxene, with the formula NaFe 3+ Si 2 0 6 -
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been intrusive tuffisites that have been replaced by carbonate minerals, or they may be genuine igneous carbonatites. Quartz veins, and iron and copper sulphides, also occur in these 'steep belts.' The steep belt of western Bressay is continued south into South Mainland through Helli Ness and Sandwick as far as No Ness (HU444211) and Leven Wick (HU410216) by a belt of intensive faulting and carbonate mineralization. This belt contains dykelike masses of carbonate, carbonate net-veining, and narrow, irregular zones of tuffisite breccia (Mykura, 1976). Various workable copper and iron ores occur in association with these carbonate veins. The carbonate rocks do not contain the niobium and rare earth elements 2 that are characteristic of carbonatites; instead they are rich in Ni, As, Cr, and Cu. Carbonate veins with traces of ore minerals are found all along the east coast of Mainland from Nesting (about HU454540) to Levenwick, and even as far south as Fair Isle (HZ210720), about 114 km south of the centre of The Firth. In discussing the nature of these 'steep belts' and their relation to the Shetland craters, one may say first that the belts are more or less radial to The Firth. However, Bressay is about 40 km south of the centre of The Firth, and Levenwick is about 60 km south of this centre. It may be added that the whole of the east coast of Shetland, from Sumburgh Head to north-east Yell, is almost a perfect straight line. It looks like a fault, although some geological outcrops appear to be continuous across it. Second, these breccia dykes, carbonate veins, intrusive tuffisites, and barytes mineralization are all well known from impact and cryptoexplosion structures. The carbonate (or carbonatite) dykes and barytes mineralization of Richat are famous, and they present an interesting geological puzzle in themselves. The Avon area in Missouri, the Urach tuffisites in Swabia, and the Slate Islands impact structure in Ontario all have breccia dykes, and there is an intrusive carbonate in the Slate Islands structure (Sage, 1978).
2
It may be noted that Ytterby, the original source of the rare earths, is three miles (5 km) outside Stockholm and therefore inside the huge Uppland impact structure.
The Shetland Craters
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What, then, do we make of these similar rocks, which appear to be associated with The Firth? Are they genuine, if unusual, igneous rocks, and if so how are they related to the rest of the geological history of the Shetland Islands? Alternatively, are they igneous rocks intruded along an impact-created fracture; or could they be products of impact-induced acoustic fluidisation intruded along pre-existing faults or fractures? The distance to which these veins and ore deposits are intruded must cast doubts on any interpretation that relates them to a putative impact in The Firth. Nonetheless the association looks interesting. The fact that St. Magnus Bay and The Firth form a pair of similarly sized basins is significant for interpretations of their origin. There are several examples of pairs of terrestrial impact structures, such as the Clearwater Lakes, the Ries-Steinheim pair in Germany, probably Jeptha Knob and Versailles in Kentucky, and the Richat-Semsiyat pair in Mauritania. Likewise a few examples are known of double asteroids in near-Earth orbits, such as 4,179 Toutatis and perhaps 4769 Castalia. It is plausible that St. Magnus Bay and The Firth were formed by the impact of just such a binary asteroid. The work of Hills and Goda (1993) suggests that asteroids 400-600 metres in diameter would be required to form basins 11 to 14 km in diameter. There are no reports of shatter cones, high-pressure forms of silica (coesite and stishovite), or other evidence of shock metamorphism in either St. Magnus Bay or The Firth. Perhaps drilling of the sea floor near the centres of these basins would lead to the discovery of shocked basement rock, but in the absence of such evidence their impact origin must remain doubtful. If, nonetheless, we adopt the impact hypothesis, we must conclude that it is highly unlikely that the only crater-forming impact on the British Isles during the Phanerozoic aeon were those that produced these two Shetland craters. The total area of the Shetland Islands is only 1,425.86 km 2 , less than 0.6% of the area of the United Kingdom; and the late Tertiary period occupies < 5 % of Phanerozoic time. Thus it is worth searching the mainland of Great Britain for other impact structures of pre-Miocene age. However, it remains a strange chance that may have led these two giant meteorites or small asteroids to score a direct hit on this isolated archipelago. One wonders how many other such
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impactors missed the Shetland Islands and created impact basins that now lie concealed beneath the waters of the North Sea. But, for the time being, these submarine impact structures are safe from discovery; and it is time to turn to our study of the peak-ring structures and circular domes of the English Midlands and the Welsh Borders.
CHAPTER
Midlands Geology it is a hard matter for friends to meet; but mountains may be removed with earthquakes, and so encounter. As You Like It, III, ii, 195-7 England, from Trent and Severn hitherto, By south and east 7 Henry IV, III, I, 75-6 The Midlands Platform is part of the Midlands Micro-Craton, which forms the foundation of England. From Powys and Shropshire to Norfolk this platform consists of Precambrian volcanic, plutonic and sedimentary rocks, overlain by a relatively thin blanket of Palaeozoic and Mesozoic sedimentary rocks. In the Midlands proper the largest area is covered by Permo-Triassic rocks, mostly sandstones, mudstones and evaporites. Over much of the Midlands the Phanerozoic sediments are <500 metres thick and the Precambrian basement outcrops in a number of small, scattered inliers, the largest of which are the Longmynd in Shropshire and Charnwood Forest in Leicestershire. In spite of its name, the Midlands Micro-Craton actually extends well beyond the boundaries of the Midlands, into Wales, Northern England, East Anglia, and the North Sea, and even into Belgium. Its total area, including the North Sea and Belgian sections, is probably at least 90,000 km 2 .
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Part I: Impacts and Geology
The Micro-Craton is roughly triangular. Its north-western margin runs from South Wales (between Carmarthen and Tenby) northeast through Mid-Wales and Shropshire (the Church Stretton Fault), then through Cheshire and the Manchester conurbation and into Yorkshire, probably to the region of York and Ripon. The margin is displaced left-laterally by the northwest-southeast faults of the Manchester-Bolton area; the line of these faults passes through the Ashby Anticline of southern Derbyshire and western Leicestershire and through Charnwood Forest. The eastern margin of the craton is less well defined, since it is under the North Sea. It probably runs approximately NNW-SSE some distance from the present east coast from Holderness to East Anglia, and so into Belgium, where it forms the eastern margin of the concealed Brabant Massif, to the north of the Ardennes. This eastern margin may even lie as far east as the northward continuation of the line of the lower River Rhine and the Ijsselmeer (Zuider Zee). To the south the Midlands Micro-Craton (here called the London Platform) dips southward into the Hercynian (Variscan) mobile belt of southern England and the south-west Peninsula. The southern margin of the platform is marked by a line of coalfields, the exposed South Wales, Forest of Dean and Bristol coalfields and the concealed fields of Oxfordshire and Kent. The eastern part of the platform, in Essex and Hertfordshire, appears to be occupied by a Lower Palaeozoic sedimentary basin, which may be a westward extension of the Brabant Massif of Belgium. To the west and north the Precambrian foundation of the Midlands Platform is overlain respectively by the Lower Palaeozoic rocks of the Welsh basin and the Palaeozoic rocks of the Pennines and the Lake District island arc. It must be emphasised that during most of its history (perhaps to as late as Jurassic and Cretaceous times), the Midlands Platform has formed high ground and was a source of sediments to the surrounding shallow seas. During Palaeozoic and Triassic times the highest ground of the Midlands Platform appears to have been in the present Fen District and East Anglia, in striking contrast to the present topography. To the north of Charnwood Forest the Midlands Platform is interrupted by a concealed Carboniferous sedimentary basin
Midlands Geology
in
called the Widmerpool Gulf,1 which separates it from the Peak District of Derbyshire. This basin extends westwards from near Grantham along the line of the Triassic basin (the Needwood Basin) through Derby and Uttoxeter that separates the Leicestershire Coalfield from the Nottinghamshire Coalfield. It contains thick deposits of Carboniferous Limestone, Millstone Grit, and Coal Measures; the limestone and the grits appear in outcrops at Breedon on the Hill and at Melbourne, north of the Leicestershire Coalfield. These rocks were probably deposited in the valley of a westward flowing river between the hills of the Midlands to the south and those of Derbyshire to the north. The Widmerpool Gulf is actually the southernmost of four eastwest rifts or sedimentary troughs in northern England and southern Scotland. The others are the Mid-Pennine or Craven 2 Trough; the Northumberland Trough; and the Scottish Midland Valley. These troughs were occupied by rivers flowing from the rising Hercynian mountains of Europe and were filled by clastic sediments (the Millstone Grit series) derived from the erosion of these mountains and by Coal Measures. In the Clitheroe region the Mid-Pennine Trough contains as much as 7,000 metres of Carboniferous sediments and might be better called the MidPennine geosyncline. South of the Widmerpool Gulf, the Midlands Platform may be divided into three parts: the Welsh Borders Platform; the industrial Midlands; and the East Midlands and East Anglia. The geological boundary between the Welsh Borders and the industrial Midlands is formed by the north-south tectonic structures of the Malvern Hills and the Abberley Hills. The boundary between the industrial Midlands and the East Midlands is less clearly defined in geological terms, but it may be taken as being marked by the outcrop of the Triassic-Jurassic boundary from north-east of Leicester to south of Leamington Spa. The Welsh Borders Platform has the shape of a right-angled triangle, with an area of about 5,000 km 2 . The north-western side,
Earned after the village of Widmerpool (SK630280), 12 km SSE of Nottingham, ^ h i s is a Celtic name, which may mean 'garlic'; there is no imputation of cowardice!
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Part I: Impacts and Geology
the hypotenuse, probably extends from east of Llandeilo northeastwards to near Newport, in Shropshire; it is defined by the line of the Careg Cennen to Church Stretton disturbance. (Careg Cennen appears to be somewhere between Llandeilo and Llandovery.) The eastern side of the triangle extends south from Newport, through Bridgnorth and along the Abberley-Malvern line to near Huntley, in Gloucestershire. The southern side of the triangle extends westward from Huntley, north of the Forest of Dean Coalfield and the South Wales Coalfield, to east of Llandeilo. The Welsh Borders Platform is surrounded by orogenic belts: it is bounded in the north-west by the folded Ordovician and Silurian rocks of the Cambrian Mountains, in the south by the Hercynian mobile belt of South Wales, and in the east by the steeply folded and thrust belt of the Malvern Hills and the Abberley Hills. In contrast the Palaeozoic rocks that rest on the Uriconian and Longmyndian basement (about 600 Myr old) show little sign of tectonic disturbance. The Lower Palaeozoic rocks outcropping along the north-western margin of the platform, such as the Ordovician quartzites of the Stiperstones, west of the Longmynd, and the Silurian limestones of Wenlock Edge, are characteristic of tectonically stable regions, such as continental platforms or passive margins; they are in striking contrast with the turbidites and volcanic rocks of the Welsh Basin. However, the largest area of the Welsh Borders, from Shropshire southwards, is occupied by flat-lying Devonian rocks (mostly Lower Old Red Sandstone), which are up to 2,000 metres (7,000') thick. These rocks form most of Herefordshire, and also extend into South Wales, where they form the Black Mountains and the Brecon Beacons. In Herefordshire, in particular, the Malvern Hills and the Woolhope Dome are the only two inliers that break through the Devonian cover to expose Lower Palaeozoic and Precambrian rocks. The volume of the Devonian rocks of the Welsh Borders (the product of their thickness and the area that they cover) is about 10,000 km 3 . This volume is equivalent to the erosion of a layer of rock 500 metres thick (nearly half the height of Snowdon) from the whole area of Wales. The great thickness and extent of Devonian rocks in the Welsh Borders are in clear contrast to their almost complete absence from the industrial Midlands and
Midlands Geology
m
the East Midlands, and suggests that the Welsh Borders Platform was part of a separate microplate that was joined to the Midland Craton along the Malvern-Abberley line during the Hercynian orogeny. The geology of the east Midlands and East Anglia, east of the River Soar, is quite different from that of the Welsh Borderlands. Instead of the thick Devonian cover, the Precambrian basement (largely volcanic and plutonic rocks ranging from ~540 to 700 Myr old) is here overlain by a few hundred metres of undisturbed Mesozoic sedimentary rocks, Jurassic in Northamptonshire and Cambridgeshire, and Cretaceous in Norfolk and Suffolk. Palaeozoic rocks are absent from most of this region, except in north Norfolk along the southern shore of the Widmerpool Gulf and in the concealed Lower Palaeozoic basin of the London Platform. Permo-Triassic rocks are also absent, and during the Triassic period East Anglia appears to have formed high ground that supplied sediments to the lower-lying country farther west. In spite of the differences in stratigraphy the Welsh Borders Platform and the East Midlands and East Anglian Platform are similar in that both consist of essentially undisturbed Phanerozoic sedimentary rocks (Jurassic and Cretaceous east of Leicester, Devonian and Lower Palaeozoic in the Welsh Borders) lying on the stable Precambrian basement. These platform areas have thus enjoyed freedom from gross tectonic disturbance and deep erosion since the end of the Precambrian ~540Myr ago. Such an area is the most likely part of Britain to record the formation of impact structures. The geology of the Midlands proper, between Leicester and the Malvern Hills, is different in both stratigraphy and structure. Here the post-Hercynian cover consists largely of Permian and Triassic conglomerates, sandstones, marls and evaporites, which rest on a Palaeozoic sedimentary sequence up to 3,000 metres thick, which itself rests on the Precambrian basement (see Table 1). The Palaeozoic sequence is dominated by Carboniferous and Cambrian rocks, with subordinate Silurian; the Ordovician and Devonian 3 3
Lower Old Red Sandstone occurs in the Trimpley and Heightlngton inliers in the Wyre Forest Coalfield, west of Kidderminster. These rocks are severely folded and faulted.
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Part I: Impacts and Geology
Table 1. Stratigraphy of the central Midlands Platform System
Series
Triassic
Keuper Marl Waterstones Keuper Sandstone
Permo-Triassic
Upper Mottled Sandstone Bunter Pebble Beds Lower Mottled Sandstone
Per mo - Carboniferous
Enville Group
Carboniferous
Upper Coal Measures Middle Coal Measures Lower Coal Measures Millstone Grit Carboniferous Limestone
Devonian
Old Red Sandstone
Silurian
Thickness (m) 200 -50
0-100 0-150 110-240 0-300 210-1070 76-700 90-370 40-370 0-150 0-300 0-500 0-500
Cambrian
Stockingford Shales Hartshill Quartzite
850 240
Precambrian
Nuneaton Volcanics Charnian
-200? >2600
are almost absent. The rocks themselves Include shale, quartzite, sandstones, limestone and coal, all sediments characteristic of continental platforms and passive continental margins. However, there are great variations in the thicknesses of the various formations. For example, the Coal Measures are up to 800 metres thick in the Warwickshire and South Derbyshire Coalfields, and > 1,000 metres thick in the South Staffordshire Coalfield, but in much of Leicestershire, from Market Bosworth eastwards, the entire Carboniferous is absent and the Triassic rests directly on Cambrian shales. The Cambrian itself is ~ 1,200 metres thick in the Nuneaton area, but at Charnwood Forest, only about 20 km away, the Triassic rests on Precambrian rocks. The PermoCarboniferous Enville beds are > 1,000 metres thick in Warwickshire and 300 metres thick in South Staffordshire, but they appear to be unknown in Leicestershire.
Midlands Geology
m
The Permo-Triassic cover of the industrial Midlands is interrupted by a number of inliers where Palaeozoic and even Precambrian rocks have been uplifted by thousands of metres. Among these are the South Staffordshire and Warwickshire coalfields, the Ashby anticline of south Derbyshire and west Leicestershire, and Charnwood Forest. These inliers are associated with folding and faulting, and with a variety of breccias. In addition, there are several occurrences of Palaeozoic intrusive rocks in the Midlands; although these intrusions are of small area, their presence provides evidence of a number of igneous episodes. The Cambrian rocks of Nuneaton and Dosthill, and at Merry Lees Colliery (about SK470058), south of Charnwood, are intruded by sills of hornblendic rocks up to 30 metres thick; these rocks have been described as diorites, microdiorites and camptonites (augite-plagioclase lamprophyres) and are regarded as being of Ordovician age. Other diorites and granodiorites occur at Mountsorrel, between Leicester and Loughborough; at Ives Head (SK478170) and Shepshed in the north of Charnwood Forest, and at Stockshouse Farm borehole (SK466021) near Desford, south of Charnwood. The Mountsorrel Granodiorite has Devonian K-Ar ages of 403 ± 18Myr and 368 ± 17Myr; the Ives Head microgranite also has a Devonian age, 374 ± 13Myr, whereas the Shepshed diorites (only 1.3 km from Ives Head) have a Carboniferous (upper Visean) age of about 335 Myr. A thin bed of Carboniferous Limestone lies directly on the diorite at the Stockshouse borehole. Basaltic or doleritic sills up to about 50 metres thick occur in the Coal Measures of the Forest of Wyre Coalfield (north of Kidderminster), in South Staffordshire, and around Whitwick and Ellistown in the Leicestershire Coalfield. These sills do not intrude the overlying Permo-Triassic New Red Sandstone, and they are therefore probably Upper Carboniferous or Permian. The Whitwick dolerite, which intrudes the Middle Coal Measures near the Thringstone Fault, has an age of 243 ± 1 1 Myr, which is consistent with a Permian or Lower Triassic age. There are three anomalous features of these igneous rocks: the presence of diorite, a typically orogenic igneous rock, in a continental platform setting; the great diversity of ages, implying the occurrence of at least four widely separated igneous episodes in
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Part I: Impacts and Geology
this stable terrane; and the absence of any clear connection with igneous activity elsewhere in England and Wales. These tectonic disturbances and igneous episodes are unexpected in the context of the stable foundation of the Midlands Micro-Craton and are in striking contrast to the stability of the Welsh Borders Platform and the East Midlands. Evidence of Hercynian folding along the southern shore of the Widmerpool Gulf is understandable, but the horst-and-graben tectonic deformation in the centre of the platform may require another cause. What this cause was will appear from an examination of the stratigraphy and structure of the four main inliers.
The Ashby Inlier Come, you shall have Trent turn'd. I Henry TV, III, I, 135 The Ashby structure (see Figure 3) is the most easterly of the three Carboniferous uplifts of the industrial Midlands. The blanket of Keuper Marl that extends from the Pennines to the Warwickshire and South Staffordshire coalfields is here interrupted by a nearly circular inlier that exposes the so-called Keuper Sandstone 1 2 and the Lower Triassic or Permian Bunter Sandstone (or Upper Mottled Sandstone) and Bunter Pebble Beds. These Triassic rocks surround the Coal Measures of the Leicestershire and South Derbyshire coalfields, which form the core of the inlier. In the north-west and north the circumference of the inlier is outlined by the arc of the River Trent from Burton upon Trent to north of Melbourne (about SK390270). The inlier is centred near SK335195 (52°46.3'N, 01°30.2'W), about 4 km north-west of Ashby-de-la-Zouch and 3 km east of 1
Strlctly the 'Keuper Sandstone' belongs to the Anisian stage of the Middle Triassic, and thus corresponds stratigraphically to the Muschelkalk of the European Triassic. ^ h e discovery of Muschelkalk-type marine sediments in the overlying Waterstones of Cheshire (Anderton et ah, 1979) suggests that the 'Keuper Sandstone' may be partly Lower Triassic (Scythian).
m
El
Part I: Impacts and Geology
Swadlincote. Its outer rim passes through Burton upon Trent in the north-west, and Repton and Foremark in the north, and then north of Melbourne in the north-east to the Carboniferous Limestone outcrops of Breedon-on-the-Hill and Breedon Cloud in the east. The rim then passes through Normanton le Heath in the south-east, Measham in the south, Netherseal in the southwest, Caldwell in the west, and so back to the River Trent. The diameter of the inlier is about 17 km. The inlier interrupts the Keuper Marl, implying that uplift has occurred since the Triassic period. Topographically the Ashby inlier forms an isolated group of hills, rising 60-70 metres above the plains of west Leicestershire to the south, and partly separated from the higher hills of Charnwood Forest to the east by passes at heights of 150-160 metres asl. The highest ground is in the north and the east, where the Triassic rocks outcrop, the culminating point being the summit of Pistern Hill (SK347199) at 184 metres above sea level. In the south and south-west, where the Coal Measures are the dominant rocks, the hills are lower, reaching a maximum height of only about 130 metres, and the outer rim of the inlier is poorly defined topographically. On a first examination of the 'Ten-Mile" (1:625,000) Geological Map the Ashby structure looks like a good candidate for a complex impact structure, with the geological inlier forming the central uplift. Since the inlier interrupts the Upper Triassic Keuper Marl it appears that the impact and uplift occurred after the Triassic period, and a fortiori after the Hercynian orogeny. However, a more detailed examination shows that the first impression is misleading. There is no concentric topography of circular or arcuate ridges and valleys, and the Keuper Sandstone and the Bunter rocks do not form concentric outcrops. The inlier is asymmetric, with the Coal Measures outcropping to the south of its centre and the Triassic rocks dominating the northern part. Moreover, the Coal Measures of the Leicestershire and South Derbyshire coalfields in the south do not form a simple dome. Finally, the oldest rocks exposed in the inlier, the Millstone Grit of the Melbourne area and the Carboniferous Limestone outcrops of Ticknall, Calke, and Breedon, occur in the north-eastern sector of the inlier rather than at its centre.
Fig. 3. Topographic map of the Ashby uplift (Derbyshire-Leicestershire). Scale ~1:100,000. Reproduced by permission of the Ordnance Survey.
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Lat
52° 45' N
A 512
Ashby-de-ta-Zouch 5 km or 3 miles A SO
Long 1° 25'W
Fig. 4. Topographic map of the Charnwood uplift (Leicestershire). Scale ~1:52,000. Reproduced by permission of the Ordnance Survey.
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The Ashby Inlier
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Another possibility is that the Ashby structure is a tectonic window where the Palaeozoic foundation is exposed by a gap in the Triassic cover. The structures thus exposed in the two coalfields are of interest, and they merit description. The Leicestershire Coalfield forms a structural basin, bounded in the north-east by the Thringstone Fault. This fault separates the coalfield from the uplifted Millstone Grit and Carboniferous Limestone in the north of the field (near Ticknall and Calke), and from the Precambrian rocks of Charnwood Forest in the south (near Whitwick, Coalville, and Ibstock). The coalfield extends to the south-south-east under the Triassic cover as far as Merry Lees (about SK470058), 3 km north of Desford and a few kilometres south of the Precambrian outcrops of Charnwood Forest. The total thickness of coal is approximately 372 metres, about 204 metres of Lower Coal Measures and 168 metres of Middle Coal Measures; the Upper Coal Measures are absent. The South Derbyshire Coalfield is synclinal in structure, and appears to occupy a graben. The coalfield has been severely folded and faulted, more so than the Leicestershire Coalfield has been. A particularly important feature is the NW-SE Boothorpe Fault, which passes through Swadlincote, WoodvUle (SK314192), and, of course, Boothorpe (SK320175). This fault to a large extent forms the eastern margin of the coalfield; it has a downthrow to the south-west of about 300 metres (1,000'). The coal deposits are thicker than those in the Leicestershire Coalfield, amounting to a total of about 850 metres. Of this the Lower and Middle Coal Measures make up about 366 metres each, and the Upper Coal Measures about 120 metres. The western margin of the exposed coalfield passes near Castle Gresley, Overseal, and Donisthorpe. It may be defined by a SSEward continuation of the Coton Park Fault (about SK270195). This fault has a downthrow to the west of about 200 metres. The line of this fault is continued to the south by a deep trough, filled with Triassic rocks, between Charnwood Forest and Nuneaton. This trough is also followed by the valley of a pre-glacial tributary of the River Soar, which flowed south-south-east from west of Shackerstone (SK374067) to west of Hinckley. The South Derbyshire Coalfield continues westwards under the Triassic as a concealed coalfield around Walton-on-Trent
m
Part I: Impacts and Geology
(216180). This 'Western Extension' is severely faulted. Ford (1972) suggests that this extension is continuous with the Warwickshire Coalfield, about 10 km to the south. Between the Leicestershire and South Derbyshire coalfields is the Ashby Anticline. This is a Hercynian fold, which runs nearly north-south SK370210 (Heath End, west of Staunton Harold Hall) to 370180, and then south-south-east to about 390130 (east of Normanton le Heath, near the south-east rim of the inlier). It should be noticed that the Ashby Anticline runs to the east of the centre of the inlier. The axis of the anticline was excavated in Triassic time to form a deep gorge separating the South Derbyshire and Leicestershire coalfields. The Cambrian sedimentary rocks (Stockingford Shales) found by boreholes around Market Bosworth lie on a SSE-ward continuation of the line of the Ashby Anticline. It is therefore possible that this fold is not restricted to the Ashby inlier but continues to the south-east under the Triassic cover. The base of the Coal Measures is not exposed in the South Derbyshire Coalfield. However, to the north of the Leicestershire Coalfield the tectonic uplift has brought the underlying Millstone Grit and Carboniferous Limestone to the surface in a group of small inliers. These rocks were probably deposited near the southern shore of the Widmerpool Gulf, or in the southern part of the gulf itself. The Millstone Grit outcrops around and west of Melbourne (SK385255), at a height of about 100 m above sea level (asl), and is overlain by Keuper Sandstone and Keuper Marl (Middle and Upper Triassic). The Carboniferous Limestone occurs in two sets of outcrops. The western outcrops are at Ticknall (SK355240), Calke (370225), and Staunton Harold (Dimmisdale) (385215); the eastern outcrops are at Breedon-onthe-Hill, Breedon Cloud, and Grace Dieu (about SK435181), north of Charnwood Forest. The limestone (or, more accurately, dolomite) succession at Breedon Cloud (at 70-110 m asl) is between 200 and 300 metres thick, and ranges from the C2-zone (the Chadian or the Arundian stage of the lower Visean epoch) to the Dl-zone (the Asbian stage of the upper Visean). The limestone beds are vertical or dip steeply to the west, towards the Ashby structure. The reason for this steep dip is unknown; it has been suggested that the limestone
The Ashby Inlier
IH
outcrops form the flank of a diaplric structure. The limestone outcrops are overlain and entirely surrounded by Triassic sandstones and marls. The Carboniferous Limestone outcrops around Ticknall (also at about 100 m asl) consist of fiat-lying limestones, dolomites and shales belonging to the D2-zone. This zone corresponds to the Brigantian stage, the highest stage of the Visean epoch. These rocks are probably thinner than those of the Breedon Cloud outcrop; by analogy with occurrences of Carboniferous Limestone in nearby boreholes they may amount to only 20-30 metres. The Ashby inlier is almost completely surrounded by the Upper Triassic Keuper Marl, which occupies most of western and southern Leicestershire, the Trent Valley as far north as Derby and Nottingham, and, to the west, the Needwood Basin of central Staffordshire. Directly to the north of the inlier, at Ingleby (SK350270) on the River Trent, the base of the Keuper Marl is about 40 metres asl. This measurement makes it possible to estimate the total structural uplift of the lowest rocks of the inlier, the Carboniferous Limestone of Breedon Cloud. Allowing about 200 metres for the limestone, 372 m for the Coal Measures, and 150-200 m for the total of the Bunter Pebble beds and the Keuper Sandstone, we find SU= 200+ 372 + - 2 0 0 + (110-40) = 850-900m. So far the evidence is consistent with the Ashby Structure's being a tectonic window that exposes the Hercynian structures of the underlying Palaeozoic terrane. However, one formation implies that there has been more tectonic uplift than appears in the pattern of outcrops, and also casts doubts on the post-Triassic date for the uplift event. This formation is the Moira Breccia, which rests directly on the Coal Measures and is overlain by either Bunter Pebble Beds or Keuper Marl. This breccia is named after the Leicestershire village of Moira (SK315156); 52°44'N, 01°32'W), between Oversea! and Ashby-de-la-Zouch, but it is not restricted to this locality; it has also been found in the Coton in the Elms borehole (about SK246152, 7 km west of Moira) and in the Battleflat borehole (about SK440110) near Ellistown, south-west of Charnwood and about 13 km ESE of Moira. This 'Battleflat Breccia' is outside the Ashby inlier altogether, and is more likely to be related to the Charnwood structure.
HBSJSBB
Part I: Impacts and Geology
The Moira Breccia consists chiefly of clasts of Cambrian quartzite; it was once thought to be of Permian age and contemporary with the Enville and Clent breccias farther west, but is now regarded as a Lower Triassic scree deposit, contemporary with the Lower Mottled Sandstone (Hains and Horton, 1969). I shall present evidence later that the Lower Mottled Sandstone is actually Permian, and that the Moira Breccia is therefore also of Permian age.) However, the provenance of the clasts that make up the breccia is uncertain: the nearest existing outcrop of Cambrian quartzite is in the Nuneaton and Hartshill district, 16 km from Moira; and at Moira itself the breccia is separated from the underlying Cambrian by at least 800 metres of Coal Measures and about another 800 metres of Cambrian shales. A distance of 16km is too far for scree material to be transported, and the thickness of the local Coal Measures makes it unlikely that there was a Cambrian outcrop near Moira at the time that the Breccia was deposited. If, on the other hand, the Ashby Anticline is interpreted as an impact structure, the Moira Breccia can be regarded as being a part of the central dome that was uplifted and brecciated at the same time. Moreover, the presence of an extensive breccia deposit is at least consistent with a violent origin for the Ashby Structure. The amplitude of the structural uplift then amounts to about 1.6 km, consistent with a diameter for the structure of about 20 km. If the Moira Breccia is regarded as an impact breccia, its stratigraphic location between the Upper Coal Measures and the Bunter Pebble Beds implies an age for the impact structure between Early Triassic and Late Carboniferous (about 240-300 Myr). This age is inconsistent with the post-Triassic age deduced from the apparent uplift of the Bunter Pebble Beds relative to the Keuper Marl of the Trent Valley. It is possible that this apparent uplift is deceptive, that the Keuper Marl is a playa or sebkha sediment deposited on a desert plain below the residual Permo-Triassic and Carboniferous hills of the Ashby-de-la-Zouch area. The present hills would then constitute an exhumed landscape, like the hills of Charnwood Forest. Alternatively, the post-Triassic uplift may have been only a minor tectonic adjustment after the main uplift event.
The Ashby Inlier It may be added that other quartzite breccias similar to the Moira Breccia are known elsewhere in the Midlands. These include the Hopwas Breccia, west of Tamworth (about 6 km from a small outcrop of Cambrian shales at Dosthill), and the concealed Nechells Breccia near Birmingham. These breccias are not close to existing Cambrian outcrops, and it is interesting that where Cambrian quartzites do appear at the surface they are not associated with breccias similar to the Moira and Hopwas breccias. These facts, the large amount of tectonic uplift required to bring Cambrian quartzites to the surface, and the existence of an extensive breccia deposit, imply that the Ashby inlier could, after all, be the central uplift of an impact structure, and that this hypothetical structure is between Carboniferous and Triassic in age, rather than being post-Triassic. It is also possible that the impact hypothesis is false, and that the Ashby inlier is a tectonic window that exposes the Hercynian structures of the Palaeozoic rocks, the Moira Breccia is an ordinary scree deposit, and the Carboniferous rocks of the South Derbyshire and Leicestershire coalfields, with their Lower Triassic cover, were uplifted after the Triassic period by ordinary local tectonic processes. If so, the Ashby inlier does at least constitute one more piece of the jigsaw that composes the English Midlands. There is another possibility, that the Ashby Structure forms only one part of a much circular landform, the Midlands Basin. This landform will be described in detail in Chapter 8. Another part of the Midlands basin is the Precambrian inlier of Charnwood Forest, and it is this inlier that we must now examine.
CHAPTER
Charnwood Forest The largest of the Precambrian uplifts of the Midlands is the classical terrain of Charnwood Forest. The Precambrian ('Charnian') outcrops appear on the geological map as a nearly semi-circular area 13-14 km in diameter, extending from near Thringstone (SK435172) in the west to Swithland Wood (SK543111) and Groby (SK520073) in the east. The centre of the circle defined by these Charnian outcrops is near SK470110, north of Stantonunder-Bardon and north-west of Markfield; the geographical co-ordinates of this central point are about 52°41.5'N, 1°18.5'W. The Charnian succession is at least 2,600 metres thick; it has been divided into three groups, called, from bottom to top, the Blackbrook, Maplewell and Brand groups. The Blackbrook group, which is >900 metres thick, consists mostly of water-lain volcanic tuffs. The overlying Maplewell Group (1,400 metres) is also mostly volcanic, and consists of tuffs and volcanic agglomerates. The Brand Group (about 300 metres) is mostly sedimentary. The Maplewell Group has been divided into four formations, called the Felsitic Agglomerate, the Beacon Hill Beds, the Slate Agglomerate, and the Woodhouse and Bradgate Beds. In the west and north-west, on Bardon Hill (SK460132) and the Warren Hills (461148) and northwards towards Grace Dieu Manor (437179), the Beacon Hill Beds are replaced by coarse agglomerates and volcanic breccias, which are called 'Bomb Rocks'. There are also intrusive 'porphyroids,' which are probably dacite or rhyodacite
19
Charnwood Forest
III
lava flows. The 'porphyroids' at High Sharpley (SK446170), west of Blackbrook Reservoir, are described as having been intensely sheared and crushed (Hains and Horton, 1969). This deformation may be the result of tectonic movements; but intense shearing, crushing and brecciation associated with structural uplift are also characteristic of impact structures. The last rocks of the Charnian succession are the intrusive 'markfieldites,' which are named after the village of Markfield (SK487101), on the south side of Charnwood. These rocks are oversaturated porphyritic microdiorites, consisting of plagioclase and hornblende phenocrysts in a graphic groundmass of quartz and alkali feldspar (Hatch, Wells and Wells, 1965). The type markfieldite of Cliffe Hill, west of Markfield, has been dated at 540 ± 57 Myr. This is essentially the same Ediacarian or Cambrian age as the South Leicestershire diorites and the Ercall Granophyre of Shropshire. The structure of Charnwood Forest is an open north-west to south-east anticline, plunging to the south-east. The dips of the rocks are mostly between 10° and 20°. In spite of this modest deformation the rocks have a strong cleavage, which strikes obliquely across the anticline. The Charnian rocks are overlain by the Keuper Sandstone and Keuper Marl; the erosion of these rocks is uncovering the rugged landscape that was buried by these Upper Triassic sediments. The Keuper rocks also largely conceal the Palaeozoic rocks that border Charnwood Forest. These include the Carboniferous rocks of the Widmerpool Gulf to the north and of the Leicestershire and South Derbyshire Coalfield to the west. In the north-west of Charnwood Carboniferous Limestone and Coal Measures occur within a few hundred metres of Charnian outcrops, but the contact between the Carboniferous and the Charnian is not exposed anywhere. Specifically, in the area of Thringstone and Whitwick the Coal Measures are separated from the Charnian by the Thringstone Fault, and 1-2 km to the northeast, at Grace Dieu Priory (about SK435181), a small outcrop of dolomite (probably of the D2 zone) occurs within a few hundred metres of the most north-westerly Charnian outcrop. It is not known whether the concealed contact between the Carboniferous Limestone and the Charnian is an unconformity or a fault.
MM
Part I: Impacts and Geology
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There are breccias at Grace Dieu, but Ford (1972) interprets them as a local facies of the basal Triassic rocks (probably Keuper Sandstone). It is interesting, though, that the location of these 'breccias' is about 1 km from the sheared and crushed Precambrian rocks of High Sharpley. At Shepshed, about 5 km east of Grace Dieu the Charnian rocks have been thrust against the Carboniferous diorite intrusions, showing that the Precambrian has been uplifted by at least several hundred metres relative to the Carboniferous and that this uplift was later than the intrusion of the diorite. West of Charnwood Forest at least 26 metres of Carboniferous Limestone have been found in a borehole at Ellistown (about SK428115), in the Leicestershire Coalfield. However, only about a kilometre east of Ellistown, the Battleflat borehole found the Keuper Marl lying directly on a quartzite breccia identified as the Moira Breccia. The nearest outcrop of Cambrian quartzite is 19 km away near Nuneaton, and borehole explorations have not found intact quartzites near Charnwood. The Coal Measures of the Leicestershire Coalfield extend underground to the south-south-east as far as Merry Lees (about SK470058), a few kilometres south of the Charnian outcrops of Groby and Bradgate. At this point the south-eastern extension of the Thringstone fault separates the Coal Measures to the southwest from vertical Stockingford Shales (Cambrian) to the northeast; these shales have been invaded by diorite sills. These Cambrian rocks are not exposed at the surface, and there is no indication of the location of the contact between the Cambrian rocks and the Charnian rocks near Groby. The vertical dip of the Cambrian implies that this concealed contact is a fault rather than an unconformity. The presence of these vertical Cambrian rocks faulted against Coal Measures indicates that the Charnian and Cambrian rocks have been uplifted relative to the Carboniferous. The rocks of Charnwood Forest have experienced hydrothermal activity and mineralization. The hydrothermal minerals present include quartz, calcite (CaC0 3 ), chlorite and epidote in the Charnian rocks; and dolomite, galena (PbS), sphalerite (ZnS), fluorite (CaF2), barytes (BaS04) and calcite either in the Triassic rocks or at the base of the Triassic. Circulating magnesium-rich
Charnwood Forest
EH
groundwater or hydrothermal waters have precipitated deposits of the magnesian clay mineral palygorskite. Pyrites (FeS2) and marcasite (also FeS2) are also present, and there are small quantities of molybdenite (MoS2), copper minerals, and gold. The Mountsorrel Granodiorite contains veins of bitumen, along with the more normal hydrothermal minerals; this bitumen was probably derived from overlying sedimentary rocks. Hydrothermal mineralization of the type found at Charnwood is of course always present in volcanic areas, but it can also occur in large impact structures, either through heating of groundwater by a large body of impact melt, or as a result of uplift of hot rock from deep in the Earth's crust. Such a hot central uplift will have exactly the same properties as a low-temperature igneous intrusion of the same size, and will produce hydrothermal alteration of the rocks of the impact structure. Thus the presence of hydrothermal minerals does not always serve to distinguish between impact structures and igneous complexes. However, the hydrothermal mineral assemblage can provide a measure both of the temperature of the circulating groundwater at the time of the deposition of the minerals and of the depth of the hydrothermal system below the Earth's surface. Certainly some of the hydrothermal minerals of Charnwood have been found in large impact structures, particularly the Manson structure (D = 35km) in Iowa (McCarville and Crossey, 1994). For example, the impact breccias and other rocks of Manson contain quartz, calcite, chlorite, and epidote. Lead and zinc sulphides (galena and sphalerite) occur in Siljan (Sweden); sphalerite and pyrites are present in Serpent Mound (Ohio); and pyrites is also present in Crooked Creek (Missouri) (Masaitis, 1992). Fluorite occurs in the Ries, Hicks Dome, and Charlevoix; and barytes has been found in the Richat structure and in several of the American cryptoexplosion structures (Fudali, 1969; Woolley et al, 1984). Another mineral family that is important in impact structures is the zeolites, which may constitute up to 10% of the volume of impact breccias (Masaitis, 1992). However, I have not yet found any mention of zeolites in or around Charnwood or in any part of the Midlands, not even in Firsoff s (1971) useful book Gemstones of the British Isles.
Em
Part I: Impacts and Geology
At Merry Lees Colliery the Whitwick dolerite (which was originally an olivine dolerite) has been altered to a porcellanous breccia of bleached shale fragments in a carbonated basaltic matrix (Ford, 1972). This description is similar to that of the dolerite sills or dykes of Tin Jouker, in the west of the central uplift of the Richat structure, which have been largely coverted to carbonate (Blanc & Pomerol, 1973). In some large impact structures (D s= 20 km) the rocks have been shattered so as to acquire a pyroclastic texture or have been melted so as to resemble either lavas or plutonic igneous rocks. It is therefore possible that some of the intrusive 'igneous' rocks of the Charnwood area are actually impact melts that have penetrated into fractures below the floor of an impact structure. The large-amplitude localised uplift of the Precambrian basement, the shearing, crushing and brecciation of the Charnian and later rocks, and even the pre-Triassic hydrothermal alteration, are all consistent with the interpretation of Charnwood Forest as the central uplift of an impact structure. If so, the diameter of the central uplift (13-14 km) implies that the full diameter of the structure is D~27km. However, there are no geographical or geological features around Charnwood that can be even tentatively identified as the outer rim of the putative impact structure, nor is there any clear stratigraphic break confined to the Charnwood area that makes it possible to date the impact. For these reasons it may be better to examine the possibility that the Charnwood uplift, together with the Ashby Anticline, is part of an even larger circular structure that occupies most of the English Midlands. This circular structure will be the subject of the next chapter.
The Midlands Basin — A Cometary Impact Structure? Grinds hard stones to meal. J.R.R. Tolkien, The Hobbit They wept like anything to see, Such quantities of sand. Lewis Carroll, Through the Looking Glass The largest of the arcuate s t r u c t u r e s of the English Midlands is defined in the north, n e a r Ashbourne, by the contact of t h e Triassic (New Red Sandstone) rocks with the Carboniferous rocks of the Derbyshire Peak District, a n d in the west by the contact of the Coal Measures of the Coalbrookdale a n d Forest of Wyre coalfields with the Devonian a n d Lower Palaeozoic rocks of Shropshire. It is only in the north a n d west t h a t t h e rim of this s t r u c t u r e is actually exposed; elsewhere it is concealed by Permo-Triassic a n d J u r a s s i c rocks. However, in the east this rim is probably marked by the western edge of the Precambrian b a s e m e n t east of Leicester. T h e circular s t r u c t u r e defined by t h e p a t t e r n of pre-Triassic outcrops also a p p e a r s in t h e large-scale topography. The rim of
mm
Part I: Impacts and Geology
this Midlands Basin is formed in the north, in Derbyshire, by the chain of hills extending west from Ashbourne south of Stoke-onTrent Ada Longton, by the low hills south-east of Market Drayton that separate the Midlands from the Cheshire Basin and by the hill complex west of Telford, where the River Severn has carved a gorge through the Palaeozoic rocks at Ironbridge. Farther south the rim is continued past Bridgnorth by the line of Devonian hills west of the River Severn as far as Stourport, north-east of the Abberley Hills. Beyond Stourport, however, the rim of the basin disappears beneath the Triassic rocks of the Worcestershire Plain, and can no longer be traced. To the east of Ashbourne the north-eastern rim of the basin probably passes between Derby and Nottingham. South of the River Trent the hypothetical east rim is concealed by the Jurassic rocks of eastern Leicestershire and of Northamptonshire, but evidence from boreholes suggests that it lies east of Leicester and west of Market Harborough. The line of the Middle Lias scarp from Husbands Bosworth (north-west of Naseby) to Edgehill probably approximates to the rim of the basin. The diameter of this structural and topographic basin is probably about 90 km. Its centre is near Grid Reference SP160950 (latitude 52°33'N, longitude 1°46'W), south-east of Sutton Coldfield (nominally at Ox Leys Farm, near Grove End). Inside the basin is a second, nearly circular, ring of hills, concentric with the rim of the basin. It is this ring of hills that is largely occupied by the towns of the Midlands conurbation. Starting in the north, one can trace the ring from Cannock Wood (SK045129) west through Cannock (SJ960100), Four Crosses (953092) and Featherstone (930049) to Wolverhampton. South of Wolverhampton the line of hills is well defined, passing east of Wombourne (S0877949 and 885919), west of Dudley (Barrow Hill, S0916896), to Brierley Hill (920870) and Wychbury Hill (919818); then between Hagley and Halesowen to the Clent Hills (934803), Walton Hill (943798), Chapman's Hill (972779), and the Lickey Hills (Beacon Hill, S0985761), south of Rubery. East of Rubery the line of hills is less well-defined, but it can be traced via Rowney Green (SP040712) and Tanworth-in-Arden (113705) to Hockley Heath (153728), where it is interrupted by the valley of the River Blythe. East of the River Blythe the line of
The Midlands Basin — A Cometary Impact Structure?
lES
hills continues west and north of Kenilworth and Coventry to the high ground of the Cambrian and Precambrian outcrops northwest of Nuneaton. The line of hills ends at Baddesley Ensor (SP268988), 26 km from Cannock Wood, and the circle therefore remains incomplete in the north. However, the topographic ring is sufficiently clearly defined for its dimensions to be measured. Its diameter is about 45 km, almost exactly half the diameter of the outer ring; and its centre is near to Grid Reference SP120920 (latitude 52°31.5'N, longitude 1°49.5'W), about 5 km W40°S of the centre of the outer ring. This position locates it in the Erdington district of north Birmingham, about 3 km north-east of the famous Gravelly Hill interchange on the M6 (better known as Spaghetti Junction!) The inner and outer rings are not exactly concentric, and the ring structure is therefore slightly asymmetric. In the north-east (the Nuneaton and Charnwood sector) the inner ring is about 30 km from the outer rim, whereas in the south-west (the Clent Hills sector) the inner ring is only 15-20 km from the outer rim. This asymmetric position of the central uplift is also found in the Richat structure in Mauritania and in the Ramghar crater in India. The ratio of the diameters of the inner and outer rings and the near-concentricity of the two rings are consistent with the hypotheses that the inner ring of hills is the central uplift of a peak-ring impact structure, and that the topographic rim is the outer rim of the structure. The suggestion that the English Midlands are the site of an Upper Palaeozoic impact structure 90 km in diameter is obviously speculative and controversial. To explore this possibility further one must examine the geology of the region. The topography of the inner ring described above reflects the geological structure. The eastern and western sectors of the ring consist of uplifted Coal Measures and older rocks, which form the Warwickshire and South Staffordshire coalfields.1 The industrial x
As a result of changes in the administrative counties, large areas of both these coalfields are now in the West Midlands, and the South Staffordshire Coalfield extends into Worcestershire. I shall use the names of the coalfields to avoid implying, for example, that the Clent Hills are in Staffordshire or that Birmingham is still in Warwickshire.
m
Part I: Impacts and Geology
cities of the Midlands are founded, both literally and metaphorically, on these uplifted deposits of coal. In the north of this inner ring a breccia consisting of Cambrian quartzite occurs at Hopwas (SKI 75048), 3 km west of Tamworth and midway between Cannock Wood and Baddesley Ensor; the presence of this breccia suggests that the ring of uplifted Palaeozoic rocks continues beneath the Permo-Triassic between Lichfield and Tamworth. It appears that in the south of the circle, between the Lickey Hills and Kenilworth, the Mesozoic cover (Middle and Upper Triassic) is < 1 km thick, whereas farther south, towards the Worcestershire Basin, the cover thickens to > 1.5 km. Thus it is possible that the uplifted ring of Carboniferous rocks is also continuous in this concealed southern sector. It is a measure of the extent of vertical movement in this part of the Midlands that the Hopwas quartzite breccia, between 60 and 100 metres above sea level, is located only about 3 km from the Whittington Heath borehole (approximately SK165085), where Carboniferous Limestone and Old Red Sandstone, stratigraphically hundreds of metres above the Cambrian Hartshill Quartzite, have been found 1,119 metres below sea level. Both the South Staffordshire and Warwickshire coalfields are bounded by faults on their eastern and western sides, strengthening the impression of a sharply defined peak-ring structure. In the north-eastern sector of the Warwickshire Coalfield, in the Nuneaton area, the arc of the circle appears to have been cut by a north-west to south-east fault, and the outer part of the peakring has subsided beneath the Triassic (Keuper) sediments of the Leicestershire Plain. The South Staffordshire and Warwickshire coalfields have different structures. The South Staffordshire field is an anticline, which brings u p Silurian rocks and even Cambrian and Precambrian (in the Lickey Hills) near the axis of the fold, whereas the Warwickshire field is a syncline, with Upper Coal Measures, and perhaps Lower Permian, exposed at the centre. However, on the east side of the Warwickshire Coalfield Cambrian and Precambrian rocks are brought up steeply against the eastern boundary fault between Atherstone and Bedworth; these outcrops may correspond to the Lower Palaeozoic and
The Midlands Basin — A Cometary Impact Structure?
MM
Precambrian outcrops in the South Staffordshire field, on the west side of the inner ring. It is clear that the Cambrian and Precambrian rocks of the Lickey Hills and the Nuneaton hills have been uplifted well across their normal stratigraphic level; such localised uplift is characteristic of complex impact structures. The amount of this structural uplift can be estimated by adding up the total thickness of the rocks exposed in the structure. This total thickness, from the Precambrian to the Upper Carboniferous or Lower Permian of the Warwickshire Coalfield, amounts to between 3,400 and 3,900 metres. Uplift of this amplitude would be expected in an impact structure with a diameter D~40km, large enough, but less than the 90 km estimated for the total diameter of the basin. It is also possible to estimate the structural uplift by using information from boreholes. Specifically, in the Whittington Heath borehole, near Lichfield, Upper Old Red Sandstone was encountered at 1,119 metres below sea level (bsl). At a borehole in Walsall, where Silurian rocks out crop at the surface (about 140 metres above sea level), Cambrian Quartzite was found 273 metres bsl. In the Lickey Hills, Cambrian rocks outcrop at 262 metres above sea level. Allowing for an intervening 28 metres of Old Red Sandstone (Hains and Horton, 1969), one may estimate that the Cambrian at Whittington Heath is about 1,560 metres bsl, and thus that the Cambrian of the Lickey Hills has been uplifted by about 1,820 metres. Since the Cambrian of the Midlands is 1,100 to 1,200 metres thick, and Precambrian rocks (Uriconian volcanics) outcrop at Barnt Green, south of the Lickey Hills and at about 250 m asl, it may be calculated that these Precambrian rocks have been uplifted by about 1,560+1,200 + 250 = 3,010 metres, in fair agreement with the uplift estimated earlier. Like the Ashby and Charnwood uplifts, the Midlands Palaeozoic ring is associated with breccias of Upper Carboniferous or Permian age. Breccias of Cambrian quartzite and limestone occur at Hopwas, west of Tamworth, and at Nechells (SP095895), north of Birmingham, close to the centre of the inner ring. Another thin breccia consisting mostly of
Part I: Impacts and Geology Cambrian quartzite occurs north of Birmingham, but this may be contemporary with the Permo-Triassic Lower Mottled Sandstone. Moreover, coarse breccias consisting largely of clasts of Uriconian volcanic rocks occur in the Wyre Forest Coalfield (the Enville Breccias), in the Clent, Lickey and Abberley Hills, and near Kenilworth (Warwickshire). The Clent Breccias, west of Birmingham, are about 120 metres (400') thick, and contain fragments up to 60 cm (2') across; the Haffield Breccia of the Abberley Hills may be even thicker, up to 140 metres (450')Another very thick breccia is recorded at Yarhampton, near Abberley Hill, at the base of the Keuper, although this may be derived from the erosion of the Haffield Breccia. The breccia formation consists mainly of coarse angular clasts of Precambrian (Uriconian) rocks in a matrix of small rock fragments, 'marls' (calcareous clays) and rock flour (Hains and Horton, 1969). The rock fragments have been reddened by a coating of haematite. The Haffield Breccia, however, contains a greater variety of fragments, including Longmyndian shales, Cambrian quartzites, and Silurian sandstones and limestones as well as volcanic rocks, all embedded in a sandy or marly matrix. The breccias therefore differ in composition from the conglomerates of the underlying Enville Beds, which consist predominantly of pebbles of Carboniferous Limestone and Silurian rocks. Some of the breccia clasts show striations similar to those found in glacially transported rocks (Stamp, 1957); and it may be significant that the bedrock underlying the Bunte Breccia around the Ries Crater in Germany has been scoured and striated by the passage of the breccia over it (Horz, 1982). The breccia formation generally rests unconformably on the lower Enville Beds or older Carboniferous rocks, but in Warwickshire the Kenilworth Breccia is said to be conformable with the Tile Hill Group of the Enville and to be followed conformably by the clays and sandstones of the Ashow Group. In the Abberley Hills the Haffield Breccia is reported to be followed conformably at Knightwick by Bunter Sandstone. If these reports of conformable successions are correct, either there is a complete sequence up through the Enville Beds and the Breccia Group to the Bunter Sandstone, or the Kenilworth Breccia and the Haffield Breccia are of different ages. These breccia formations are generally interpreted as alluvial fans eroded from former extensive outcrops of Uriconian volcanic
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Fig. 5. Topographic map of the Midlands Basin. Scale ~1:500,000. Reproduced by permission of the Ordnance Survey.
ce Survey and the British Geological Survey.
Fig. 6. Geological map of the Midlands Basin. Scale ~1:500,000. Reproduced by permission of the Ordnance Survey and the British Geological Surve]
The Midlands Basin — A Cometary Impact Structure?
mBmm
rocks, particularly between the South Staffordshire Coalfield and Wyre Forest. The supposed existence of these Uriconian outcrops implies that the Uriconian basement has been uplifted through thousands of metres during the Hercynian orogeny. However, there are reasons for doubting this interpretation, and these doubts lead to another possible explanation of these breccias. In the first place, the breccia formations are not associated with Precambrian volcanic outcrops that they might have been derived from. For example, the breccias of the Clent Hills rest on Upper Carboniferous rocks and are 7-8 km from the Uriconian rocks of Barnt Green; the Kenilworth breccias are about 20 km from the volcanic rocks of the Nuneaton area; and the very thick Haffield Breccia, which rests mostly on Silurian rocks, is > 2 0 k m from Uriconian rocks in any direction. The absence of any nearby Uriconian outcrops becomes more impressive when one considers that the large, angular clasts of the breccias cannot have been transported any great distance, and that their source rocks must lie within, at most, a few kilometres of the breccia outcrops. Thus, even before the breccias were deposited, the Upper Carboniferous rocks that most of them rest on must have been essentially in contact with Uriconian rocks. This argument can be reversed if one considers that the known Uriconian outcrops, in the Lickey Hills, near Nuneaton, and in the Wrekin itself, are not associated with thick PermoCarboniferous breccias. Thus the alluvial fan hypothesis invokes extensive unobserved Uriconian outcrops as the source of the breccias even in the face of evidence that such outcrops do not yield thick breccia deposits. Although there are only a few small outcrops of Uriconian volcanics in the inner ring of the Midlands Basin, there are several sizeable outcrops of Silurian rocks. Moreover, the Silurian rocks, being younger, are more likely to be exposed by erosion than the Precambrian rocks. In spite of these facts, there are no breccia deposits that consist predominantly of Silurian clasts, although Silurian pebbles are abundant in the conglomerates of the underlying Enville Beds. All these facts imply that the Breccia Formation of the Enville Beds is something out of the ordinary, and that it cannot be accounted for by ordinary processes of erosion and sedimentation.
El
Part I: Impacts and Geology
A clue to its origin may be given by the fact that breccias consisting of Silurian fragments are practically unknown and Cambrian breccias are rare, b u t breccias dominated by Precambrian fragments are abundant; that is, the degree of brecciation of a rock unit is correlated with the amplitude of its stratigraphic uplift. In some respects these Permo-Carboniferous breccias resemble impact breccias, specifically the parautochthonous lithic breccias formed in the crater floors of terrestrial impact structures. According to French (1998), such breccias form irregular bodies tens to hundreds of metres in size, and are composed of angular rock and mineral fragments in a clastic matrix of smaller but similar fragments. This description can be matched almost word for word with the description of the Clent and Haffield breccias. Rock flour (finely ground rock material), which occurs in the Clent Breccia, is a common constituent of impact breccias but is rare in ordinary sedimentary breccias. According to French (1998), deep-seated parautochthonous rocks may be suddenly uplifted by hundreds or thousands of metres in the centres of large impact structures, and this rapid uplift may produce additional varieties of breccias. It is therefore possible that the Precambrian breccias of the inner ring of the Midlands structure were produced by sudden impact-induced uplift and brecciation of the deep Precambrian basement, rather than by erosion of Precambrian massifs. The Haffield Breccia, farther from the centre of the structure, may be tentatively identified as proximal impact ejecta outside the crater rim; it may be compared to the Bunte Breccia of the Ries crater. The Haffield Breccia resembles the Bunte Breccia in thickness (20 to 130 metres according to Horz (1982)), and in its greater variety of rock types; however, unlike the Bunte Breccia it is not covered by a blanket of suevite (impact melt-fragment breccia) ejecta. Discovery of evidence of impact shock in the clasts of these breccias would provide conclusive evidence that the Midlands Basin is indeed an impact structure. Unfortunately traces of shock metamorphism are rarely found in these lithic breccias (Horz, 1982; French, 1998), and the impact origin of such breccias and their associated 'crypto-explosion structures' has to be
The Midlands Basin — A Cometary Impact Structure?
m
established from the presence of more severely shocked rocks, such as suevites, pseudotachylites, and impact melts. However, in the absence of such rocks in the Midlands, a search for shock effects in the breccias offers the best hope of confirming or refuting the impact hypothesis for the Midlands Basin. If the 'Uriconian' breccias of Kenilworth, Clent, Haffield and the Forest of Wyre are true impact breccias, then the same is probably true of the quartzite breccias of Nechells and Hopwas, and of the Moira Breccia of the Ashby and Charnwood uplifts. This possibility requires a re-examination of the nature of these two uplifts. It is possible that they are Hercynian massifs that have been accidentally incorporated in the Midlands impact structure, or even that they are smaller impact structures formed simultaneously as a result of the break-up of the impactor. However, an examination of the large-scale topography and geology suggests that these two uplifts are an integral part of the large impact structure. South of Charnwood, in south Leicestershire, the occurrence at the surface of Precambrian diorites and Cambrian shales suggests the presence of another, largely concealed, uplift between the inner peak-ring and the outer rim of the basin. In the south-west, almost directly opposite to Charnwood, deformed Devonian rocks and Uriconian breccias occur in the Forest of Wyre, north of Kidderminster, providing evidence of yet another tectonic uplift. These four uplifts may be arcs of a discontinuous outer ring anticline, concentric with the outer rim; this ring anticline has a diameter of about 80 km, only slightly less than the total diameter of the basin. The apparent presence of a third ring constitutes evidence for the impact interpretation; it is difficult to imagine any terrestrial process that could create three concentric circular ring structures on this scale. The youngest rocks in the uplifted inner ring are the Upper Coal Measures (Upper Carboniferous) of South Staffordshire and Warwickshire. The oldest of the rocks that fill the basin are the Bunter (Lower Triassic) Lower Mottled Sandstone and Pebble Beds. Thus the uplift must have taken place between the Late Carboniferous and the Early Triassic epochs. The breccias tell the same story, since they overlie the Permo-Carboniferous Enville Beds and are themselves overlain by the Bunter rocks. This stratigraphic evidence is consistent with the uplift and brecciation events having occurred at the same time.
EEI
Part I: Impacts and Geology
Unfortunately in the Midlands the time interval between the Late Carboniferous and Early Triassic is only sparsely represented by sedimentary rocks, and what rocks there are contain few fossils and are difficult to date. This near-absence of datable rocks from the time of the formation of the Midlands Basin is not a coincidence; large thicknesses of sedimentary rocks accumulate during geologically quiet intervals, and it is gaps in the sedimentary record that point to tectonic disturbances. However, it may still be possible to obtain a more precise date for the Midlands structure by using what rocks there are to set limits on the ages of the youngest uplifted rocks and the oldest basin-filling rocks. In the absence of fossils, we may use the principle established by Haughton (1878), that the thickness of a body of rock can be used to estimate the time taken for its deposition. In this case one can use sedimentation rates derived from well-dated stages of the Carboniferous system to estimate the ages of the poorly dated rocks of the Upper Coal Measures and the higher rocks of the Warwickshire and South Staffordshire coalfields. It has been emphasised previously that the Carboniferous succession in the Pennines is extremely thick, probably amounting originally to about 7 km. This succession consists of rocks eroded from the Caledonian and Hercynian highlands of Europe and deposited as deltaic sediments in a subsiding Pennine trough. Episodes of mountain building led to the deposition of coarser sediments in the Pennines; thus the Millstone Grit of the Namurian (or Serpukhovian) stage is related to the Sudetic orogeny (about 330 Myr) in central Europe, and the red sandstones of the Upper Coal Measures are related to the Asturic orogeny (about 300-310Myr). Since the durations of Millstone Grit sedimentation during the Serpukhovian stage and of Lower and Middle Coal Measures sedimentation during the lower Westphalian (Moscovian) stage are known, at least approximately, one can calculate the sedimentation rates for these two stages and then use them to estimate the duration of Upper Coal Measures sedimentation. It is known that the maximum thickness of Millstone Grit in the southern Pennines is >2,000 metres (Bennison and Wright, 1969; Rayner, 1971), and that the Millstone Grit succession in the North Staffordshire Coalfield is about 1,450 metres thick (Hains and Horton 1969). Since the
The Midlands Basin — A Cometary Impact Structure?
lg£ggi«|
Serpukhovian (Namurian) age lasted about 11 Myr (327-316 Myr), the average sedimentation rate was about 180m/Myr in the Pennines and about 130m/Myr in northern Staffordshire. The Lower and Middle Coal Measures in northern Staffordshire are about 1,280 metres thick. It is estimated that the time taken to deposit these measures was about 9Myr (about 316-307Myr), so the mean sedimentation rate was about 140m/Myr. The total thickness of the Upper Coal Measures in northern Staffordshire is about 1,500 metres; from bottom to top it consists of the Blackband Group (about 335 m), the Etruria Marl (about 335 m), the Newcastle-under-Lyme Group (about 122m), 2 and the Keele group (about 670 metres or more). By using the estimated sedimentation rate of 140m/Myr, one finds that the time taken for the deposition of the Upper Coal Measures was 10-11 Myr, i.e. from about 307 to about 296 Myr. The deposition of the Keele Group took about 5 Myr, and the deposition of the Blackband, Etruria Marl and Newcastle groups took about 6 Myr. Since the Newcastle-under-Lyme Group is probably Westphalian, or Moscovian {Anthraconauta tenuis zone, according to Hains and Horton (1969) and Rayner (1971)) and the Keele Group appears to be at least partly Stephanian (or Kasimovian), the Westphalian-Stephanian boundary is at about 301 Myr. The uppermost Upper Coal Measures in England are probably near the Kasimovian-Gzelian stage boundary (formerly middle Stephanian). In the Midlands coalfields the Upper Coal Measures are succeeded (apparently without any great break in sedimentation) by the Enville Beds, named after a village (S0825867) in the Wyre Forest Coalfield. These rocks consist of largely unfossiliferous red beds and breccias; in the Warwickshire Coalfield, where they reach a maximum thickness of about 1,070 metres, they are divided into the Calcareous Conglomerate Group at the base, the Tile Hill Group (named after a hill west of Coventry, at SP280790) in the middle, and the Kenilworth Sandstone at the top. The Enville Beds are often described as Lower Permian. However, the Early Permian epoch lasted about 34 Myr (290-256 Myr) and 2
In the South Staffordshire Coalfield these rocks are called the Halesowen Group and are up to 150 metres thick.
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Part I: Impacts and Geology
consisted of four ages, and it would be useful to obtain a more precise age for these rocks. This can be done if we assume that sedimentation was in fact continuous from the Upper Coal Measures into the Enville Beds. Although it is obviously unwise to use the same mean sedimentation rate for the Enville red beds as for the Millstone Grit and the Coal Measures, adoption of a rate of 140m/Myr implies that the Enville Beds could have been deposited in 7-8 Myr, implying a possible age somewhere between 280 and 290 Myr. This is consistent with the assignment of the Kenilworth Sandstone on fossil evidence to the lowest stage of the Permian system, the Asselian stage (290-281.5Myr). These calculations, then, yield a maximum age for the Midlands basin of about 290 Myr. We can now attempt to obtain a minimum age for the structure by examining the stratigraphy of the sandstone and marls that largely fill the Midlands Basin. In the past these rocks have been described as 'Bunter sandstone,' and mostly assigned to the Lower Triassic. However, parts of the series pass laterally into Upper Permian Zechstein rocks (dolomites and limestones, marls and evaporites) and therefore must themselves be Upper Permian. The error here is that of assuming that all rocks of Bunter facies are of Early Triassic age. In fact, according to Harland et at. (1982), the lower part of the Bunter Sandstone was actually deposited during the last age of the Permian, the Tatarian (253-248Myr), and is therefore Upper Permian. It is noteworthy, then, that in Britain and north-west Europe the mass extinction at the Permian-Triassic boundary, when 96% of all species of marine animals became extinct, 3 was not accompanied by any noticeable change in sedimentation. In County Durham and along the eastern flanks of the Pennines the Upper Permian comprises the Magnesian Limestone formation, consisting of limestones, marls, and evaporites. These rocks are assigned to the Ufimian-Kazanian stage (256-253Myr), SThe recent discovery of extraterrestrial fullerenes (C60 molecules) in PermoTriassic boundary sediments implies that this mass extinction itself had an astronomical cause.
The Midlands Basin — A Cometary Impact Structure?
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the lower stage of the Upper Permian. The Magnesian Limestone becomes thinner from Durham to Nottinghamshire, and south of Nottingham it is replaced by 'a sandy pebbly bed of Bunter Sandstone type' (Rayner, 1971). This 'Bunter Pebble Bed' thus belongs to the Ufimian-Kazanian stage, and must be at least 253Myrold. In the English Midlands the Bunter Pebble Beds rest on the Lower Mottled Sandstone, a dune sandstone which is u p to 300 metres thick in Cheshire and the Wolverhampton area. These sandstones may be partly contemporary with the Magnesian Limestone deposits of north-east England; they have in fact been interpreted as sand dunes on the southern shore of the Zechstein Sea (Rayner, 1971). However, the lower part of the formation may belong to the Kungurian stage (260-256 Myr), the uppermost stage of the Lower Permian; if so, it should be regarded as belonging to the Permian Rotliegendes ('red beds') rather than to the Triassic Bunter Sandstone. Estimates based on sedimentation rates derived for continental elastics elsewhere suggest that the Lower Mottled Sandstone could have been deposited in about 10 Myr, consistent with the assignment to the Kungurian stage. Thus the youngest basin-filling rock can be dated at about 260 Myr. Thus the age of the Midlands Basin can be tentatively fixed in the time interval 290 to 260 Myr. We may thus assume an age of 275 ± 1 5 Myr, probably in the Sakmarian (281-269 Myr) or Artinskian (269-260 Myr) age. This age is in good agreement with the Saalic mountain-building episode of the Hercynian orogeny, and with the date of intrusion of the granites of Devon and Cornwall (about 280Myr). This age for the Midlands Structure has implications for the origin of the dolerite intrusions of the Forest of Wyre, South Staffordshire and Leicestershire coalfields. As will appear later, large impact structures contain great volumes of rocks of igneous appearance that have actually been melted by the impact. In the largest structures, such as Sudbury, and perhaps the Bushveld Structure and the lunar maria, these 'impact melts' almost fill the crater; their presence inevitably makes it difficult to distinguish between impact structures and genuinely igneous complexes.
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Part I: Impacts and Geology
An impact structure the size of the Midlands Basin would be expected to contain about 1,000 km 3 of melt rocks. Although there are abundant doleritic igneous intrusions in the Midland Valley of Scotland and in northern England (including the famous Whin Sill), these intrusions are Upper Carboniferous, with an age of ~300 Myr. If the suggestion (Meneisy and Miller, 1963; Hains and Horton, 1969) that the Whitwick dolerite is of mid-Permian age is correct, these Midlands dolerites may be physically related to the Midlands Basin; they may, indeed, be the surviving remnants of the impact melts, intruded into the sedimentary rocks beneath the floor of the impact basin. In the light of the evidence presented in this chapter, the northwestern limit of the New Red Sandstone and the Upper Carboniferous rocks of the English Midlands (excluding Cheshire) can be tentatively identified as the outer rim of a peak-ring impact structure with a diameter of about 90 km, with its centre located north of Birmingham. The Palaeozoic uplifts of the South Staffordshire and Warwickshire coalfields, and the associated circular ring of hills, are identified as the inner peak-ring, with a diameter of about 45 km. The uplifts of the Ashby anticline, Charnwood Forest, south Leicestershire, and the Forest of Wyre may be arcs of a discontinuous ring anticline between the outer rim and the inner peak-ring. The estimated age of the impact structure is 275 ±15 Myr, i.e. in the Early Permian epoch. The structural uplift of the Precambrian rocks of the inner peak-ring is between 3,100 and 3,900 metres, rather less than would be expected in a structure of this size. The Lower Permian breccias at the top of the EnvUle Beds are identified as parautochthonous breccias of the crater floor, or, in the case of the Hafield Breccia of the Abberley Hills, as proximal impact ejecta. I have made it clear that this interpretation of the geology of the Midlands in terms of a large impact structure rather than in terms of Hercynian tectonics is speculative, and likely to be controversial. Many readers may reject the idea out of hand on the grounds that it is unbelievable that such an impact structure could escape notice in an area as well known as the English Midlands. More cogent arguments against the impact hypothesis are that no definite impactites (such as suevites or impact melt)
The Midlands Basin — A Cometary Impact Structure?
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have been identified; that there are no signs of shock in the rocks (particularly the Coal Measures) of the central peak-ring; and that the 'missing interval' (no more than 30Myr) between the Enville Beds below and the Bunter Lower Mottled Sandstone above is too short for the signs of impact to have been destroyed by erosion. The absence of evidence of impact in the Coal Measures is particularly significant, since these rocks were < 3 km below the surface at the time of the hypothetical impact, whereas in a normal impact structure with D ~ 9 0 k m one would expect shock effects and brecciation to be observable to a depth of about 9 km. Thus the supposed impact appears to have produced only a shallow crater, with only superficial shock effects; this fact may be consistent with the reduced amount of structural uplift observed in the central peak-ring. The absence of extensive shock metamorphism and brecciation may require the abandonment of the impact hypothesis for the Midlands Basin. However, it may be possible to account for the observations by supposing that the structure was formed by the impact of a comet rather than an asteroid. This suggestion is not as arbitrary as it may sound. Recent evidence from studies of lunar and terrestrial impact structures (Shoemaker, 1998) and of comets (Bailey and Emel'yanenko, 1998; Hughes, 1998) suggests that more than half of all terrestrial impact structures with D > 2 0 k m are produced by comets, and that the fraction of cometary impact structures increases with increasing crater diameter. It may be significant that Shoemaker (1998) finds evidence for an increase in the long-term comet flux between 200 and 300 Myr BP, that is, during Permian and Triassic time. In addition, owing to the effects of atmospheric resistance, terrestrial craters differ in morphology from craters on airless bodies such as Mercury or the Moon, and craters made by cometary impacts are different from those made by asteroidal impacts (Hills and Goda, 1993). The atmospheric ram pressure on a comet or a stony asteroid is large enough to break the body into fragments, which then spread out into a swarm of smaller bodies. As the radius of this swarm increases, its thickness (in the direction of its velocity) decreases, so that the swarm of
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Part I: Impacts and Geology
meteoroids becomes flattened into a strongly oblate spheroid (a 'fat pancake' shape, in the analogy of Melosh (1989)). Hills and Goda find that the radius of the swarm of fragments of a comet increases by about 1000 metres at the point where it hits the ground. A crater with D = 9 0 k m can be formed by a cometary nucleus with an initial radius r 0 =1.0-1.2km (i.e. a diameter do = 2.0-2.4 km) and an impact speed u0 = 40kms~ 1 . Thus the swarm radius r s ~ 2 x r 0 (i.e. r s ~2km), and the cross-sectional area of the cometary nucleus is therefore quadrupled. If the volume of the swarm is constant, the thickness of the swarm is reduced to about a quarter of the original diameter (to ~500 metres), and the aspect ratio 4 a ~ 8 . Other authorities give larger inferred initial radii for the impacting body required to produce a 90-km crater, in the range r 0 = 1.6-2.0km (do = 3.2-4.0km). However, the same principles apply to these larger bodies; for r0 = 2.0km the final diameter of the meteorite swarm is d ^ e k m , so that dg~ 1.5 x do, and oc~3.5. Impacts by fragment swarms with such large aspect ratios produce craters that are shallower than those formed by the more coherent stony or iron asteroids, and that have either flat floors or, at least, reduced central uplifts. This description is in good agreement with the shallow topography and superficial impact damage observed in the Midlands Basin. One may adopt a simpler model by using the rule of thumb that the diameter of a crater is about 20 times the diameter of the object that created it. The implied diameter of the 'Midlands impactor' is then 90 k m / 2 0 = 4.5 km. Given that the swarm diameter dg is about 2 km larger than the original diameter, it follows that do ~ 2.5 km, and thus r0~ 1.2 km. The increase in the diameter of the crater due to the break-up of the impactor is, of course, compensated by a reduction in the depth. The hypothesis of a cometary impact is consistent with deduced impact rates. Impacts by bodies with radii of 1.0-1.2 km probably occur somewhere on Earth at mean intervals of about 0.4 to 2Myr, and in the British Isles at mean intervals of about 600 to 2,800 Myr. The higher rate implies that the discovery of
4
The ratio of the largest diameter of the swarm to its smallest diameter.
The Midlands Basin — A Cometary Impact Structure?
Ill
one such Phanerozoic crater in the British Isles is consistent with expectations, particularly if the flux of comets was indeed higher during the period from 200 to 300 Myr BP (Shoemaker, 1998). The fact that an impact capable of forming a crater the size of the Midlands Basin probably occurs about once per million years implies that such impacts do not cause world-wide catastrophes. The four ages of the Early Permian had an average length of 8.5 Myr, much longer than the mean time between the formation of such craters. Hills and Goda (1993) suggest that the impact of a body with r 0 ~ 1.2 km could eject enough dust into the upper atmosphere to darken the Earth and cool the climate. It may therefore be significant that the inferred age of the Midlands Basin coincides with the great Permo-Carboniferous Ice Age. According to Hills and Goda, cometary impacts are more likely than asteroidal impacts to cause fires over large areas. However, the hypothetical Midlands impact probably did not start extensive fires, because in the almost lifeless Permian desert there was hardly anything to burn. To summarise: are the arcuate rim of the Permo-Triassic outcrop, the circular ring of hills corresponding with the Palaeozoic uplifts of the South Staffordshire and Warwickshire coalfields, and the Permo-Carboniferous breccias the results of uplift and erosion of Hercynian mountain ranges, or are they the traces of the impact of a comet in the Midlands of England? It is now time to turn from the complexities of the Midlands to the remaining areas of geological stability, first westwards to the Welsh border country of Herefordshire and Shropshire, and then eastwards to the London Platform.
The Herefordshire Domes from the banks of Wye and sandy-bottom'd Severn 1 Henry IV, III, I, 66-7 come to you in sheep's clothing, but inwardly they are ravening wolves. Matthew, 7, 15. The ancient county of Herefordshire (now incorporated in the new county of Hereford and Worcester) forms the hilly country of the Welsh Borders west of the Hercynian mountain remnants of the Malvern Hills and the Abberley Hills. The soil is mostly loam, and the county is almost entirely agricultural. Geologically, Herefordshire forms part of the stable terrane of the Welsh Marches, and its surface geology is dominated by sedimentary rocks of the Devonian system, more so indeed than the geology of Devon itself, which is largely Carboniferous. Although the Welsh Marches are surrounded by orogenic belts, the Longmynd and the Cambrian Mountains to the north-west, the Malvern and Abberley Hills to the east, and the South Wales and Forest of Dean Coalfields to the south, the tectonic waves associated with these orogenic belts appear to have broken against
m
The Herefordshire Domes
•gill
the rigid basement of the Marches, without disturbing the rocks of the platform. In Herefordshire itself there are only four significant disturbances. The first two are the Vale of Neath Disturbance and the Swansea Valley Disturbance, which run north-east from the western part of the South Wales Coalfield. The Neath Disturbance is marked by the SW-NE line of hills between Grosmont (Gwent) and Hereford; beyond Hereford the disturbance strikes north-eastward along the valley of the River Frome to Knightswick (or Knightsford Bridge), where the River Teme breaks through the line of the Malvern and Abberley Hills. The Swansea Valley Disturbance lies west of the Neath Disturbance: it follows the line of the River Tawe north-east of Swansea, and then probably continues to the line of the River Wye between Glasbury (SO 174392) and Hay-on-Wye, and then to Titterstone Clee Hill, east of Ludlow. Although these two disturbances are of Hercynian age, their NE-SW strike suggests that they have an earlier, Caledonian, history. The other two disturbances are the Woolhope Dome, east of Hereford, and the Hope Mansell Dome, on the Gloucestershire border in the north of the Forest of Dean. These two circular domes have already been mentioned in Chapter 3 as possible impact structures, and they form the subject of this chapter. The Woolhope Dome is located south of the A438 between Hereford and Ledbury. It is a topographic as well as a structural dome; and its circular shape is obvious on the Ordnance Survey 1: 50,000 map (sheet 149). This circular shape is most apparent in the north-western semi-circle of the dome, where it is defined by the Rivers Wye, Lugg, and Frome. This is the semi-circular arc from Fownhope (S0581343) through Mordiford (571375), Prior's Frome, Dormington, Perton, Stoke Edith, Tarrington, Durlow Common, Putley (646376), and Woolhope Cockshoot (631372). The topography of the south-eastern semi-circle of the dome is less clear. East of Fownhope the southern margin of the dome is formed by a fault, which brings up the Ludlow (Upper Silurian) and older rocks of the dome against the Downton Series rocks of the Lower Old Red Sandstone. This fault strikes about eastsouth-east through Overdine (600337), Lower Buckenill, Sollers
HI
Part I: Impacts and Geology
Hope (613332), Foxhalls, and Dean's Place (637314). Along the eastern margin of the dome the high ground of Marcle Hill (631355) is continued southwards by Ridge Hill (231 metres, 628339), and then to the south-south-east by more high ground east of the River Wye as far as May Hill (296 metres, S0696212) in Gloucestershire. This high ground consists of the Ludlow rocks of the Woolhope Anticline, which connects the Woolhope Dome with May Hill. The most striking topographic feature of the Woolhope Dome itself is a semi-circular ridge in the north of the dome. This ridge forms the high ground of Backbury Hill (225 metres), Tower Hill, and Seager Hill (S0617387, 264 metres), which is the highest point of the dome. This ridge is continued southwards by the high ground of Marcle Hill and Ridge Hill, which form the eastern margin of the dome. Inside this ridge, the centre of the dome is occupied by another hill (188 metres), with its peak at about S0594366, in Haugh Wood, between the villages of Mordiford and Woolhope1. This 'central hill' is actually about a kilometre west of the geometrical cente of the dome, which is located at about SO604370 (52°01.8'N, 02°34.6'W), between the villages of Checkley and Woolhope, and just east of Sharpnage Wood. Measurements of the Ordnance Survey map yield a diameter for the topographic dome of 6.9±0.1km. The dome is almost exactly circular; the east-west diameter is formally 6.88 km, and the north-south diameter is 7.01 km. The difference of 130 metres between these two diameters is less than the errors of the measurements, and <2% of the average diameter of the dome. The river system of the Woolhope Dome can be described briefly. The northern half of the interior of the dome is drained by a river that rises at SO608371, north-east of Sharpnage Wood, and flows west, following the outcrop of the Middle Silurian Wenlock Shale between Seager Hill and the central hill to join the River Lugg (itself only about 400 metres from its confluence with the River Wye) at Mordiford (568374). East of Mordiford this stream follows the line of a NE-SW fault between Haugh Wood and Bear's Wood (584378). The east of the dome is drained by a river that rises north of 1
The name 'Woolhope' probably means something like 'blind valley with wolves'; it has nothing to do with sheep's clothing.
The Herefordshire Domes
m
Woolhope and flows southward, also along the outcrop of the Wenlock Shale, to join the River Wye at How Caple (607302). Another river rises on the east side of the central hill and flows south-west to join the River Wye near Fownhope. Geologically the Woolhope Dome is an inlier of Silurian sedimentary rocks. The rocks occur in more or less concentric outcrops dipping away from the centre of the dome, with the oldest rocks at the centre. These oldest rocks are upper Llandovery (Lower Silurian) sandstones (the Haugh Wood Beds), which are overlain and encircled by the lower Wenlock (Middle Silurian) Woolhope Limestone. In its turn the Woolhope Limestone is overlain by the Wenlock Shale and the Wenlock Limestone. This limestone is overlain by the Upper Silurian Elton Beds, 2 the Bringewood Beds (or Aymestry Limestone), the Leintwardine Beds (or Dayia Shales), and the Whitcliffe Flags. The highest hills of the dome (Backbury Hill, Seager Hill, Marcle Hill, and Ridge Hill) all occur on the Bringewood Beds. The whole succession is overlain by the rocks of the Downton Series (Lower Old Red Sandstone). This series is now regarded as Upper Silurian rather than Lower Devonian; Harland et al. (1982) and Odin (1994) assign the Downton Series to a Pridoli epoch, named after a location in Bohemia. It is admittedly rather unfortunate that the lithological change from the Llandovery-WenlockLudlow marine succession to the Downton (or Pridoli) Lower Old Red Sandstone does not coincide with the palaeontologically defined Silurian-Devonian boundary. However, it m u s t be remembered that Ludlow itself is the classical site of a conformable transition from the Ludlow Series to the Downton Series. Moreover, the inclusion of the Downton Series in the Silurian helps to emphasise that there is no sharp break in the succession, and that the rocks of the Downton Series, which form the low-lying valleys of the Rivers Wye, Lugg, and Frome, are as much part of the Woolhope inlier as are the rocks of the Llandovery, Wenlock and Ludlow Series. The entire succession exposed in the Woolhope Dome consists of sedimentary rock; there are no volcanic or intrusive rocks anywhere in the dome. 2
Formerly k n o w n a s t h e Lower Ludlow Shales.
m
Part I: Impacts and Geology
Although the Woolhope Dome as a whole is circular to within the errors of measurement, the outcrops of the lower rock formations are not. For example, the outcrops of the Haugh Wood Beds and the Woolhope Limestone are elongated in an east-west direction; and the outcrops of the Elton Beds and the Bringewood Beds are elongated to the north-west and southeast. Moreover, the dome is markedly asymmetric. The outward dips of the rocks are steeper in the west and south of the dome than in the north and east; as a result the outcrops of the higher Ludlow Series beds are much broader on the east and north sides of the dome, outside Marcle Hill and Seager Hill, than they are in the west and south. A characteristic feature of the central uplifts of impact structures is that the outward dip of the rocks increases towards the centre of the structure. This centripetal increase in dip appears to be present, to some extent, in the Woolhope Dome, particularly in the north-east and south-east sectors. In the north-east sector the mean outward dip increases from 14.9°±6.5° in the Whitecliffe Beds to 26.5°±12.5° in the Wenlock Shale, and then decreases to 13.2°±2.4° in the Woolhope Limestone and 18°±8.5° in the Llandovery Series. In the south-east sector the mean outward dip increases from 9.3°±3.4° in the Whitecliffe Beds to 32.1°± 18.3° in the Wenlock Shale, and then decreases to 26.1°±17.2° in the Woolhope Limestone and 18°±8.5° in the Llandovery Series. However, there is no significant radial variation in the dip of the rocks in the north-west sector of the dome; and in the south-west sector the outward dips decrease towards the centre of the dome, from 44°± 1.5° in the Bringewood Beds to 18°±8.5° in the Llandovery Series. 3 The largest dip measured in the Woolhope Dome is 89° in the Wenlock Shale near S0595376, on the north-north-east side of the dome {Earp and Hains, 1971). It may be argued, then that the radial variation of the dip of the sedimentary rocks is consistent with the hypothesis that the Woolhope Dome is an impact structure, but it does not provide compelling evidence for this hypothesis. ^ h e dips measured for the Llandovery Series are for the whole dome, and are not divided into sectors.
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map of the Woolhope Dome (Herefordshire). Reproduced by permission of the Ordnance Survey and the British Geological Survey.
Fig. 8. Geological map of the Woolhope Dome (Herefordshire). Scale ~1:48,000. Reproduced by permission of the Ordnance Survey and the British Geolog
The Herefordshire Domes
19
In the south-west and south of the dome, between the musically named Bagpiper's Tump (570370) and Dean's Place (637314), the margin of the dome is formed by a number of faults. In many places these faults cut out the higher Ludlow beds, and bring the Downton Series down against the Bringewood Beds or the Elton Beds. In some places, for example east of Fownhope and east of Sollers Hope, the Downton Series is actually faulted against the Wenlock Limestone. This 'southern boundary fault' is not the only fault that affects the Woolhope Dome. In fact, the whole dome is cut up by faults, particularly in the north-west and in the south. Most of the faults cut the harder Haugh Wood Beds and Woolhope Limestone in the centre of the dome, and the Ludlow beds around its circumference; there are fewer faults in the soft Wenlock Shale. In the centre of the dome, in particular, the faults disrupt the concentric pattern of the outcrops and make them irregular. This disruption of the outcrops is also observed in the Serpent Mound (Ohio) impact structure (Baldwin, 1963; Reidel et al, 1982). Finally, many of the faults around the circumference of the Woolhope Dome also cut the beds of the Downton Series; this observation confirms that these beds form part of the Woolhope structure. Most of the faults that cut the dome are more or less radial, and are therefore related to the dome itself rather than to the regional tectonic structure. However, there are two exceptional faults, or fault zones, with a NE-SW (i.e. Caledonian) trend. The more westerly of these runs north-east from near Mordiford, across the north-west sector of the dome, towards Tarrington, north of the dome. This fault throws down to the north-west, bringing Ludlow-stage Elton beds against the Wenlock Shale west of Checkley; near Mordiford it displaces the western margin of the dome to the right. The second fault extends from the east end of the central hill (west of Woolhope) to Woolhope Cockshoot on the east side of the dome, where it brings the Downton Series to the south down against the Bringewood Beds to the north. This fault displaces the eastern margin of the dome, again to the right. The total thickness of the Llandovery to Ludlow succession in the Woolhope Dome is about 820-900 metres (Earp and Hains, 1971). This is similar to the 660 metres of Llandovery to Ludlow
m
Part I: Impacts and Geology
rocks in the Wenlock-Ludlow area of Shropshire, and much less than the >2,500 metres in the Welsh geosyncline. To the south of the Woolhope Dome, Silurian (Ludlow) rocks are also exposed in the elongated Woolhope Anticline. This anticline extends for 16-19 km to the south-south-east as far as the May Hill inlier (S0693214), in Gloucestershire, where Llandovery and Wenlock rocks appear again. Sedimentary rocks of the Ludlow series also outcrop about 6.5 km north of the Woolhope Dome in Shucknall Hill (173 metres), at S0595434. This small Silurian inlier lies north of the Neath Disturbance, which is followed by the valley of the River Frome. The central hill of the Woolhope Dome is formed by the resistant Llandovery Sandstones (Haugh Wood Beds) and Woolhope Limestone. Away from the centre of the dome the soft Wenlock Shale and Elton Beds (Lower Ludlow Shales) have been eroded to form circular valleys, whereas the intervening hard Wenlock Limestone forms a circular ridge between them. Finally the Bringewood beds and the overlying Ludlow rocks form the outer arcuate ridge of Backbury Hill, Seager Hill, and Marcle Hill. Thus the dome consists topographically of concentric arcuate or circular ridges surrounding a central hill. This topographic pattern is essentially similar to that found in the central uplift of the Richat Dome in Mauritania, although Richat is much larger (D~42km), and better exposed, than the Woolhope Dome. The amount of uplift can be estimated from the observation that west of the Woolhope Dome the River Wye, which is here 40-50 metres above sea level, flows over Lower Old Red Sandstone, probably of the Downtonian (Pridoli) stage. The lowest rocks of the Silurian succession are stratigraphically 820-900 metres below the base of the Old Red Sandstone, but they outcrop in the central hill at a topographic height of 188 metres, i.e. about 140 metres above the valley of the River Wye. Thus these upper Llandovery rocks have experienced a structural uplift (SU) of at least 960-1040 metres. It is more difficult to estimate an upper limit for the uplift. In this part of Herefordshire the Downton Series of the Lower Old Red Sandstone is about 500 metres thick, so the top of the Downton Series is about 1,300-1,400 metres above the Llandovery sandstones. If the Old Red Sandstone of the Wye valley is at the
The Herefordshire Domes
HB1
top of the Downton Series the total uplift could be as much as 1,540 metres. However, the Wye appears to flow through the lower part of the Downton Series, and therefore the uplift is probably nearer to the lower limit of 960-1,040 metres estimated above. This pattern of structural uplift, with concentric circular outcrops of rock and radial faulting of the central uplift, is characteristic of terrestrial impact structures formed in sedimentary terranes, such as the Wells Creek structure in Tennessee. Moreover, the pattern of faulting in the Woolhope Dome is remarkably similar to that found in the central uplift of the Gosses Bluff impact structure in Australia. The central uplift of Gosses Bluff is about 5 km in diameter, and is therefore similar in size to the Woolhope Dome. However, this central uplift forms only a small part of the whole Gosses Bluff structure, which is no less than 22 km in diameter. Therefore, if the two structures are comparable, the similarity between them suggests that the Woolhope Dome is actually the central uplift of a much larger impact structure. If the Shucknall Hill inlier is taken to represent the outer rim of this impact structure, the original diameter would have been about 13 km, or much the same as that of Wells Creek (D=14km). The structural uplift at Wells Creek is very similar to that of the Woolhope Dome; according to Baldwin (1963), the uplift of the dome is 2,500' (760 m), and the downthrow of the inner ring syncline (sx) is 700' (210m), yielding a total vertical displacement of 3,200' (970m), essentially the same as the 960-1,040 m obtained for Woolhope. On this interpretation the river valleys that surround the dome to the west, north and east could be identified with the ring syncline observed in other terrestrial impact structures. The hilly country west of the River Wye and north of the River Frome (including Shucknall Hill), and, farther east, around Little Marcle and Much Marcle, might then represent the former outer rim of the structure. This suggestion, that the Woolhope Dome is only the central uplift of a larger impact structure, is consistent with the amount of structural uplift. According to Grieve (1987), the structural uplift (SV) at the centre of an impact structure is related to the diameter (D) of the structure by the formula SJ7=0.06xD 1 \ or equivalently D= 12.9 x SU° 91 , where D and SUare both measured
UBI
Part I: Impacts and Geology
in kilometres. Thus a structural uplift St/3= 0.96-1.04 km implies a total diameter for the structure of D ^ 12.3-13.5 km. The association of the Woolhope Dome with the Woolhope Anticline and the Vale of Neath Disturbance suggests that the dome is related to ordinary tectonic structures; if so, it is probably of endogenous origin rather than being an impact structure. However, it has been observed that several crypto-explosion and impact structures are found at the intersection of fault zones (Gallant, 1964; Nicolaysen and Ferguson, 1990); this is true, for example, of the Crooked Creek (Missouri) impact structure, which is located at the intersection of the Cuba and Palmer faults. These intersecting faults may therefore have been created by the impact, rather than marking weak points in the Earth's crust where crypto-volcanic explosions could occur. Some such interpretation might explain the location of the Woolhope Dome at the intersection of the Woolhope Anticline and the Neath Disturbance. The Woolhope impact may have produced a fracture (now marked by the Woolhope Anticline) in the direction of the stress concentration formed by the May Hill Anticline, south of the Malvern Hills. On the other hand, if the Woolhope Dome is of tectonic origin rather than being an impact structure, the anticline may have developed as a branch of the Malvern-May Hill monocline in the direction of the stress concentration represented by the dome. It should be remembered that about 10" 4 of the energy of craterforming impacts goes into seismic waves (Melosh, 1989), and that in a large impact this seismic energy is equivalent to that of a large earthquake. For example, the energy released by the formation of a 13-5-km impact structure is about 10,000Mtons (lOGtons, or 4 x 10 19 J), and the seismic component of this energy is equivalent to an earthquake with a magnitude of about 7.2. Faulting associated with an earthquake of this size might well extend the 20 km to the May Hill inlier, if not the 100 km to Neath. It must also be remembered that in a country with the varied geological history of the United Kingdom, any random impact is likely to be within a few kilometres of an entirely independent tectonic structure, which might even be re-activated by the impact. If the Woolhope Dome is in fact an impact structure, it has been deeply eroded, and the distinctive signs of impact have been
The Herefordshire Domes
i8nPJlsSI
destroyed. The stratigraphic evidence is insufficient to give any indication of the age of the structure. It must be postDowntonian (t=£410Myr), and is probably post-Devonian (t=£ 360 Myr). Since the Old Red Sandstone in the Welsh Borders may be as much as 2,200 metres thick and the thickness of the Silurian succession in the Welsh Borders is 820-900 metres, the Haugh Wood Beds (Llandovery sandstones) at the centre of the dome may have been covered by 2^3,000 metres of Silurian and Devonian sediments at the end of the Devonian period. Such a thickness of cover rocks would have been enough to protect the lower Silurian rocks from the effects of the shock of an impact, and these rocks would therefore probably not show clear signs of shock metamorphism and brecciation. Anderson and Owen (1980) regard the Woolhope Dome and Anticline as Hercynian structures and therefore probably Upper Carboniferous (i=290-330 Myr). However, if this part of Herefordshire was covered by a sufficient thickness of PermoTriassic New Red Sandstone or Chalk, the Dome could be a Mesozoic or even a Tertiary impact structure. In this connection, it may be mentioned that the intersecting faults of the dome are still seismically active. Davison (1924), in his History of British Earthquakes, describes earthquakes in the Hereford region in 1853, 1863, 1868, 1896, 4 and 1924. This seismic activity implies that the faults are of geologically recent origin, rather than being of Hercynian age; this young age for the faults would be consistent with the interpretation of the Woolhope Dome as a Mesozoic or Tertiary impact structure. The Woolhope Dome shows the same structural features as authentic terrestrial impact structures, such as Wells Creek, of the same size. However, the dome is not known to show the distinctive features of impact structures, such as mega-brecciation, shatter cones, or microscopic evidence of shock deformation, and it is therefore necessary to consider the possibility that the dome is of endogenous origin. There are no igneous rocks associated with the dome, and it may therefore be concluded that it is not of igneous origin. The circularity of the topographic dome is evidence
4
There are, however, some odd features of the 1896 earthquake (see Appendix 1).
Part I: Impacts and Geology against it being an anticline; veiy few anticlines are anything like circular, and the location of the dome on the essentially undeformed Welsh Borders Platform is also evidence against a tectonic origin. The hypothesis of a sedimentary diapir can also probably be excluded. The Woolhope Dome is certainly not a salt dome, for the simple reason that there are no evaporites anywhere in the Lower Palaeozoic rocks of the Welsh Borders. Diapiric uplift of other sedimentary rocks, for example shales, is possible but unlikely; the dome is too large both for the thickness of underlying Lower Palaeozoic sediments and in relation to the ring syncline. Moreover, sedimentary diapirs generally occur in deep sedimentary basins or the foredeeps of orogenic belts, not in the shallow sedimentary cover of stable continental platforms. If the Woolhope Dome is not of igneous, tectonic or diapiric origin, it may yet be an impact structure. If so, the absence of the stigmata of impact implies that it is a 'hypo-astrobleme', exposed at an erosional level below the reach of shock. Because of the absence of evidence of shock, the dome can be considered only a possible impact structure; nevertheless, a detailed study of its rocks may yet yield the required positive evidence of impact. The second and smaller of the tectonic domes of Herefordshire is the Hope Mansell Dome, which is located in the Forest of Dean 16 km south of the Woolhope Dome and 5 km south-east of Rosson-Wye; its centre is at about SO635204 (about 51°52.8'N, 02°31.8'W). The diameter of the Hope Mansell Dome is about 4 km; it is the smallest of the impact structures identified in this section of this book. The structure is essentially the same size as Kofels Hollow, and is slightly larger than the New Quebec Crater (D=3.44km), Steinheim Basin (D=3.4km), and Flynn Creek (D=3.6km). Unlike the Woolhope Dome, the Hope Mansell Dome forms a topographic depression. In a Landsat photograph the structure appears as part of a circular, almost crater-like, hollow, open to the north and partly enclosed by a crescentic ridge. Morphologically this hollow could be taken for a glacial corrie, 5 were it 5
In fact the Forest of Dean and the Hope Mansell Dome were very close to the SE edge of the Pleistocene ice sheets, so there may have been some glacial erosion.
The Herefordshire Domes
m
not for the associated up-doming of the strata and the presence of a central hill. The summit of this central hill is between 168 and 183 metres above sea level, whereas the encircling ridge reaches a maximum height of 279 metres on the east side of the hollow. The Hope Mansell Dome consists of younger rocks than the Woolhope Dome. The central hill consists of Lower Old Red Sandstone. This is overlain and encircled by less resistant Upper Old Red Sandstone marls, which have been excavated to form the topographic depression. These marls are in turn overlain by hard Carboniferous Limestone and Upper Coal Measures, which form the encircling ridge. Although the Hope Mansell Dome is small it is none the less worthy of note. In particular, the hydrology of the dome presents an interesting example of river capture. There are two streams draining the basin. One rises in the south-east of the depression and flows north-west past Hope Mansell (625195) and then north, to the south and west of the central hill. The other stream rises east of the central hill, north of Puddlebrook, and flows north and north-west past Pontshill (637219), north of the central hill. The obvious (and presumably the original) course for these two streams is northwards to Weston-under-Penyard (631232) and then westwards towards Ross-on-Wye along the valley used by the dismantled railway line between Ross-on-Wye and Gloucester. Instead the Pontshill stream turns south-west and joins the Hope Mansell stream at Frogmore (629218), and then the combined river flows westward through a valley that cuts the high western rim of the depression (which here reaches 203 metres) to join the River Wye at S0584196, south of Walford. As a result of this river capture, the high ground of Chase Wood and Penyard Park, north of the river, is cut off from the rest of the Forest of Dean. The stratigraphy of the Hope Mansell area provides evidence that makes it possible to estimate the structural uplift of the dome (see Table 2). The summit of the central hill of the dome, which consists of Lower Old Red Sandstone, is at h~ 180 metres, whereas the base of the Coal Measures to the east and west of the depression is at about 150-200 m. The intervening Upper Old Red Sandstone is between 60 and 180 metres thick, and the
Part I: Impacts and Geology Table 2. Stratigraphy of Hope Mansell Dome region, Forest of Dean Series
Formation
Upper Carboniferous Upper Carboniferous? Lower Carboniferous
Upper Coal Measures Upper Drybrook Sandstone Lower Drybrook Sandstone Whitehead Limestone Crease Limestone Black Rock Limestone Lower Limestone Shale Tintern Sandstone Quartz Conglomerate
Upper Devonian
Thickness (m) 500-600 70-105 -30 20-30 70 40 60-150 6-30
'Carboniferous Limestone' succession (from the Lower Limestone Shale to the Upper Drybrook Sandstone) is between 230 and 270 metres thick. Thus the base of the Upper Old Red Sandstone (and the top of the Lower Old Red Sandstone) is between 150270-180 = 300 metres and 200-230-60 = 90 metres below sea level, so that the structural uplift {SU) of the Lower Old Red Sandstone of the central hill is between 270 and 480 metres. This estimate for the structural uplift is in fair agreement with the 200-300 metres of uplift cited for the Steinheim Basin, with D = 3 . 4 k m (von Engelhardt, 1972, reprinted in McCall, ed., 1979) and the 350 metres of uplift at Flynn Creek, with D=3.6km. These figures are rather larger than those obtained from the formula of Grieve (1987) for the structural uplift-diameter relation, namely SLT=0.06 x D 1 1 . This formula yields uplifts of 230m, 250 m and 280 m for Steinheim, Flynn Creek and Hope Mansell respectively. However, there is another possible interpretation of the Hope Mansell Dome. It lies between two of the north-south synclines of the northern Forest of Dean, the Wigpool Syncline to the east and the Howie Hill Syncline to the west. The rocks of these synclines form the east and west limbs of the encircling ridge. Moreover, the high ground of Penyard Park and Chase Wood in the northwest of this ridge is formed of Devonian Lower Old Red Sandstone rather than of Carboniferous Limestone or Coal Measures. This Old Red Sandstone must dip southwards under the Carboniferous rocks of the Forest of Dean. Moreover, the Lower Old Red Sandstone of the central hill is continuous to the north,
The Herefordshire Domes
Bi
where the topographic basin opens onto the Herefordshire plain, rather than being overlain by Upper Old Red Sandstone and Carboniferous rocks. These observations imply that the Hope Mansell structure is not a true dome but a steeply plunging anticline between the Wigpool and Howie Hill synclines; if so, it cannot be an impact structure. This chapter has examined two reported domal uplifts in Herefordshire. The Hope Mansell Dome appears to be an anticline in the Carboniferous rocks of the Forest of Dean that brings the underlying Old Red Sandstone to the surface. However, the Woolhope Dome stands out among tectonic domes for its nearly circular shape, its abrupt uplift, and its isolation from other tectonic disturbances; if there is an example in Britain of the classical crypto-explosion structure, it is surely the Woolhope Dome. These last chapters have discussed possible impact structures in the Midlands Platform and the Welsh Borders. The next chapter will look farther east, at the Tertiary rocks of the London Platform.
CHAPTER
EE The Rochford Basin — A Digression into Essex This is my own, my native land! Sir Walter Scott, The Lay of the Last Minstrel, VI, 1. And there fell a great star from heaven, and it fell upon the third part of the rivers, Revelation, 8, 10 The Rochford b a s i n is t h e nearly circular topographic b a s i n t h a t s u r r o u n d s t h e small town of Rochford 1 (TQ874907), between t h e T h a m e s e s t u a r y a n d t h e e s t u a r y of t h e River C r o u c h in s o u t h e a s t Essex; t h e a r e a is covered b y s h e e t s 168 a n d 178 of t h e O r d n a n c e Survey 1: 50,000 Series a n d by Sheet 2 5 8 / 2 5 9 of t h e 1: 5 0 , 0 0 0 Series of t h e Institute of Geological Sciences. S o u t h e n d Airport is inside t h e b a s i n . The floor of t h e b a s i n is flat (mostly 1 0 - 1 5 m e t r e s above sea level), b u t to t h e s o u t h , west a n d n o r t h t h e b a s i n is encircled by ^Hunting dogs' ford,' the source (by back-formation) of the name of the River Roach.
The Rochford Basin — A Digression into Essex
Em
the low hills of Southend-on-Sea, Prittlewell ('babbling brook'), Westcliff-on-Sea, Eastwood, Hockley, Ashingdon, and Canewdon. These hills continue westwards and south-westwards to the high ground (up to 84 metres) of Rayleigh (TQ805906) and Thundersley (TQ787886); these are called the Rayleigh Hills. The hill-slopes that form the southern and western margins of the basin are fairly steep, even where they are not the sides of river valleys; they may be degraded Pleistocene sea-cliffs. Such steep slopes are found south of the A127 (the Arterial Road) between Eastwood and Prittlewell (around TQ850893), west of Stroud Green (around 853906), and between Hawkwell and Hockley (between 848921 and 844922). However, the topography of the basin is mostly subdued, and this fact makes its size uncertain; but its diameter is probably between 6 and 7 km. The centre of the basin is near TQ890910 (51°35'N, 0°44'E). The Rochford basin essentially forms a low-lying embayment in the relatively high ground between the Thames Estuary and the River Crouch. To the north and south of the basin the hills of Canewdon and of Southend-on-Sea extend 6 km east of the steep slopes of Eastwood and Stroud Green on the west rim of the basin. The floor of the basin consists of London Clay (belonging to the Ypresian stage 2 of the Lower Eocene), with a cover of Upper Pleistocene sands, gravels, and river brick-earths. The surrounding hills also consist of London Clay, overlain by Claygate Beds (17-23 metres thick) and by the Lower Bagshot Beds, which form the highest parts of the hils. The Lower Bagshot Beds probably belong to the upper Ypresian stage, and are about 49-50 Myr old; in this part of Essex they are about 23 metres thick. The London Clay rests on the Woolwich Beds (Lower Eocene or Palaeocene) and the Thanet Beds (Palaeocene), which in total are 26 to 55 metres thick. The Thanet Beds rest unconformably on the Upper Chalk. The Lower Eocene rocks that form the Rayleigh Hills are also overlain by Pleistocene deposits. In particular, the valleys that drain from the hills into the Rochford Basin contain deposits of 2This same clay occurs at Ypres in Belgium, notorious as a scene of trench warfare during the First World War.
m
Part I: Impacts and Geology
'head', a solifluction deposit formed under perlglacial conditions during the Pleistocene ice ages. In addition, there are deposits of brick-earth in the hills that form the western and southern margins of the Rochford Basin itself. (Deposits of brick-earth also occur north of the River Crouch, to the north of Burnham-onCrouch; they are not restricted to the neighbourhood of the Rochford Basin.) The Rochford Basin is entered and drained by five main streams: Prittle Brook (the source of the name 'Prittlewell'); the Eastwood Brook; the River Roach; and two streams that enter the basin from Hockley and Hawkwell ('hook-shaped stream') in the north-west. These streams rise west of the basin, in the hills of Thundersley, Rayleigh, and Hockley, and they flow slightly south of east to enter the basin. It may be inferred that the rivers that drain the Rayleigh Hills existed before the deposition of the Pleistocene periglacial sediments. For example, east of Thundersley Prittle Brook cuts through deposits of 'head' (overlying Claygate Beds) at 55-60 metres above sea level, whereas Bagshot Sands outcrop on the surrounding hills at heights of >80 metres. Thus the valley must have been cut to a depth of at least 20-25 metres before the onset of periglacial conditions. So far, the Rochford Basin might well be a basin of combined marine and fluvial erosion. However, there are two anomalous features of the basin, one morphological and the other geological, which suggest a different origin. The morphological feature is connected with the drainage of the Rayleigh Hills and the Rochford basin. In the west of the region the five streams listed above flow parallel to one another, and slightly south of east. This parallelism is itself rather remarkable, to the extent that in the three largest streams (Prittle Brook, the Eastwood Brook, and the River Roach) changes in direction occur in the same parts of the course of each stream. However, on entering the Rochford Basin, these streams change direction and flow towards the centre of the basin, thus forming a centripetal drainage pattern. Thus the two streams from Hokley and Hawkwell join and flow south-east, meeting the River Roach in Rochford itself (TQ876903); and the Eastwood Brook turns north-east and joins the River Roach on Rochford Golf
The Rochford Basin — A Digression into Essex Course (TQ870900). Prittle Brook also changes though where it flows through Prittlewell itself from the basin by an east-west ridge. In spite of turns and flows north and then east to join the TQ891901, near Stambridge Mill.
S direction, even it is separated this, the brook River Roach at
The morphology of the Rayleigh Hills and the Rochford Basin may be compared with that of the Althorne Hills of the Dengie Hundred to the north, between the River Crouch and the Blackwater Estuary. The streams flowing east from the Althorne Hills remain essentially parallel after they have emerged onto the Dengia and Bradwell Marshes; there is no central embayment in the Althorne Hills that corresponds to the Rochford Basin. The geological anomaly is the Rochford Basin is the presence, near its centre, of a small inlier of Chalk. This Chalk outcrop occurs on the north shore of the tidal River Roach, at about Grid Reference TQ893902 (51°34.7'N, 00°44.0'E); this is about 1.8km east of Rochford and about 600 metres east of Stambridge Mill (887903) and Broomhills. The outcrop is very small, about 20 metres by 15 metres, and about 1 metre thick; it is not marked on the geological map, which shows only tidal flat deposits at this point. According to the geological section the top of the Chalk should be at a depth of at least 110 metres (and at most 160 metres) below sea level. The top of this outcrop is about 1 metre above the high-tide mark. The Chalk outcrop is very rich in flint pebbles, some of them large enough to be called cobbles, and this implies that it is part of the Upper Chalk, which in this part of Essex is about 85 metres thick. However, it is of unusual character; instead of being the pure chalk familiar from the Downs and the Chiltern Hills, it is interstratified with what appears to be clay-with-flints, a familiar Tertiary deposit. At the west end of the outcrop a surface layer of Chalk about 10 cm thick overlies 0-30 cm of grey clay with abundant flint pebbles, which itself overlies 25-30 cm of weathered white chalk with flint. Near the centre of the outcrop the main chalk bed overlies a bed of very darkcoloured clay with flint pebbles. Near this part of the outcrop the chalk bed passes laterally eastwards into clay with flints, although the chalk reappears farther east. There may be a slight dip of the rocks to the east or north-east, but this is very small,
Part I: Impacts and Geology certainly < 5°. However, some internal planes dip more steeply, at about 10°. The Chalk outcrop does not show any of the defining features of impact; it is not brecciated, and it does not contain shatter cones. On the other hand the mere presence of a Chalk outcrop at this point is inconsistent with the stratigraphy and structure of this part of Essex, and requires a localised uplift of between 110 and 245 metres. The appearance of the rock, with its interstratified clay beds, is unusual for the Chalk; it might even be taken for a melange* or olistostrome, a jumbled mass of fragments from a wide range of rock types (Whittow, 1984). Such melanges are thought to be formed by the slumping and flow of unstable rock masses at a continental margin, but the low-lying coastal plain of Essex, near the centre of the stable London Platform, is the last place where one would expect to find such a rock. The situation may be comparable with the flow of incompetent clays in the Silverpit impact crater (Stewart & Allen, 2002) in the North Sea (see Epilogue). Alternatively this might be an outcrop of glacial boulder clay or till, but the Pleistocene ice sheets are not thought to have reached this part of Essex. The nearest known outcrops of boulder clay are in the Hanningfield area (TQ 729958), about 17 km from this Chalk outcrop. Moreover, the nearest mapped outcrop of Chalk to the north is 46 km away, a large distance for ice to transport a coherent mass of chalk and clay 20 metres long. The small size of the Chalk outcrop (no more than 300 m2) suggests that it is a detached block of the Cretaceous 'basement' rather than part of a coherent basement uplift. If so, its presence makes a point of similarity with the Decaturville (Missouri) impact structure, which contains a detached basement block of Precambrian pegmatite with an exposure of only a few square yards (Baldwin, 1963; Snyder and Gerdemann, 1965). It may be significant that the Decaturville structure is 6 km in diameter, and is therefore very nearly the same size as the Rochford Basin. Moreover, the structural uplift in the centre of Decaturville (-160 metres) is similar to the structural •^he Italian name for this rock is argille scagliose, or 'scaly clay,' which is a fair description of this outcrop.
The Rochford Basin — A Digression into Essex
row
uplift of 110-245 metres deduced for the outcrop of Chalk in the centre of the Rochford Basin. However, this Chalk outcrop is not as severely deformed and brecciated as the rocks of the DecaturvUle structure (Baldwin, 1963). The Chalk uplift is not the only structural feature associated with the Rochford basin. A geological section on the IGS map (Sheet 258/259) from the easternmost point of Canvey Island to Canewdon and to Bridgemarsh Island in the River Crouch shows uplifts of the Woolwich and Thanet Beds and the Chalk beneath Canewdon and Westcliff-on-Sea, on the rim of the basin, and a syncline in these beds in the basin itself, west of Rochford. There are also shallow synclines beneath Canvey Island and the River Crouch, outside the basin. The amplitudes of these structures are about 40 metres, and they extend to depths of at least 170 metres below the land surface. The top of the Chalk west of Rochford is 150-160 metres lower than the Chalk outcrop by the River Roach, 3.2 km to the east. Obviously a single section is not sufficient to prove the presence of ring synclines or of a ring anticline; the structures shown by this section may be ordinary east-west anticlines and synclines. However, the presence of these tectonic structures in the Chalk suggests that the Rochford Basin has a structural origin rather than being merely a basin of erosion; and the section is consistent with the existence of a ring syncline in the basin itself surrounding the central Chalk uplift, of a ring anticline (corresponding to the rim of the 'crater') beneath the encircling hills, and of an outer ring syncline beneath the estuaries of the River Thames and the River Crouch. There is no gravity anomaly or geomagnetic anomaly associated with the Rochford basin, and the absence of any such anomalies constitutes evidence against the impact hypotheses. However, the absence of gravity and geomagnetic anomalies may be the result of erosion of the shocked rocks of the crater floor, and it is consistent with the small amplitude of the structural deformation. In spite of the central uplift of Chalk, there is no central hill in the Rochford Basin; on the contrary the River Roach flows right across the centre of the basin, and thereby exposes the Chalk inlier east of Stambridge Mill. The same state of affairs is found
Em
Part I: Impacts and Geology
in the Wells Creek Impact structure in Tennessee, where Wells Creek (which gave its name to the structure) flows directly across the central uplift. Thus the fact that the River Roach crosses the presumed central uplift of the Rochford Basin does not necessarily prove that the basin is not an impact structure. It does, however, suggest that the basin has been severely eroded and that it is probably an old structure. This eroded state raises the question of the age of the basin. If the basin is assumed to be an impact structure, it must be younger than the Claygate Beds and Bagshot Beds of the Rayleigh Hills, that is, it is < 50 Myr old. Equally, since the floor of the basin is covered by Upper Pleistocene (< 1 Myr) sands and gravels, the basin must be older than these sediments. Use of the simple rule of thumb that the age of an impact structure is the mean of the ages of the youngest remaining pre-impact rocks and the oldest exposed post-impact rocks yields an age for the Rochford Basin of 20-25 Myr, that is, Late Oligocene or Early Miocene. The stratigraphic evidence, such as it is, may also be consistent with this age. It must be emphasised that the fact that there are no sedimentary rocks in this part of Essex to represent the time interval from the end of the Ypresian stage at ~50Myr to the Pleistocene epoch does not mean that nothing happened during this time. On the contrary, it is likely that Middle and Upper Eocene and Oligocene sediments were deposited in the London Basin, as they were in the Hampshire Basin, and that they were eroded as a result of uplift during the Miocene epoch. The Eocene and Oligocene rocks of the Hampshire Basin were derived from the erosion of highland areas to the west, and the changes in sedimentation can be used to date periods of uplift in western Britain. Thus the sandy Bagshot and Bracklesham beds of Hampshire (of Ypresian and Lutetian age) were deposited in response to an episode of uplift towards the end of the Early Eocene. In general terms, the Hampshire Basin sediments were derived from the highlands of Devon and Cornwall, whereas the sediments of the London Basin were derived from the Welsh highlands. If uplift in Wales was contemporary with uplift in the South-West Peninsula, it is possible to estimate the thickness of
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er's earths in SE England and the location of the putative Surrey crater. 'nance Survey and the British Geological Survey.
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The Rochford Basin — A Digression into Essex
HH
the vanished Eocene and Oligocene rocks of the London Basin from the thicknesses preserved in the Hampshire Basin. In the Hampshire Basin the Bagshot Sands (which correspond to the Lower Bagshot Beds of the London Basin) are about 40 metres thick, and the Bracklesham Beds (corresponding to the Middle and Upper Bagshot Beds) are about 180 metres thick; these beds include the famous coloured sands of Alum Bay, in the Isle of Wight. In the London Basin the total thickness of the Bagshot Beds is about 120 metres. The Bracklesham Beds are followed in the Hampshire Basin by about 120 metres of Barton Beds and Lower Headon Beds, of Bartonian (upper Middle Eocene) age. Above these are about 210 metres of Priabonian (Upper Eocene) and Lower Oligocene sediments. The Late Eocene and Early Oligocene interval represented by these sediments corresponds to a time of intense mountain-building in the Swiss Alps, and to a period of uplift both in the South-West Peninsula and in Wales; hundreds of metres of Oligocene sediments occur in the Tertiary basins of Bovey Tracey and Petrockstow (Devon) and in the Mochras Basin, south of Harlech. It is therefore plausible that erosion of the resurgent Welsh highlands led to the deposition of a similar thickness of Upper Eocene and Lower Oligocene sediments in the London Basin to that found in the Hampshire Basin, i.e. about 200 metres. Unfortunately no Upper Oligocene sediments are preserved in the Hampshire Basin, and one can only speculate as to the thickness of such sediments that may have been deposited, and then eroded during Miocene uplift. Similarly, we do not know whether Lower Miocene sediments were deposited anywhere in Britain, and then eroded during the later Miocene. One might guess that the now-eroded Upper Oligocene may have amounted to the same thickness as the preserved Lower Oligocene of the Hampshire Basin, namely about 150 metres; but it must be emphasised that this is nothing more than a guess. If one adds all these hypothetical thicknesses together and includes 20 metres of Claygate Beds and 30-40 metres of upper London Clay, one finds that the total thickness of Tertiary sediments overlying the London Clay now exposed in the floor of the Rochford Basin may have amounted to 500-650 metres in Late
Em
Part I: Impacts and Geology
Oligocene or early Miocene times (about 24Myr ago). If the basin was formed by impact at about this time, the shocked rocks forming the crater floor would have been eroded away during the Miocene epoch to expose the uplifted but unshocked root of the crater. Thus we may tentatively suggest an age of about 20-25 Myr for the impact that may have formed the Rochford Basin. If so, the basin is of similar age to the Haughton impact structure in Arctic Canada, and probably rather older than the Nordlinger Ries and Steinheim Basin in Germany. It will be remembered that the total depth of structural deformation found at other astroblemes is ddef~ 0.2-0.3 xD, where D is the diameter of the structure. This implies that the deformation at the Rochford Basin should extend to a depth of 1.2-1.8 km below the floor of the hypothetical crater. The proposed amount of post-Oligocene erosion would then amount to nearly half of d^ef. The diameter of the Rochford Basin indicates that, if it is an impact structure, it falls into the transitional impact regime for stony asteroids, where the crater is formed by the impact of a flattened swarm of fragments rather than by a single body, and where the crater itself is broad and shallow with a very low and narrow outer rim. Melosh (1989) cites the Decaturville impact structure in Missouri as an example of such a crater. Since the Rochford Basin is essentially the same size as Decaturville, it is possible that structural deformation there is relatively shallower than in larger impact structures. This interpretation of the morphology of the Rochford Basin makes it possible to use the work of Hills and Goda (1993) to estimate the size of the asteroid that may have formed the basin. The object turns out to be unexpectedly small; its diameter was probably about 200-230 metres, and the energy of the impact was between 200 and 2,000 Mtons (8 x 10 17 to 8 x 10 18 J). This object was probably similar in size to the asteroids that made the Rio Cuarto crater field in Argentina and Kofels Hollow in Austria. Such an asteroid probably collides with the Earth about once per 2,000-3,000 years, and therefore a crater similar to the Rochford Basin may be formed in the British Isles about once per
The Rochford Basin — A Digression into Essex
m
3-5 million years. This estimated rate is consistent with the proposed Oligocene or Miocene age for the basin. 4 This analysis implies that impacts capable of making craters the size of the Rochford Basin are not particularly rare on the geological time scale, and that probably 10-20 such craters have been formed in the British Isles since the end of the Cretaceous period. It is therefore possible that the Rochford Basin itself is one such crater, which has survived the ravages of erosion and glaciation that have destroyed other Tertiary craters. However, the geological and geomorphological evidence that it is an impact structure is inconclusive; detailed examination is required to find out whether the outcrop of Chalk beside the River Roach is part of a true central uplift, or whether the concealed anticlines and synclines in the underlying rocks form part of a system of ring anticlines and synclines.
4
Counts of lunar craters suggest a rate of about 1 per lOMyr, implying that there should be 6-7 Tertiary craters of this size in the British Isles.
CHAPTER
Fuller's Earth and Bagshot Sands — A Surrey Crater? I dare meet Surrey in a wilderness, Richard II, IV, i, 74 An enormous hole had been made by the impact of the projectile, And the sand and gravel had been flung violently in every direction... The heather was on fire eastward, and a thin blue smoke rose against the dawn. H.G. Wells, The War of the Worlds T h e circular landforms described in t h e previous c h a p t e r s have all b e e n identified a s possible i m p a c t s t r u c t u r e s from their morphology, their circular s h a p e a n d t h e p r e s e n c e of a more or less well defined central uplift. Although geological evidence c a n be a d d u c e d for t h e impact interpretation of t h e s e s t r u c t u r e s , their ages c a n be only roughly estimated by interpolation between t h e y o u n g e s t r o c k s of t h e d i s t u r b a n c e a n d t h e oldest u n d i s t u r b e d r o c k s t h a t r e s t on it. The p r e s e n t c h a p t e r will deal with a different type of s t r u c t u r e , w h o s e p r e s e n c e is inferred from t h e p r e s e n c e of possible impact
H21
Fuller's Earth and Bagshot Sands — A Surrey Crater?
j|t||
ejecta interstratified with normal sedimentary rocks rather than from the presence of a circular landform or exposed uplift. On the other hand, the lithostratigraphic age of these ejecta, and of the impact structure itself, can be determined to within the width of a fossil zone. These ejecta are the Aptian (mid-Lower Cretaceous) fuller's earth outcrops of south-east England. These 'fuller's earths', which occur in the Lower Greensand formation, are pure montmorillonitic 1 clays that were formerly used for cleaning clothes. The English stratigraphy of the Aptian stage has been thoroughly studied, and corresponds to the Lower Greensand. From bottom to top the Aptian formations of southern England are the Atherfield Clay, Hythe Beds, Sandgate Beds, and Folkestone Beds (probably mostly Albian). The whole Aptian stage in England testifies to marine transgression, in contrast to evidence from other parts of the world of a fall in sea level. This marine transgression is probably the result of local tectonic subsidence. In fact the Aptian stage, and the fuller's earth zone in particular mark the opening of a strait through Bedfordshire and Buckinghamshire connecting the North Sea Basin and the Wessex-Weald basin. The fuller's earth beds occur in the Sandgate Beds, which belong to the nutfieldensis zone of the Upper Aptian; counting from oldest to youngest this is the sixth of the seven zones that form the Aptian stage. The fuller's earth clays have been identified as altered pyroclastic rocks, perhaps originally of trachytic composition. However, no actual volcanoes that could have erupted these pyroclastics have ever been identified; Anderton et al. (1979) speculate that these volcanoes lay west of Britain or even on the London Platform. This last suggestion sounds improbable, since the London Platform has been essentially stable since the beginning of Mesozoic time or even earlier, but it is not impossible. According to Harland (1982) the Aptian age lasted from 119 to 113Myr BP; however, later research has given different dates, of 124.5 to 112Myr and 121.0±1.4Myr to 112.2±l.lMyr. If the
^ontmorillon, in France, the presumed source of the mineral name 'montmorillonite,' is at 46°26'N, 0°50'E, 64km north of Rochechouart, the site of the largest impact structure in France.
EH
Part I: Impacts and Geology
seven stages of the Aptian are assumed to be the same length, the nutfieldensis zone, and the fuller's earth deposits, can be dated to about 114± 1 Myr. These Aptian fuller's earths are not restricted to England, but occur in other parts of the world, and it is plausible that the British and European deposits are related to volcanic activity associated with the opening of the North Atlantic Ocean and with block faulting in the Narrow Seas around the British Isles. The problem is to identify the volcanoes, and one way of doing this is to examine the distribution of their putative products. At present outcrops of Aptian fuller's earths occur in East Sussex west of the River Cuckmere (approximately TQ510100); in Kent between West Mailing and Yalding (west of Maidstone, at approximately TQ700550; in Surrey between Dorking and Reigate (several outcrops in the general area of TQ200500), and at Nuffield (about TQ310505) itself, 3 km east of Redhill; in a small Aptian outcrop south of Faringdon in southern Oxfordshire, at about SU290920; and in two outcrops in Bedfordshire, around Woburn (SP950325) and in the Clophill area (approximately TL080385). Fuller's earths do not appear to be present in the small Aptian outcrops east of Abingdon and between Melksham and Devizes. The total extent of the known outcrops is therefore about 120 km from north to south and 140 km from east to west, with the largest distance between outcrops being about 150 km. According to Anderton et at. (1979) the nutfieldensis zone is not present in either Norfolk or Devon, so it is not possible to say whether fuller's earths were ever deposited there. The Aptian stage is represented in the Yorkshire Wolds (Speeton Clay) and in the North Sea, but fuller's earths are absent. Fuller's earths also appear to be absent from the Lower Greensand and contemporary rocks of Dorset, Hampshire, the Isle of Wight, and the English Channel south of Beachy Head. It is not clear whether Aptian rocks are present in the Celtic Sea south of Ireland, but the apparent absence of fuller's earths from the western Aptian outcrops of Dorset and Hampshire casts doubt on the suggestion that the volcanic sources were to the west of Britain. It is interesting that the extant outcrops of fuller's earth are on or inside the circumference of a circle of radius ~75 km with its centre in north-west Surrey, in the Chertsey-Camberley area (very
Fuller's Earth and Bagshot Sands — A Surrey Crater?
iH
roughly SU950650, or about 51°23'±3'N, 0°40'±5'W). This centre is marked by the large outlier of Middle Eocene Bagshot Beds of west Surrey, north Hampshire, and south Berkshire; this outlier is roughly circular, and has a mean diameter of about 20-25 km. The apparent association between the deposits of fuller's earth and this isolated Eocene sedimentary basin suggests that the Bagshot Beds were deposited in a lake on the site of a Lower Cretaceous impact structure, and that the Aptian fuller's earths are the altered ejecta from this impact. According to Melosh (1989), an impact structure about 23-30 km in diameter would yield an ejecta thickness of about 0.1 m (10 cm) at r~75km. This estimated diameter is consistent with the observed diameter of the outcrop of the Bagshot Beds. It must also be remembered that the ejecta pattern from terrestrial impact structures is influenced by the interaction of the ejecta with the atmosphere. If this hypothetical Surrey impact structure is 20-30 km in diameter, the impact energy is 10-100 Gtons (4x 10 1 9 -4x 10 2 0 J), far in excess of the ~150-Mton energy required to blow off the atmosphere over the site of the impact, 2 It is therefore difficult to assess the likely distribution of impact ejecta. The Ries crater is similar in size, and this impact hurled tektites > 400 km from the crater, but no tektites are known to be associated with the English fuller's earths; from this point of view a volcanic interpretation may be preferable. On the world-wide scale, the Aptian age was a time of great changes in plants and animals. Flowering plants, which had appeared during the immediately preceding Barremian age, became abundant; and cycads, ginkgoes and bennettitales almost died out. The brachiosaurid dinosaurs died out, and the first placental mammals appeared. At about the same time as these extinctions occurred the eruptions of the Rajmahal Traps, in north-east India. These eruptions lasted for about 2Myr around 116± 1 Myr, in the middle of the Aptian age, and probably about 2Myr before the deposition of the English fuller's earths. Thus the
2
Melosh (1989, p. 212) points out that no known volcanic eruption, not even Krakatau or Tambora, has ever produced an atmospheric blow-out plume, and that this fact sets an upper limit of ~150Mtons on the energy of these eruptions.
I
Part I: Impacts and Geology
Aptian extinctions, like the greater extinction event 50Myr later at the end of the Cretaceous period, were accompanied by flood basalt eruptions. But the volcanic eruptions were not the only Earth-shaking events of the Aptian age. There are seven or eight known impact structures with ages probably between 110 and 120Myr, four of them in Europe. The largest of the seven terrestrial impact structures is the Carswell structure (D=39km, t= 115±10Myr) in Saskatchewan, 3 but even this is overshadowed by the great lunar crater Tycho (D=87km), which is thought to be of the same age. The existence of so many impact structures in this age range may be evidence of an impact cluster, which itself may have been part of the cause of the Aptian extinctions; it is at least consistent with the possible existence of another impact crater of the same age in Surrey. I have already mentioned that the previously separate marine basins of the North Sea and southern England were joined together at the same time (to within the observational errors) as the deposition of the fuller's earth, and that this strait is within 80 km of the hypothetical Surrey impact structure. Could this union of the two basins be related to the formation of the impact structure? It has been pointed out (Emanuel et at, 1995) that giant impacts can produce 'hypercanes' (super-hurricanes); the atmospheric blast wave from an impact is, meteorologically, a very strong wind. A rough estimate suggests that the wind speed from an impact of the required magnitude might be 1,500-5,000 mph (0.7-2.lkms" 1 !) Such a wind would obviously produce enormous waves (hundreds of metres high) in the surrounding seas, quite apart from any tsunami generated by impact-induced seismic waves; these waves might flood and destroy the isthmus separating the two basins. And so it seems that the war of the worlds came not only to England but to exactly that part of England that H.G. Wells chose for the beginning of the Martian invasion of Earth.
3
It may be more than coincidence that the Deep Bay crater, with an age of 100 ± 50Myr, is only 450 km from the Carswell structure.
Fuller's Earth and Bagshot Sands — A Surrey Crater?
ill BMW
This possible crater in Surrey is the only one known in Britain that is certainly of Mesozoic age. In the next chapter we shall go north to Scotland, to examine another Palaeozoic circular landform; if this is an impact structure it is the oldest, and probably the largest, known in the British Isles.
CHAPTER
Gabbro, Granite, and Grampians The earth hath bubbles, as the water has, And these are of them. Macbeth, I, ii, 79-80. Macbeth! Macbeth! Macbeth! Beware Macduff Macbeth, IV, I, 71 The Grampian Highlands, between the Highland Boundary Fault and the Great Glen (Gleann Mor), form the most mountainous part of the British Isles; there are seven mountains and five subsidiary tops higher than 4,000' (1,219 metres). These highlands consist of Caledonian granites and Dalradian metamorphic rocks. However, the north-eastern part of the region, in the former country of Aberdeenshire (now Grampian), is much less mountainous, even though it consists of the same highgrade metamorphic rocks as the mountain country to the west. Specifically, there are only two mountains (The Buck and Pressendye) higher than 2,000' (610 metres) east of longitude 3°W and north of the River Dee, although west of 3°W dozens of peaks rise above the 2,000' contour. Even more striking, only one mountain (the Hill of Fare) exceeds 1,000' (305 metres) east of longitude 2°30'W and north of the River Dee. This change in
Gabbro, Granite, and Grampians
M
topography is very marked to anybody travelling west from Aberdeen towards the Cairngorms, or east towards Aberdeen. Examination of a satellite photograph of the British Isles shows that these Aberdeenshire lowlands are encircled in the west by three concentric semi-circular mountain arcs, extending from about south-south-east to about north-north-west. These mountain arcs are fairly well defined, and they clearly cut across the large-scale structures of the Grampian Mountains. The arcs are centred on or near the granite mountain of Bennachie 1 (1,733'= 528 metres), which is located at NJ660225 (57°17'N, 02°34'W), west of Inverurie and about 31 km from Aberdeen. The inner of the three mountain arcs passes through Corrennie Moor (NJ620095), Coiliochbar Hill (503162, 1,747' = 532m), the Correen Hills (1,588' =484m), Wishach Hill (1,375' = 419 m), Hill of Foudland (NJ603332), and the Hill of Tillymorgan (NJ652347), east of the A96. This arc is about 24 km in diameter; it is centred on Bennachie. The middle ring includes the Hill of Fare (NJ672028), Lumphanan 2 (583042), Pressendye (490088), The Buck (412233), Black Hill (1,656' = 505m), Grumack Hill (1,723'=525m), and Muckle Long Hill (1,285'= 392 m), and so towards Huntly. This arc is about 42 km in diameter; it is centred slightly north of Bennachie. The outer arc is the best defined of the three. It starts near Aberdeen and passes through Kerloch (N0697879), Mulnabracks, Hill of Cat (N0484871), Cock Cairn, Broad Cairn, Morven (NJ377039), the Ladder Hills, Livet Water, Carn a Bhodaich, and so to Glen Fiddich, east of Glen Rinnes and Dufftown. The diameter of this arc is about 66 km (perhaps as much as 70 km); its centre is near to or slightly north of Bennachie. There may even be a fourth, outermost, mountain arc, consisting of the mountains between Glen Clova and Glen Esk in the :
It has been suggested that Bennachie is the original Mons Graupius ('crooked mountain'), where the Roman army of Agricola met the Picts under Calgacus in AD 81 (or AD 86). 2 Where Macbeth, the last Gaelic-speaking king of Scotland was killed, in AD 1057.
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Part I: Impacts and Geology
south, Lochnagar and the White Mounth, the Cairngorm Mountains, and the Hills of Cromdale in the north. The radius of this arc is about 55km, implying a diameter D~ 110km. These mountain arcs do not occur in north-east Aberdeenshire, north and east of Bennachie. However, the northern sector of the third arc should touch the Moray Firth coastline near Macduff; the north-eastern sector should pass through the Buchan district inland of Peterhead and Fraserburgh; and the eastern sector should follow the North Sea coastline north of Aberdeen. This circular or semi-circular structure, consisting of the lowlands of Aberdeenshire and the surrounding mountain arcs, will be called the East Grampian Basin throughout the rest of this chapter. Geological maps show that the NE-SW strike of the Dalradian metamorphic rocks of the southern Grampians changes north of the Lochnagar granite, on the outermost mountain arc, and that the average strike farther north is NNE-SSW. However, between Ballater and Huntly the strike of the Dalradian rocks is concave towards Bennachie; the radius of this concave arc is about 37 km. Thus the topographic arcs reflect the geological structure. However, the mountain arcs described above do not consist of the same rocks throughout their length; for example, the third (outer) arc consists of granite south of the River Dee and of Dalradian metamorphic rocks in the Ladder Hills. It should be mentioned that the Siluro-Devonian 'Newer Granites' of the Grampian Highlands are concentrated in the eastern Grampians, and are most numerous around the southern and western margins of the East Grampian Basin. Few granites occur north of Bennachie, except for the Strichen and Peterhead granites, 40-50 km to the north-east of the centre of the basin. At this radius these granites would be between the third and fourth rings, if the rings could be identified in this north-eastern sector. There is, however, a more remarkable and more significant feature of the geology of the East Grampian Basin, namely the fact that the mountain arcs encircle the so-called 'Younger Gabbros' of Aberdeenshire. These igneous intrusions outcrop over a distance of 75 km as seven separate masses: the Morven-Cabrach,
Gabbro, Granite, and Grampians
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Huntly, Insch, Haddo House, Maud, Arnage and Belhevie gabbros. The Insch gabbro, which is the largest of the seven, is near the centre of the basin, and lies 6 km north of Bennachie; it is 44 km long and has an area of about 250 km 2 . The Morven-Cabrach and Huntly masses are associated with the mountain arcs to the west; and the Maud, Haddo House, Arnage and Belhevie masses lie near the expected location of the east rim of the structure. These separate gabbro masses are regarded as parts of a single sheet that was intruded into the Dalradian metamorphic rocks and that has now been dismembered by tectonic deformation and erosion. The original area of this intrusive sheet was 600 square miles 3 (1,600 km 2 ), and its thickness was at least two miles (3 km) (Mercy, 1965). If these dimensions are accurate the original volume of the intrusion was 5,000 km 3 . It is so large that it might be described as a lopolith. It is of interest that the volume of the Younger Gabbros is larger than the volume of the Carboniferous Clyde Plateau Basalts (1,250 km3) and of the Tertiary Antrim Plateau Basalts (800 km 3 ). These gabbros are also much larger than the Tertiary gabbro intrusions of the Inner Hebrides. In spite of the large volume of the Younger Gabbro intrusions, there are no volcanic rocks associated with them; the focus of volcanic activity in the Dalradian succession is around Loch Awe, in the South-West Highlands. Moreover, geologists do not appear to have been able to account for these intrusions within the framework of the Caledonian orogeny as a whole. Geological books generally restrict themselves to describing the Younger Gabbros, or merely mentioning their existence, without attempting to give any geophysical reason for their presence in this part of the Grampian Highlands; this fact suggests that the gabbros are seen as geologically anomalous. The gabbros are about 490 Myr old. They were intruded after the main metamorphism of the Dalradian rocks, between about 480 and 510 Myr ago, but they have suffered tectonic dislocation and are themselves intruded by granites. In addition to the Younger Gabbros, which outcrop inside the mountain arcs of the East Grampian basin and were intruded 3
This is 0.6% of the total area of the United Kingdom!
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Part I: Impacts and Geology
after the metamorphism of the Dalradian rocks, there are metamorphic 'Older Gabbros', which occur around the western rim of the structure and have been altered to epidiorite or hornblendeschist (amphibolite). These rocks extend from Portsoy, on the Moray Firth, to the Morven-Cabrach mass of the Younger Gabbros. The 'Younger Gabbros' are layered basic intrusions, and consist predominantly of norite, a plutonic rock in which orthopyroxenes (particularly hypersthene) dominate over clinopyroxenes. Other rocks found in these intrusive masses include peridotite, picrite, troctolite, olivine-gabbro, syenogabbro, and quartz-gabbro; in addition, anorthosite occurs at Portsoy, in association with the 'Older Gabbros'. The intrusive sheet is therefore differentiated. Hatch, Wells and Wells (1965) suggests that norites are produced through the assimilation of argillaceous (clayey) country rocks by a gabbroic magma. This interpretation is confirmed for the Younger Gabbros by the presence between the norite and the Dalradian metamorphic rocks of a wide zone of contaminated norite, which contains large numbers of metamorphic xenoliths in the process of assimilation. In many respects the Younger Gabbros resemble lopoliths such as the Bushveld Complex in South Africa and the Sudbury Igneous Complex of Ontario. The estimated volume of the Younger Gabbros is about 60% of the volume of the Sudbury Igneous Complex (>8,000km 3 ). The forms of the intrusions are similar; the same rocks (norites and ultrabasic rocks) occur in all three complexes; and there is the same assimilation of the country rock by the norite magma. However, there are also differences. The Bushveld and Sudbury complexes contain granophyres and ignimbrites (the Rooiberg4 Felsite of the Bushveld and the Onaping Formation of Sudbury), which are not present in the Younger Gabbros. Also, the larger igneous complexes are rich in metal ores: the Bushveld is the main source of chromium and the platinum metals; and Sudbury is the Earth's main source of nickel and an important source of copper and the platinum metals. These metals are not found in the Younger Gabbros, although 4
'Red Mountain', in Afrikaans. The Rooiberg Felsite has an estimated volume of 3xl0 5 km 3 (Grieve and Cintala, 1992}.
Gabbro, Granite, and Grampians
II
there is a record of chromite in serpentine in Aberdeenshire (Wilson et ah, 1946, cited by Duff, 1965). However, it is interesting that chromite has been mined from the ultrabasic rocks and serpentines of Unst, in Shetland, and that iron and copper are also present in the Shetland Islands (Duff, 1965). The resemblances between the Younger Gabbros of Scotland and the Sudbury and Bushveld Complexes are significant, because Sudbury is now regarded as an established impact structure (Dietz, 1964; Grieve et at, 1991), and the Bushveld is also considered to be a possible impact structure (Dietz, 1962; Hamilton, 1970; Rhodes, 1975). The 'igneous' rocks of Sudbury have been identified as impact melt and fallback breccia (Grieve, Stoffler, and Deutsch, 1991), and the same hypothesis may be applicable to the rocks of the Bushveld (Grieve and Cintala, 1992). The East Grampian Basin can also be identified in the map of the metamorphic zones of the Highland; it corresponds to the region of Buchan-type metamorphism, which is characterised by the minerals chlorite, 5 andalusite (ALjOSiOJ, cordierite (Mg2Al4Si5018), and sillimanite (Al2OSi04). The highest grade of metamorphism (the sillimanite) is found in the encircling mountain arcs; the cordierite and andalusite zones are found in the interior of the structure. In comparison with the Barrovian metamorphism found elsewhere in the Grampian Highlands, the Buchan-type metamorphism is produced by high temperatures (500-700°C) but low pressure (<5kbar=0.5GPa, corresponding to depths of about 18 km 6 ). Thus the Buchan metamorphism was probably associated with tectonic uplift and exhumation of the Dalradian rocks in a rising nappe (Johnson, 1965). The high-temperature metamorphism may have been caused by the Younger Gabbro igneous intrusions. To express this slightly differently, the Buchan area appears to have been the site of a positive thermal anomaly (a 'hot spot') during the Caledonian orogeny.
5
From the Greek chloros - 'green,' not from the element chlorine. The chlorites are common metamorphic minerals, characteristic of the greenschist facies. ^ h i s estimated depth gives an idea of the extent of post-orogenic, or postimpact, erosion.
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Part I: Impacts and Geology
The Buchan-type metamorphism is believed to be post-tectonic (that is, it is later than the folding of the Dalradian rocks), and to be rather older than the intrusion of the Younger Gabbros. Although the Buchan mineral assemblages 'probably occur at a high structural level' (Johnson, 1965), the Buchan area is now part of the Boyndie Syncline, which is occupied by Upper Dalradian rocks, with Lower Dalradian rocks to the east and west (Mercy, 1965). All these phenomena may be explicable by the hypothesis that the East Grampian Basin is an impact structure with D~ 110 km, and that the Younger and Older Gabbros, instead of being igneous rocks, are the remnants of the impact melt sheet. The structure would therefore be directly comparable to, although much smaller than, Sudbury (D=200km) and the Bushveld Complex (D ~ 500 km). The circular shape of the East Grampian Basin and its lack of relation to the trends of the Grampian Mountains are obviously consistent with its being an impact structure. The resemblances between the Younger Gabbros and the Sudbury and Bushveld complexes are easily understood on the impact hypothesis. The absence of volcanic activity associated with the Younger Gabbros is also understandable if the gabbros are actually part of an impact melt sheet. In particular, the assimilation of the country rocks observed in all three complexes is a characteristic feature of impact melts because they are superheated, that is they are at temperatures well above the melting point of rocks. Igneous magmas are at temperatures near to their melting point, and cannot assimilate large volumes of country rocks without solidifying. The presence of xenoliths of country rock at the base of the 'intrusion' is also characteristic of impact melt sheets (Grieve, 1987; French, 1998, particularly Figs. 6.9 and 6.10, pp. 86-7). However, the estimated volume of the Younger Gabbros (5,000 km3) seems too large for an impact structure of the size of the East Grampian basin (D~ 110 km). Specifically, its volume is much larger than the volumes of impact melt in the Manicouagan (1,200 km3) and Popigai (1,750 km3) impact structures, although both have diameters of about 100 km and are
Fig. 12. Topographic map of the East Grampian Basin. Scale ~1:480,000. Reproduced by permission of the Ordnance Survey.
he East Grampian Basin. ed by permission of the Ordnance Survey and the British Geological Survey.
Fig. 13. Geological map of the East Grampian Basin. Scale ~1:700,000. Reproduced by permission of the Ordnance Survey and the British Geological Survey.
Gabbro, Granite, and Grampians
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therefore nearly the same size as the East Grampian basin. Moreover, according to Grieve and Cintala (1992), the volume of impact melt (V™) is related to the diameter of the crater by the relation Vm°cDtc3-85, where Dtc is the diameter of the transient cavity. However, the volume of the Younger Gabbros is 60% of the volume of the Sudbury Igneous Complex, although, according to the relation of Grieve and Cintala, it should have only 10% of the volume of the Sudbury Complex. French (1998) adopts a slightly different formula for the volume of impact melt, namely Vm = 0.0004XD 3 - 4 . According to this formula, for the East Grampian Basin (D= 110km) Vm = 3,500km 3 , and Vm for Sudbury (D~200km) is about 27,000 km 3 . The unexpectedly small observed volume of the melt sheet at Sudbury may be due to erosion of the outer parts of the sheet. The thickness of the Younger Gabbros (3 km) also seems excessive for the size of the East Grampian Basin; according to French (1998) the impact melts sheets of West Clearwater Lake (D=32km) and Popigai (D= 100 km) are hundreds rather than thousands of metres thick. Indeed the Younger Gabbros appear to be thicker than the Sudbury Igneous Complex (2.5 km). This discrepancy suggested that the thickness of the Younger Gabbros has been over-estimated, or at least that the quoted thickness is a maximum value rather than an average over the whole gabbro sheet. If the average thickness is in the region of 1-2 km, the volume of the Younger Gabbros would be about 1,600-3,200km 3 , reasonably consistent with the expected volume of impact melt for a structure of the size of the East Grampian Basin. The thermal anomaly and the Buchan-type metamorphism associated with the East Grampian Basin may be related to heating of the underlying rock by the impact melt sheet, and also to impact-induced uplift of hot rock from the lower continental crust. The high structural level of the Buchan metamorphism may be evidence for this impact-induced uplift; and the fact that the metamorphism followed the tectonic folding may imply that it is related to the impact rather than to the earlier orogenic activity. Even the fact that the Buchan region is now part of the Boyndie Syncline may be related to the impact. Very large impact
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Part I: Impacts and Geology
structures, such as Popigai, Vredefort and Sudbury, tend to have central depressions, often surrounded by an uplifted ring (peakring structures), rather than central peaks. These central depressions may be formed through impact-induced melting of the floor of the crater, and consequent collapse of the central peak. However, in the East Grampian Basin it is difficult to distinguish morphologies and structures that may be related to the hypothetical impact from those produced by Caledonian tectonic activity. Nevertheless, it is possible to understand the distribution of the Younger Gabbros on the hypothesis that the East Grampian Basin is an impact structure. On this interpretation the Insch gabbro mass is the remnant of the impact melt body occupying the central depression, whereas the other six masses originally occupied a 'ring syncline' outside the encircling ring of peaks. The absence of granophyres and 'ignimbrites' (fallback breccias) from the East Grampian Basin may be the result of erosion of the upper levels of the impact melt sheet. In addition, granophyres may be absent because the impact melt sheet was not thick enough to produce such rocks by differentiation. If so, the absence of granophyres is consistent with the view that the volume of the impact melt sheet is, as expected, less than that of the impact melt sheets of Sudbury and Vredefort. There is another way of investigating the origin of the Younger Gabbros, by means of their isotopic ratios. If these gabbros are actually impact melt rock they were produced by the melting of the rocks of the continental crust. If they are true igneous rocks they are the products of partial melting of the upper mantle and of intrusion of the resulting magma into the crust. Briefly, the initial strontium isotopic ratio 8 7 Sr/ 8 6 Sr (ft,) in mantle melts cannot exceed 0.704, whereas rocks derived from melting of crustal rocks can have much higher initial values of 8 7 Sr/ 8 6 Sr (Brown and Musset, 1981). For example, the average value of ft/ in the rocks of the Sudbury Igneous Complex is 0.707, and the ratio ranges upwards to 0.7175. These ratios prove that the rocks of Sudbury were produced by melting of the Earth's crust, rather than being derived from the mantle. In addition, the neodymium isotope ratio 143 Nd/ 144 Nd can be used to measure the length of time that a rock has been part of the continental crust. The
Gabbro, Granite, and Grampians
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'crustal residence times' (or Nd model ages) for the rocks of Sudbury are between 2.5 and 2.9 Gyr; these ages are much larger than the age of the Sudbury structure itself (1.85 Gyr), and imply that the rocks of the Sudbury Igneous Complex were produced by melting of the Archaean country rock. The same principles can be applied to the Younger Gabbros of the East Grampian Basin. If the initial 8 7 Sr/ 8 6 Sr ratios (Rj\ for these rocks are < 0.704, they are igneous rocks derived from the Earth's mantle; if f?/> 0.704, the rocks were produced by the melting of crustal rocks. Likewise, if the rocks are mantle magmas, their Nd model age should be about 490 Myr, the age of their intrusion. If the rocks are impact melts, their Nd model ages should be the ages of the source rocks from which the Dalradian sediments were derived. This age is quite uncertain, but it is probably > 800 Myr. Thus these isotopic measurements are capable of distinguishing between ordinary igneous rocks and impact melt rocks. If the East Grampian basin is an impact structure its location in an active orogenic belt is unusual, although the discovery of lake Kara-Kul in the mountains of Tajikistan shows that such things do happen. Tectonic uplift of the Grampian Mountains after the formation of the impact structure has resulted in unusually deep erosion. The sillimanite gneiss that occurs in the south-west of the Buchan metamorphic region and of the East Grampian Basin may have formed at depths of 18-24 km; thus since Early Ordovician time this thickness of overlying rock has been removed by erosion to expose this gneiss. (The implied erosion rate is 1 metre/20-27 kyr, or 37-50 m/Myr; this rate is in fair agreement with the sedimentation rates derived in the chapter on the Midlands structure.) It is thus understandable that most of the impact melt sheet has been destroyed. However, the floor of the impact melt sheet, in geological terms the contact between the Younger Gabbros and the Dalradian metamorphic rocks, survives in places; and evidence of impact might be obtained from these 'floor rocks'. By analogy with Sudbury, shatter cones should be present to a distance of about 30 km from the centre of the structure, that is, both north and south of the Insch gabbro mass. Moreover, Dalradian xenoliths or clasts in the base of the Insch gabbro (or norite) mass may show evidence of the planar deformation features that are characteristic of shock
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Part I: Impacts and Geology
metamorphosed rocks; however, such deformation features may have been annealed by the heat of the overlying molten rock (Grieve and Cintala, 1992). It has already been stated that the Younger Gabbros are about 490 Myr old, and this may be taken as the age of the East Grampian Basin. This age is rather less than that of the main deformation and metamorphism of the Dalradian rocks (514-480 Myr), and is consistent with the geological evidence that the gabbros and the Buchan metamorphism are post-tectonic. However, the close coincidence between the ages of the East Grampian basin and the Caledonian orogeny is interesting in the light of suggestions that episodes of tectonic activity occur at intervals of 26-30 Myr, and that they are related to episodes of bombardment of the Earth by comets and asteroids (Clube and Napier, 1990). The age of the Younger Gabbros places them stratigraphically near to the Cambrian-Ordovician boundary. It is thus of interest that three extinction events, called biomere boundaries, are recorded during the Upper Cambrian epoch; one of them marks the Cambrian-Ordovician boundary itself (Palmer, 1982). The age of the Younger Gabbros thus suggests that the impact that supposedly produced the East Grampian Basin was responsible for one of these boundary events. However, the stratigraphy of the Dalradian and the Cambro-Ordovician time scale are not yet certain enough for the East Grampian Basin to be reliably connected with a particular stratigraphic level. If, as has been suggested, the uppermost Dalradian rocks are Lower Ordovician, the East Grampian Basin, if it is an impact structure, must also be of Ordovician, rather than Cambrian, age. It should also be noted that even if one of these three biomere boundary events was caused by the East Grampian impact, we still have to look for two large Upper Cambrian impact structures to explain the other two! If the East Grampian Basin is confirmed to be an impact structure, it will be the largest Phanerozoic impact structure in Europe, and indeed one of the largest in the world. It may be difficult to believe that such a large impact structure could escape the attention of geologists in a country like Scotland; one might as well believe that a breeding colony of an unknown species of large animal could live undiscovered in Loch Ness! However, the
Gabbro, Granite, and Grampians
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age and location of the East Grampian structure mean that it has suffered both erosion and tectonic deformation, so that its form is no longer recognisable. Indeed the sheer size of the East Grampian basin may have hindered its recognition; it is simply too large to be seen as a whole by geological study, and its discovery had to await the advent of photography from satellites. Moreover, the East Grampian Basin does not show the typical geological features of smaller impact structures, namely the central uplift and the concentric outcrops of outward-dipping rocks; instead it appeared to be a layered basic igneous complex. It has only recently come to be recognised that impact melts can mimic conventional igneous rocks, and that supposed igneous complexes may actually be impact structures. Indeed, it should be understood that effectively all impact structures with D3= 100 km will contain large bodies of apparently igneous rocks, and therefore the scarcity of known impact structures in this size range may be due more to their having been mistaken for igneous complexes rather than to the actual absence of such structures. This will be particularly true in parts of the world that are in only the early stages of geological exploration. It seems, at least, that the supposition that most of the Earth's really large impact structures have already been discovered is likely to be very wide of the mark. If its estimated diameter of 110 km is correct, the East Grampian basin occupies nearly 4% of the area of the United Kingdom, and a full 12% of the area of Scotland. This is much larger than the percentage areal coverage of 0.1-0.2% that was predicted in Chapter 2. This consideration again implies that giant impact structures may be more abundant than has been supposed, and that terrestrial impact rates have been underestimated. These last eight chapters have described and examined eight structures in Great Britain, ranging in diameter from 4 to 110 km, whose geology provides some evidence of impact. The next chapters will look at a number of circular landforms of obscure origin where there is less geological evidence available.
CHAPTER
Other Circular Structures All things from eternity are of like forms And come round in a circle Marcus Aurelius, Meditations Thou hast spoken right. Tis true. The wheel is come full circle; King Lear, V, iii, 175-6. Besides the circular structures described in the last eight chapters, for which there is some geological evidence for impact, there are also a number of circular or arcuate structures in the British Isles that are of obscure origin. These structures will be listed and briefly described in this chapter, but there is no necessary implication that any of them is of impact origin. These circular or arcuate structures have been identified from a satellite photograph or from maps; the geological evidence bearing on their origin is equivocal, but none of them show the characteristic circular inlier with surrounding concentric rings of younger rocks. The first of these arcuate structures is the great bay in the south coast between Coverack (Cornwall) and Bolt Head. This
Other Circular Structures bay defines an arc of a circle whose centre is located in the neighbourhood of SX280020 (or about 49°50'N, 04°23'W), about 58km east of Lizard Point. The diameter of this circle is about 110 km. Geologically this bay forms part of the Western Approaches Basin, a sedimentary basin that contains Tertiary rocks overlying Palaeozoic rocks of the Hercynian mobile belt. It is encircled to the north by the Devonian grits, sandstones and slates of South Devon and the Cornish Peninsula, and by the metamorphic rocks of Start Point and the Lizard Complex. There is no concentric arrangement of sedimentary outcrops around the bay. A particularly interesting feature of the bay is an outcrop of garnetiferous gneiss in the Eddystone Rock, south of Plymouth. This rock, which has been a notorious destroyer of ships and lighthouses, probably belongs to the east-west gneiss belt that forms Start Point and Bolt Head in Devon. To the east of this 'Eddystone Bay' is the second arcuate structure, namely Lyme Bay. The margin of this bay, from Brixham to Chesil Bank and the Isle of Portland, forms an arc of a circle centred near SY300530 (about 50°22'N, 02°58'W). The diameter of this circle is about 75-80 km. The coast of Lyme Bay forms one of the classical geological areas of England, but the structures on land are not related to the bay itself. In his book Bombarded Earth, Gallant (1964) identified twelve examples of arcuate or circular stretches of coastline, including some as far afield as Antarctica and Patagonia. It is strange that he should have overlooked these two examples in south-west England. The next circular structure forms the western part of the Hampshire Basin; it lies east-north-east of Lyme Bay and is mostly on land. Its rim is defined by the margin of the Chalk outcrop from Wimborne Minster (Dorset), through Cranborne, Fordingbridge, Redlynch, Whiteparish, Romsey, Eastleigh, and the west end of Portsdown Hill. The centre of this circle is near SZ330920 (about 50°44'N, 01°30'W), either in south-west Hampshire or in the western Solent between Lymington and Yarmouth. The diameter of the circle is about 60 km. It will be seen that the basin should include the Isle of Wight, but its circular shape is marred by the east-west Isle of Wight Monocline.
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Part I: Impacts and Geology
Geologically the Hampshire Basin is a deep structural basin, containing Palaeogene sedimentary rocks overlying and surrounded by Chalk. This is exactly the opposite pattern to that of a complex impact structure, but it is what is expected of a tectonic basin. It is certain that these three basins, 'Eddystone Bay,' Lyme Bay, and the Hampshire Basin, are not impact structures. For one thing their diameters are much larger than those predicted in Chapter 2 for British impact structures. Moreover, the geology of these basins is probably inconsistent with an impact origin. However, they possess some interesting features, which may shed some light on their origin. First, the basins form a chain, striking ENE-WSW and decreasing regularly in size from west to east. Second, the granite intrusions of the Southwest Peninsula, from Carnmenellis to Dartmoor, form an arc concentric with the coast of 'Eddystone Bay'. Third, the eastern margin of the Dartmoor granite intrusion is concentric with the western shore of Lyme Bay. These facts suggest that the basins have some connection with the Hercynian orogeny. If so, one might venture to predict the existence of Hercynian granites beneath the Mesozoic cover in Dorset, north of Lyme Bay, and in Hampshire and Wiltshire north of the Hampshire Basin. One interpretation of the Hercynian orogeny in southern England and South Wales is that the entire mobile belt has been thrust northwards along a decollement surface from a destructive plate margin to the south (perhaps in Brittany). It may be tentatively suggested that the granites of Land's End, the Seven Stones Reef (about SW050230), and the Scilly Islands form part of the circumference of a fourth circular bay, west of Lizard Point. However, there are not enough identified points of the circumference of this supposed bay for either its centre or its diameter to be measured accurately. It must be emphasised that this part of the Hercynian mobile belt is still poorly understood; and a study of these three basins may provide information bearing on its tectonic history. Another lesson to be learnt from these three basins is that circular arcs can be produced by tectonic processes; they are not solely consequences of impacts! The Isles of Scilly themselves present some interesting features. It may be seen from a map that the outer islands of Gugh,
Other Circular Structures
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St. Agnes, Annet, Samson, Biyher, Tresco, St. Martin's and the Eastern Isles form a semi-circular arc partly surrounding the central island of St. Mary's. Thus St. Mary's forms, as it were, a central peak, with the outer islands forming an encircling northwestern rim. The sea passages of St. Mary's Sound, The Road, Crow Bar and Crow Sound might then be interpreted as the north-western sector of a ring syncline. The centre of the semicircle formed by the outer islands is near Grid reference SV918115 (49°55.4'N, 06°17.7'W), near Holy Vale on St. Mary's. The diameter of the 'central peak' is 2.5 km (north-east to southwest) by 3 k m (north-west to south-east). The radius of the 'ring syncline' formed by the inner arc of the outer islands is 3-3.5 km. The radius of the 'outer rim formed by the outer arc of the outer islands is about 6 km; thus the implied diameter of this 'outer rim' is about 12 km. The ratio of the diameter of the 'central peak' to that of the outer rim is then 0.21-0.25, similar to the relative diameter (-0.22) of the central peak in complex impact structures of the same size. However, it would be extremely speculative to interpret the Isles of Scilly as part of an impact structure. The only evidence for impact is morphological, the semi-circular shape and the diameter ratio between the 'central peak' and the supposed outer rim. The absence of the south-eastern sector of the circle is a serious difficulty for an impact interpretation. The circular shape of the island group may be of purely geological origin, perhaps related to concentric zoning of the granite that composes the islands, and variations in its susceptibility to erosion. However, if the islands do mark the site of an impact structure, it is curious that such structures should have been identified in both the most north-easterly and the most south-westerly island groups of Great Britain. Brookesmith (1991) presents a submarine contour map of the Scilly Islands, which shows that the -20-metre contour forms a roughly semi-circular island with a diameter of about 11 km, with a peninsula extending to the south-west, towards Bishop Rock. This landform is consistent with the Scilly Islands being the surface manifestation of a roughly circular granite intrusion rather than an impact structure. The sharp cut-off to the southeast is probably due to a north-east to south-west fault (i.e. the 'Caledonian' trend) from the Eastern Isles to St. Agnes; this could be either a normal fault or a transcurrent fault.
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Part I: Impacts and Geology
Farther north, Wales provides two examples of arcuate or circular structures. The first is St. Brides Bay, in Dyfed (formerly Pembrokeshire). The centre of the bay is located near SM785170 (about 51°50'N, 05°13'W), and the bay is 15-16km in diameter. The geology of the bay is complex, and consists of Precambrian rocks, Cambrian, Ordovician and Silurian rocks (including the Skomer Volcanic Series), Old Red Sandstone, Millstone Grit, and Coal Measures. The Precambrian and Lower Palaeozoic rocks form the promontories of St. David's Head to the north and the Marloes Peninsula to the south, with the less resistant Coal Measures forming the lower ground east of St. Brides Bay. The bay is therefore regarded as an erosional embayment, carved out of the Coal Measures (Anderson and Owen, 1980). The second Welsh structure is in northern Powys, and is both larger and more puzzling than St. Brides Bay. It can be seen well enough on satellite photographs, and can be identified on a map if one knows where to look. It forms the relatively low ground of the Vale of Powys, south of the Berwyn Mountains. The course of the River Severn between Carsws and Welshpool (y-Trallwing) defines its southern margin, and Llanfyllin is near its north-eastern margin. The course of the River Vyrnwy below Dolanog defines a sort of inner ring, and the centre of the structure is near Llanfair Cereinion (SJ105060, or 52°39'N, 03°20'W). The entire circle is 25-30 km in diameter. This area is less mountainous than most of Wales. It is mostly <400 metres above sea level. In contrast, Clun Forest, to the south, is at 400-600 m above sea level; Plynlimmon, to the south-west, reaches 752 m; and the Berwyn Mountains, to the north and north-west, reach 827 m in Moel Sych. This circular structure consists chiefly of Silurian sedimentary rocks, although Upper Ordovician rocks occur in the north and east. Whether from lack of exploration or for some other reason, the Caledonian tectonic structures of Wales appear to die out at the margin of this circle, leaving an apparently undeformed area inside it (see Bennison and Wright, 1969, p. 153, and Anderson and Owen, 1980, p. 76). The origin of this circular structure remains obscure. The apparent absence of Caledonian tectonic deformation suggests that it marks an area that is underlain by rigid basement, perhaps
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a Precambrian massif like the Longmynd or Charnwood Forest. No more can be said in the absence of detailed studies of the structure. Another circular depression has been identified in the north of Hertfordshire, partly enclosed by the chalk hills of Letchworth and Offley. This large basin, about 8.5 km in diameter, in turn surrounds a group of hills (the 'central uplift'), which form the site of the small market town of Hitchin. The basin is open to the north of Hitchin and Ickleford, between Letchworth and Pirton, where it is drained by the River Hiz. The central uplift is mostly clearly defined by the rivers and dry valleys that surround Hitchin; its diameter is about 2.5 km, or about 30% of the diameter of the basin. The centre of the structure defined by the diameter of the central uplift is in Hitchin Town Centre, probably between the parish church (St. Mary's) and Windmill Hill on the east side of the town and within a few hundred metres of the OS Grid Reference TL187291. The geographical co-ordinates of the centre are 51°56.7'N, 00°16.5'W. An interesting feature of the basin is the presence of a partial ring of low hills between the rim of the basin and the central uplift. These hills include Halfway Hill in the south-west (midway between Hitchin and Offley), the hills south of Little Wymondley in the south, some hills west of Graveley in the east, and probably the Highover Hills between Hitchin and Letchworth in the north-east. This ring of hills has a diameter of about 5.5 km, about 65% of the diameter of the basin; it is concentric with the outer rim and with the 'central uplift', and may represent a ring anticline. If so, its relative diameter is in good agreement with those of the ring anticlines of the Richat structure (about 72%) and Wells Creek (70%). Morphologically, then, this 'Hitchin Basin' bears a close resemblance to a complex impact structure with a central uplift and ring anticline. However, the geological evidence contradicts this interpretation. Specifically, most of the 'central uplift', particularly Windmill Hill and the northern part of Hitchin, consists of glacial deposits rather than Chalk. These deposits occupy a deep north-south glacial channel carved through the Chalk. Moreover, the altitudes of the mapped geological boundaries between the Lower, Middle and Upper Chalk are essentially constant, and
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show no sign of structural uplift either inside the Hitchin basin or in the 'central uplift'. The predicted uplift for an impact structure with D = 8 - 9 k m is SU= 600 ± 3 0 metres; such an uplift would probably bring Palaeozoic rocks to the surface. (It may be added, though, that the uplift in the Decturville (Missouri) impact structure (D=6km) is only 160 metres, <40% of the predicted uplift. The impact hypothesis for the Hitchin Basin might be saved by two assumptions, namely that the basin is deeply eroded and that its present level of exposure is almost at the lower limit of impact-induced deformation, and that the structural uplift and the depth of deformation have been reduced by the atmospheric break-up of the impactor. This hypothesis has already been invoked for the Midlands structure and the Rochford Basin. Normally impact-induced deformation extends to a depth of about 20% of the diameter of the structure, i.e. to d~ 1.7 km for the Hitchin basin. However, the depth of deformation is much less in impact structures formed by the flattened 'meteorite swarms' produced by atmospheric break-up. If we suppose that the Hitchin Basin is a genuine impact structure, it can be dated only as pre-glacial (>0.5Myr) and postCretaceous (<65Myr). However, Hitchin is only 80 km from the Rochford Basin, and it therefore possible that the two basins were formed by a double impact. If so, the same reasoning that was used for the Rochford Basin implies that the Hitchin Basin was formed during the Late Oligocene or Early Miocene epochs, about 20-25 Myr ago. Nevertheless, the radical changes to the local geomorphology brought about by glacial erosion and deposition imply that it is equally likely that the entire basin is the product of Pleistocene glaciation and that the morphological resemblance to a complex impact structure is only coincidence. Still in England, but farther north, another circular structure can be identified in the Vale of Pickering, south of the North York Moors. This appears as a complete circle on satellite photographs. Its northern rim is formed by the arc of the North York Moors through Pickering, Kirkbymoorside, and Helmsley. The southern rim is formed by the Howardian Hills as far as Malton and Norton. There is no eastern rim; to the east the basin is open to the valley of the upper River Derwent.
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The basin formed by the Vale of Pickering is about 20 km in diameter. Its centre is near TA700780 (54°11'N, 00°55'W), about 3 km east of Nunnington. The geology and morphology of the Vale of Pickering are extremely interesting. The Vale itself consists of Upper Jurassic Kimmeridge Clay under a thick cover of glacial deposits; the surrounding hills consist of Middle Jurassic deltaic rocks, contemporary with the oolitic limestones of southern England and the Midlands. The Vale of Pickering is thus a structural basin or trough as well as a topographic depression. South of Helmsley the Gilling Gap forms a fault trough, with throws of up to 300 metres. The drainage pattern of the basin is centripetal, with the rivers that flow from the North York Moors converging towards the centre of the basin. The main river of the basin is, however, the Derwent, which rises near Scarborough and Filey and then flows westwards, away from the North Sea, into the Vale of Pickering. This unusual behaviour is a consequence of the Pleistocene Ice Age. During the most recent (Devensian) glacial stage, the eastward valley from the Vale of Pickering to the sea was blocked by the North Sea ice sheet, and the water draining from the North York Moors and from the ice sheet itself was trapped between the ice and the hills surrounding the Vale to form Glacial Lake Pickering. The trapped water eventually escaped through Kirkham Gorge, south of Malton, between the Howardian Hills and the Yorkshire Wolds. It is not clear whether the circular shape of the Vale of Pickering seen on the satellite photograph is simply illusory, or whether it is entirely a consequence of the Pleistocene history of Glacial Lake Pickering. Again, the tectonic history of the Pickering Trough does not appear to be entirely understood. The most northerly 'circular' structure in England is a complex arcuate structure in the northern Pennines, between the River Tyne and the River Wear. This structure consists of concentric ridges and river valleys centred on Blanchland Moor (NY946526, or about 54°51'N, 02°05'W), in Northumberland. Blanchland Moor is located 4.8 km west of Derwent Reservoir. The outer arc of this structure is defined by the River West Allen north of Coalcleugh (NY803450) and by the valley of the upper River Wear from Cornriggs (NY847414) to Stanhope
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(NY992392). The hills east of the River West Allen and north of the River Wear, from Carr Shield (820483) through Middlehope Moor (869430) and Stanhope Common (957419) to Collier Law (NZ017417) define a concentric arc inside this inner arc. The valley of the River East Allen, from Allendale Town (NY835558) to Allenheads (860454), and the valleys eastward through Rookhope (940427) and Waskerley Reservoir (NZ025442) form a third arc, which itself encloses the arc of hills between Hexhamshire Common (NY863552), Allendale Common (870505), Bolt's Law (950455), and Muggleswick Common (NZ018483). Finally Devil's Water and the River Derwent west of the Derwent Reservoir form an innermost valley arc west and south of Blanchland Moor. These topographic arcs cannot be identified north and east of Blanchland Moor, but the radius of the outer (West AllenWeardale) arc to the south and west is 15-16 km. However, the hills north of Alston, such as Whitfield Moor (NY733539), and the hills south of the River Wear, from Burnhope Seat (NY788375) to Pawlaw Pike (NZ009322), may form an outermost arc, with a radius of about 21 km. Geologically, this is mostly Millstone Grit country. However, Carboniferous Limestone is exposed in the valleys of the Rivers East Allen and West Allen and the River Wear; and Blanchland Moor itself consists of Coal Measures. North of the River Tyne the outer (Weardale-West Allen) arc is continued by the contact between Millstone Grit and Carboniferous Limestone from Haydon Bridge (west of Hexham), Wall (Hadrian's, of course), and Great Whittington. It is interesting that this arcuate structure lies inside the outcrop of the Great Whin Sill, the largest dolerite intrusion in Britain. This North Pennine arcuate structure may be related to the Lower Devonian Weardale Granite. Geological activity appears to have focused on this area for a long time; the Upper Carboniferous Whin Sill (about 301 Myr) is lOOMyr younger than the Weardale Granite, and the northern Pennines are noted for their Carboniferous mineral deposits. The ninth of these arcuate structures, the North Down structure, is also one of only two in Ireland. Its eastern margins are defined by the east and west shores of Strangford Lough and the Ards Peninsula, which form three concentric arcs. The centre of
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these arcs is located near Saintfield (54°28'N, 05°50'W), between Belfast and Downpatrick. The radius of curvature of the western shore of Strangford Lough, which forms the inner of the three arcs, is about 12.5 km; that of the eastern shore of the lough is about 18 km; and that of the east coast of the Ards Peninsula, which forms the outer arc, is about 25 km. Thus the radius of the outer arc is about twice the radius of the inner arc. This is the ratio found in peak-ring impact structures between the radius of the outer rim and the radius of the central uplift (Melosh, 1989). Interpreted as components of an impact structure, Strangford Lough would be an arc of the ring syncline, and the Ards Peninsula would be an arc of the encircling ring anticline or outer rim. It is, however, more difficult to identify arcuate structures west of Strangford Lough. The shores of Belfast Lough to the north and of Dundrum Bay to the south may constitute part of the outer ring. Likewise, the lowlands of Dundonald (between Belfast and Newtonards) and the valley of the River Lagan between Belfast and Lisburn may constitute the northern part of the inner ring, and the granite hills of Slieve Croob (the source of the River Lagan) may form its south-eastern part; but these are only points on an otherwise poorly defined circumference. Geologically most of North Down and the Ards Peninsula consist of Lower Silurian (Llandovery) sedimentary rocks, with small inliers of Upper Ordovician rocks. However, Strangford Lough itself, the Dundonald lowlands, and the valley of the River Lagan are underlain by Permo-Triassic sandstones, and the hills west and north of Belfast belong to the Tertiary basalt plateau of Antrim. According to Anderson and Owen (1980) the PermoTriassic rocks of Strangford Lough and the Lagan valley occupy a Tertiary tectonic basin or trough. This Strangford Lough trough is probably of the same age as the Loch Ryan basin in south-west Scotland, and the Penrith trough and Solway Basin in northwest England. All these tectonic depressions were probably produced by crustal extension and thinning related to the Tertiary volcanic activity in western Scotland and Northern Ireland and to the opening of the North Atlantic Ocean. Sharp (1971) also mentions a group of lakes, resembling the Siljan Ring, in central Ireland as a possible impact structure.
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This may be the ring of lakes that encircles most of County Longford and the western part of Westmeath and includes Lough Ree to the west and Loughs Kennel, Owel and Derravaragh to the east. This ring is about 45 km in diameter, and is therefore similar to the Siljan Ring; its centre is at about 53°37'N, 7°42'W. There is no geological evidence for impact; the local rock is Carboniferous Limestone with a few small inliers of Old Red Sandstone. This chapter has described ten circular or arcuate structures, mostly identified from satellite photographs, in England, Wales, and Ireland. It remains to explain the origin of these structures. The North Down arcs and the three South Coast structures appear to be of essentially tectonic origin. The North Down arcs may have been created by crustal extension related to the Tertiary volcanism of Antrim and the Inner Hebrides; and the South Coast structures are probably related to Hercynian thrusting and the intrusion of the Cornubian batholith. Of the other five structures, the North Pennine arcs may be a surface manifestation of the concealed Weardale Granite; the Hitchin Basin is probably the result of glacial and fluvial erosion; and St. Bride's Bay appears to be a basin of marine erosion. The circles detected in the Vale of Pickering and in North Powys, however, do not appear to be related to the regional geology. Are these circles illusory, or are they related to concealed basement structures? It has been suggested that large arcuate or circular features on the Earth's surface are manifestations of concealed ancient impact structures in the basement rocks; these features have been called 'astrons'. It is possible that some of the arcuate features described in this chapter are such 'astrons.' However, it is difficult to understand how such basement structures could manifest themselves at the surface through a thick cover of sedimentary rocks, or how, in countries with as long a history of tectonic activity as the United Kingdom and Ireland, they could retain their circular shape. Nonetheless, it must be remembered that almost the whole of the surface area of England, Wales and Ireland consists of rocks that are <600Myr old, and that we are almost completely ignorant of the geology of the Precambrian basement of these countries. Even Scotland consists mostly of
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rocks that are
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Impacts in History
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CHAPTER
EE3 Small Craters, Airbursts, and Tsunami We have learned now that we cannot regard this planet As being fenced in and a secure abiding place for Man; We can never anticipate the unseen good or evil That may come upon us suddenly out of space. H.G. Wells, The War of the Worlds The first section of t h i s book w a s dedicated to t h e identification of large, complex i m p a c t s t r u c t u r e s of geological age, s t r u c t u r e s t h a t could be expected to r e m a i n detectable features of t h e British l a n d s c a p e for t e n s or h u n d r e d s of millions of years. This second section e x a m i n e s t h e possible effects of smaller i m p a c t s . While t h e b e s t - k n o w n meteorite c r a t e r s were all several t h o u s a n d y e a r s old, they could be regarded a s merely geological curiosities; a n d it could b e a s s u m e d t h a t meteorite i m p a c t s were of n o importance on historical or archaeological time-scales. Even t h e occurrence of t h e T u n g u s k a explosion a n d t h e SikhoteAlin cratering impact within 4 0 years a n d 2,700 km 1 of each other ^ h i s is less than the distance between Dublin and Istanbul.
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could be dismissed as merely a statistical fluke. It is indeed a measure of the lack of understanding of impact processes (and of exobiology!) that the hypothesis that the Tunguska explosion was the crash-landing of an extraterrestrial spaceship could not only be proposed but could be treated as a serious alternative to its interpretation as an asteroidal or cometary impact. The discovery during the 1980s and 1990s of large numbers of Earth-crossing asteroids, the identification of historically recent airburst events, the use of satellite photography to detect exploding fireballs in the Earth's atmosphere, and the continued formation of meteorite craters, showed that events like the Tunguska explosion are not rare on a historical time-scale, and that smaller events, such as the Sikhote-Alin meteorite fall, are common on the time-scale of a human life-span. In particular, it has become apparent that the Sikhote-Alin iron meteorite fall was not the only, or even necessarily the largest, cratering impact of the 20th century. A large meteorite fall in western Honduras in November 1996 produced a crater 50 metres in diameter and started a fire. Another fall was reported from the Nyika Plateau of Malawi in 1959 (Mossman, 1972; Graham et ah, 1985); this is said to have produced a crater 80 metres in diameter. A crater in the Okavango Delta in Botswana is 22 m in diameter, and is said to have been formed in August 1978 or in the 1930s (Henshaw, 1997). (The reported formation of a 150-metre crater in southern Botswana on 1989 May 7 appears to be a hoax.) A doubtful report from Vallecitos, in New Mexico, describes an explosion in a field that formed a crater 23 m across and 2.4 m deep during July 1983. Europe is not immune to such crater-producing impacts. A meteorite fall at Simuna (Estonia) on 1937 J u n e 1 made a crater 8.5 m in diameter (Bronshten, 1991; Steel, 1995), and the fall of an iron meteorite at Sterlitamak, in the Urals, on 1990 May 17 produced a 5-metre crater. However, the largest 20th century meteoritic event (besides the Tunguska explosion) was the Rio Curuca (Brazil) impact of 1930 August 13. This impact, which had an energy of about lMton, not only set fire to the tropical rain forest but produced a crater with D~ 1.0-1.6 km. The formation of this crater, together with the other reported 20th century cratering impacts, suggests that
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kilometre-sized craters may be formed on land on a timescale of centuries rather than millennia. Analysis of such events has made it possible to estimate the rate of formation of meteorite craters with diameters less than a few kilometres. For example, it appears that on average one 10-metre crater is formed every one or two years on land, and a 100-metre crater may be formed perhaps once per 50-100 years. Craters with D > 1 km may be formed on land on a time-scale of 100-1,000 years. Stony meteorites need to have initial radii (r0) >100 metres to reach the Earth's surface without exploding, and these meteorites (or small asteroids) produce craters with D = 3 - 4 k m . One of the smallest craters produced by a stony asteroid is the New Quebec Crater (D=3.44km). Craters of this size are probably formed about once per 1,000 years, about once per 3,000-4,000 years on land, 2 and perhaps about once per 2Myr in the British Isles. Counts of lunar craters yield slightly lower cratering rates. It has been estimated from such counts that craters with D > 5 k m (about the size of the Rochford Basin) are formed about once per 6,000 years somewhere on earth, about once per 20,000 years on land, and about once per lOMyr in the British Isles. (This estimate implies that there should be about 100 Pleistocene land craters with D> 5 km.) These estimates imply that a 10-metre crater is formed about once per 600-1,000 years in the British isles, and that 100-metre craters are formed about once per 10-40kyr. Craters with D > 1 km are formed about once per million years. No meteorite craters in this range of sizes have yet been identified in the British Isles, and there are no certain records of any craters being formed. It must be repeated that the Pleistocene glaciations must have destroyed any meteorite craters >15kyr old over most of the British Isles, and that ordinary erosion would probably destroy even a 1-km crater within a million years.
^ h i s implies that craters the size of New Quebec are formed about once per 50 kyr in Canada. Perhaps New Quebec itself is younger than the 1.4Myr usually assigned to it.
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However, there are a few equivocal records in guide-books and books of local history that may refer to meteorite falls and the formation of craters. These records will be discussed in the next chapter. Such cratering impacts are the result of the fall of iron meteorites with masses of several tons or more; these impacts in the minority, since irons constitute only about 5% of meteorites. Stony meteoroids in the same range of masses explode in the atmosphere producing 'terminal flares' (Hills and Goda, 1993), and do not yield large enough meteorites to produce craters. However, such atmospheric explosions can still cause damage on the Earth's surface. A stony meteoroid with an initial diameter of about 2 metres and a mass of 12-15 tons will produce an explosion equivalent to about 1,000 tons of TNT (1 k t o n - 4.2 x 10 12 joules). The 'terminal flare' of the fireball will have an absolute magnitude (i.e. as seen in the zenith at a distance of 100 km) of Mv — 21; at night such a flare will probably be reported as being as bright as the Sun and as lighting up the sky like daylight. (In fact most terminal flares occur at heights h~ 25-35 km, so that a flare with M v = —21 will have an apparent magnitude of n\= - 2 3 to - 2 4 to an observer directly below it.) Shoemaker (1983) estimated that airbursts with explosive yields of 1 kton occur about ten times per year on average over the whole Earth, and that the mean annual event is an explosion equivalent to about 20 kton, rather larger than the Hiroshima atom bomb of 1945. However, satellite observations indicate that there are about 100 1-kton impacts per year, and 12-25 impacts with explosive yields of 10-20ktons (Lewis, 1996). These higher estimates imply that the annual event is approximately a 100-kton explosion, that 1-megaton events occur about once a decade, and that Tunguska-type airbursts, with explosive yields of 10-20 Mton, occur about once a century. Since the frequency-magnitude relationship for fireballs is approximately JVoc m" 0 8 3 3 (Bevan et al, 1998), it can be calculated that the annual event over the British Isles is probably equivalent to a 5 to 10-ton explosion, equivalent to a 3.8-magnitude earthquake or an average tornado. A fairly conservative estimate suggests that the terminal flare associated with a 10-ton
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explosion should have M v ~ - 1 5 , that is, it should be about ten times as bright as the full Moon. The recent estimates of the frequency of kiloton airbursts suggest that such explosions occur over the British Isles about once in 20 years on average, and that 10-kton events occur about once per 100-200 years; that is, there should have been one or two 10-kton explosions since the birth of Charles Darwin (1809-82). 3 For comparison, the energy of a 1-kton explosion is equivalent to an earthquake with Richter magnitude 5.2. It may also be estimated that explosions equal to 100 tons of TNT (about 4 x 10 11 J), which are equivalent to a magnitude 4.5 earthquake and capable of causing damage on the ground, occur perhaps once per 5-10 years. However, in the course of my research I have noticed a curious thing; scientists and the authors of books on natural disasters have no difficulty in believing that buildings have been damaged and destroyed, and people and animals injured and killed, by, for example, lightning, hailstorms, tornadoes and earthquakes, but reports of damage and injury inflicted by meteorites are dismissed as obviously false, or at best are regarded as doubtful on the grounds that the reports are old and that they come from distant places. Where the evidence of death or damage resulting from a fireball explosion or meteorite fall is indisputable, the event is invariably identified as a terrestrial phenomenon (ball lightning being a favourite), apparently on no stronger grounds than the belief that meteorites do not damage buildings or kill people. There is no reason why meteorites should be incapable of harming either human beings or our property; simple physical reasoning shows that a 2-kg meteorite falling at 500 m s - 1 has a kinetic energy of 250 kJ, more than a one-ton car travelling at 45m.p.h. (20 m s - 1 ) and certainly more than enough to kill a man. About 2,500 meteorites of this mass fall per year, or about a quarter of a million per century. This unwillingness of scientists to accept documentary evidence of deaths and injuries resulting from meteorite falls appears to be indicative of a residual disbelief in meteorites, and may be significant when we come to examine 3
Shoemaker's lower estimated rate implies that 1-kton explosions occur over the British Isles about once per 100-200 years.
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some accounts of historical disasters in the British Isles that have been attributed to meteorological phenomena and earthquakes. Extrapolations of predicted impact rates suggests that a 1-Mton explosion should occur about once per 10-20kyr over the British Isles, and that Tunguska-type explosions occur about once per 200 kyr. Thus it is probable that a 1-Mton event has occurred over the British Isles or the surrounding seas since the end of the last glacial stage. These calculations indicate that damaging airbursts, with explosive yields >100 tons of TNT, are much commoner, perhaps by a factor of 100 or more, than crater-forming impacts. However, during the past ten years it has been realised that the greatest danger we have to fear from meteorites is that of marine impacts and the tsunami that they produce. Even quite modest impacts in the ocean, by objects 100 metres in diameter with impact energies of about 150Mtons, can produce tsunami with run-up heights of several metres, high enough to wreak havoc in coastal areas all round an ocean basin. Impacts of this magnitude are expected to occur somewhere on earth about once per thousand years. Even Tunguska-type impacts may be capable of producing damaging tsunami. Such impact tsunami may have been the source of many of the world's Deluge legends, and may also have given rise to one of the most powerful themes in European mythology, the story of Atlantis. Since the North Atlantic Ocean occupies about a twelfth of the Earth's surface area, 150-Mton impacts into the North Atlantic may be expected about once per 12,000 years. Thus it is not only possible but probable that the western coasts of Europe, including the British Isles, have been struck by an impact tsunami since late glacial times. The next chapter will describe a number of mysterious holes in the ground with attached legends that suggest that they are meteorite craters. The following chapter will examine the evidence for large fireball explosions over the British Isles during the historical period.
CHAPTER
Dozmary Pool and Other Craterlets The only natural freshwater lake in southern England, south of the limit of Pleistocene glaciation, is Dozmary Pool, on the granite upland of Bodmin Moor in Cornwall. The pool is located at grid reference SX195745 (about 50°32.7'N, 04°32.7'W), north of Browngelly Downs (SX196727) and about 2.5km south of the famous Jamaica Inn. The pool lies on the high ground between the River Fowey to the east and the St. Neot River, or River Lovery, to the west. The topography of this part of the moor is subdued, with no high hills or deep valleys. Dozmary Pool itself is at a height of between 260 and 270 metres above sea level (asl); it occupies a shallow north-south valley (<20 metres deep), which connects the valley of the River Fowey with that of the River Lovery. The latter valley is here occupied by the artificial reservoir of Colliford Lake. This connecting valley is narrower both to the north and the south of Dozmary Pool than at the pool itself. The pool is drained to the south-west by a stream that flows via Gillhouse Downs (SX192739) into Colliford Lake (18857380). There is no inflow into the pool, which is supplied entirely by rainfall (about 60", or 1.5 metres, per year).
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The pool is elongated from north-east to south-west, rather than being circular. The NE-SW diameter is about 520 metres, and the NW-SE diameter is about 310 metres. A reasonable mean diameter would be about 350 m; this makes the pool larger than the Odessa, Boxhole and Henbury meteorite craters, and similar in size to the Macha crater in Siberia and the St.-Imier crater in Switzerland. There is no evidence on a map scale of 1:25,000 of a raised rim to the pool. There is an arcuate bay, with a diameter of roughly 180 m, on the west side of the pool, with its centre at about SX193745. However, examination of a map of this part of Bodmin Moor suggests that Dozmary Pool is partly surrounded by low hills and ridges, rising to 284 m asl at Pinnockshill (SX189746), that form a circle centred on the pool with a diameter of about 900 m. One might speculatively interpret this partial ring of hills as the rim of a former topographic basin now occupied by Dozmary Pool. The floor of this basin is mostly covered by heath and bracken; but around SX19207425 (400 metres south-west of the pool) the ground is marshy, perhaps because the flow of groundwater is obstructed by an underlying ridge of granite farther to the southwest (19107415). This interpretation is partially confirmed by the existence of a small quarry at Grid Reference SX19347396, 486 m south of Dozmary Pool, at a height of 270 m asl. The presence of this quarry constitutes evidence that there is at least one outcrop of rock above the level of the pool. Legend has it that Dozmary Pool is bottomless(!); and, according to one guide-book, it was reported in 1533 to be 14 fathoms (84', or 26 metres) deep, still a respectable depth for an English lake. However, it appears that the pool is now shallow enough for a person to wade across it, i.e. about 1 metre deep. Either the reported depth of 14 fathoms was a gross over-estimate, or the pool was deeper, and perhaps larger, in the 16th century. The implied depth of the large 'Dozmary basin' surrounding the pool, measured from the top of Pinnockshill, is between 15 and 25 metres. Dozmary Pool is an ancient feature of the landscape; archaeological evidence suggests that it was formed during or before Neolithic time, i.e. before 2000 BC. However, the origin of the pool has not attracted much attention.
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Various possible origins can be excluded immediately. There is no evidence that Bodmin Moor was ever glaciated, and thus Dozmary Pool is not a glacial depression. It is obviously not a volcanic crater; the granite of Bodmin Mor was intruded 270-280 Myr ago, during the Early Permian epoch, and there has been no igneous activity in this part of Cornwall since that time. There is no evidence of faulting or other tectonic disturbance, and tectonic subsidence over such a small area is impossible. The local rock is insoluble granite, so the pool is not a karst feature due to solution of the rock. There is no evidence of blocking of the valley in which the pool lies by the sediments of a tributary stream, or by landslides from any nearby hills. It is possible that this valley was once the valley of a tributary of the River Lovery, and that its upper reaches have now been captured by the River Fowey; but it is difficult to see why this river capture should have left a lake in the abandoned valley, or why the valley broadens out to make room for the pool. Two possible origins remain for Dozmary Pool: it may be an artificial excavation, probably for tin or copper, like the mediaeval peat diggings of the Norfolk Broads; or it could be a meteorite crater. However, archaeological evidence suggests that Neolithic and early Bronze Age smiths obtained metals from surface outcrops rather than by quarrying or mining; besides, the granite of Bodmin Moor would be more difficult to dig than the soft peat of Norfolk. The possibility that Dozmary Pool is a meteorite crater requires a more detailed examination, particularly because of the uncertainties in the age and the original depth of the pool. If the 16th century depth measurement of 26 metres was accurate at the time, the depth-diameter ratio of the pool was about 1/13.5 (0.074), rather less than the same ratio for Barringer Crater but similar to that for other small meteorite craters. If, on the other hand, the depth of 26 metres refers to the surrounding 'Dozmary basin,' the depth-diameter ratio is about 1/35 (0.029), much less than that of typical meteorite craters. This relatively small depth and the absence of a clear external rim cast doubt on the impact hypothesis, or they imply that if Dozmary Pool is a meteorite crater it is a very old one. Estimates of erosion rates on Bodmin Moor suggest that, on the impact
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hypothesis, the 'crater' is about 300 kyr old, or of Late Pleistocene age. However, the diameter of Dozmary Pool suggests that it falls into the transitional impact regime for iron meteorites between groups of craters (such as Henbury and Quillagua) on the one hand and classical bowl-shaped simple craters (such as Barringer Crater) on the other. It will be remembered that in this transitional regime aerodynamic pressure crushes the incoming meteoroid into a flattened swarm of fragments whose impact produces a shallow, flat-floored crater with a very low, narrow outer rim. Thus, if Dozmary Pool is a meteorite crater, its shallow profile may be of primary origin, rather than being the result of a long period of erosion. The rather irregular shape of the pool, and particularly the presence of the bay on the western side, may be consistent with the impact of such a swarm; the pool may be an overlapping cluster of craters rather than a single crater. Depending on whether the diameter of the 'crater' is that of the pool itself (~350m) or that of the surrounding roughly circular depression (~900m), the 'crater' may have been formed by an iron meteorite with a diameter between 10 and 20 metres, and a mass of 4,000 to 30,000 tons. The impact energy would be between V4 and 2Mtons, much less than the energy of the Tunguska impact of 1908. None of this reasoning can be said to prove that Dozmary Pool is a meteorite crater. However, it keeps the impact hypothesis open, and may indicate that the pool is younger than it appears to be from its morphology. Moreover, it suggests that if either Dozmary Pool or the surrounding depression is a meteorite crater, it is of the same type as, for example, the Henbury crater field rather than Barringer Crater. According to some versions of the Arthurian legend, Excalibur, the sword of King Arthur, was thrown into Dozmary Pool after Arthur's death. This story may be significant, since the Welsh name of Excalibur is Caled-Fwlch, or 'lightning sword.' 1 Perhaps 1
The Welsh word fwlch (lightning) is equivalent to the Irish bolg. Thus it appears that the name of the Fir Bolg of Irish legend, who are identified with the Belgic tribes who invaded Britain in the 1st century BC, may be translated as 'lightning men.'
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the Arthurian story is based on a real event, when a flashing object that looked like a bright sword fell onto Bodmin Moor and made a crater, which is now occupied by Dozmary Pool. The fragments of iron meteorite found scattered around the crater might have been transformed by legends into the shards of a sword. Similar stories of a fiery object falling from the sky are told by Native Americans and Australian aborigines about the formation of Barringer Crater and the Henbury craters. If the story of Excalibur is indeed based on an eye-witness account of the impact, the event can probably be dated as <13kyr BP, the age of the Creswellian culture. If so, the United Kingdom, like Estonia and Poland, and probably like France, Switzerland and Austria, has a post-glacial meteorite crater. However, this interpretation is largely based on surmise, and it needs to be confirmed or refuted by geological evidence, namely by dating of the sediments of the pool, by measurement of the size and the depth-diameter ratio of the bedrock depression, and by examination of the nature of the rock underlying the sedimentary layers. Is this rock solid granite, breccia, or something else? There is little hope of finding actual meteorites associated with the pool, but a search for them might still be worth while; the impact origin of the Kaalijarv craters in Estonia was established by the presence of meteoritic fragments. Some digging around in guide-books and old reference books has brought to light three holes in the ground that may be of interest. The best documented of these are the water-filled pits called Hell's Kettles, south of Darlington. These pits are located at Grid Reference NZ109281 (54°29.6'N, 1°33.9'W). There were originally three pits, although the smallest was filled in during the 1950s. The larger of the remaining pits is about 7 0 m (eastwest) by 4 0 m (north-south), and is about 6 m deep; the smaller pit, to the south-east, is about 4 5 m x 4 0 m (north-south and east-west respectively), and is nearly circular. The third pit was much smaller than its companions and was only about 1.2 m deep. The pits are smaller than most terrestrial meteorite craters, but are larger than the Haviland (Kansas), Dalgaranga, and Sikhote-Alin craters. The local rocks are evaporites of the Upper Permian Magnesian Limestone, and this is consistent with the opinion that the pits
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were formed in 1179 by subsidence as a result of dissolution of gypsum; other such solution pits occur in the same area, and one of the Hell's Kettles pits is fed from an underground aquifer in the Magnesian Limestone. However, Short (1749) tells a different story, in which the pits were formed by an earthquake. In Dr. Short's words, on 1178 December 25 'the Earth lifted up its self like a high Tower, and so continued the whole Day, and then fell with so horrible a Noise, that it terrified all the neighbouring Inhabitants; and the Earth swallowed it up, and made there a deep Pit, which remains to this day, called Hell's Kettles.' Obviously the collapse of the ground to form a large solution pit would be very noisy, and might well be mistaken for an earthquake, but it does not appear to be consistent with Dr. Short's description of the Earth lifting itself up like a high tower. This part of the account seems more like the appearance of a bright fireball, followed by the impact of a giant meteorite; such an impact would certainly cause earth tremors, and would probably be described afterwards as an earthquake. The depth-diameter ratio of the smaller pit (about 1: 7) is near to the mean ratio for meteorite craters of this size. The presence of a group of pits is also consistent with the impact hypothesis; most impact craters of the size of Hell's Kettles occur in groups. The distance between the pits in relation to their diameter is of the same order as the distance between members of groups of meteorite craters, although the Hell's Kettles group is both absolutely and relatively more compact than other groups of craters, perhaps because outlying smaller pits have been destroyed. The work of Hills and Goda (1993) indicates that the pits could have been formed by the break-up of a stony meteoroid with an initial diameter do = 3 - 4 m or an iron meteoroid with do~2m. The initial mass would have been between about 20 and 140 tons, and the final masses of the meteorites would have been about 15-60 tons. Meteorite falls of this mass occur in Britain on a time-scale of about a millennium. The dimensions of the swarm of meteorites produced by the break-up of such a meteoroid is consistent with the area covered by the Hells Kettles group. The airburst produced by the break-up of this meteoroid would have had an explosive yield of a few kilotons (equivalent to a seismic
Dozmary Pool and Other Craterlets
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Richter magnitude M R ~5.5), and the impact of the meteorites would have produced an earth tremor with M R ~2-3. These magnitudes are consistent with the statement that the formation of the pits was accompanied by an earthquake. It appears then that Dr. Short's account and the morphological evidence are consistent with the Hell's Kettles pits being meteorite craters, but the geological evidence favours their interpretation as collapse features. In the former case, they are the only British meteorite craters with documentary evidence for their formation, and as such they deserve to be protected as a site of special scientific interest. The cathedral city of St. Albans, in Hertfordshire, has an old legend about a dragon that used to live in a 'cave' (called 'Wormenhout,' i.e. 'dragon-holt') beside a ravine outside the town. Since the dragon was an ally of the Devil, its 'cave' was largely destroyed during the 11th century by a bishop of St. Albans. This legend sounds like a garbled account of the fall of a large meteorite that produced a crater. Even readers who know Hertfordshire may find this picture of St. Albans as a town of ravines and caves rather unfamiliar, and may have difficulty in working out where 'Wormenhout' actually was in relation to the present town. Examination of the Ordnance Survey map shows a feature in Prae Wood, west of the city, described as 'Fish Ponds.' These ponds lie 400m south-east of a deep valley, a tributary of the River Ver, that separates them from Gorhambury. Their position is TL119069 (51°45.0'N, 0°22.7'W). The location of these 'fish ponds' is in agreement with the description of Wormenhout as being near to a ravine. It also seems to make little sense to have fish ponds in a wood, where they would be almost inaccessible to horses and carts, particularly in winter, and where they would be fouled by fallen leaves every autumn. Although the identification can be only tentative, it is plausible that these Prae Wood fishponds actually mark the site of the Wormenhout crater. It must be remembered that the Latin word cavus originally meant 'hollow' rather than 'cave' in its modern sense of an underground chamber with a roughly horizontal entrance. In modern planetary science a cavus is defined as 'a hollow or an
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irregular depression on a planetary surface' (Ridpath, 1997); in the past it probably applied to almost any surface depression, including the circular bowl-shaped depressions now called craters. The word 'crater' formerly meant only the orifice at the summit of a volcano, and its meaning was extended to cover the characteristic surface features of the Moon as a result of confusion of their morphology with those of volcanoes. Thus the 'cave' of Wormenhout may well have been a crateriform depression rather than the underground chamber that the word suggests nowadays. A 19th century itinerary (Kearsley, 1803} describes two 'caves' near Penrith (Cumbria), 'north of the river' (presumably the Eamont). These 'caves' are said to be named Isis Parlis, after a giant who lived in them and who destroyed men and cattle. This also sounds like a garbled account of some natural disaster, again possibly a cratering meteorite impact. However, the reference is very brief, and its interpretation is equivocal. The 'caves' are not shown on the Ordnance Survey map, and I have not come across any other reference to this giant. 2 Although it would be of great scientific interest if Dozmary Pool and the other hollows discussed in this chapter were proved to be meteorite craters, it must still be emphasised that crater forming impacts do not present a serious threat to the British Isles. The next chapter will discuss evidence for large fireball explosions during the historical period, and the possible effects of these explosions.
2
This 'giant' may be connected with an earthquake in Cumberland in 1000 AD that is said to have swallowed up people, cattle and houses (Short, 1749).
CHAPTER
Levin-Bolt and Blast The front of heaven was full of fiery shapes, Of burning cressets; and at my birth The frame and huge foundation of the earth Shak'd like a coward. I Henry IV, III, I, 14-17. and then came a blinding glare of vivid green light. Everything in the kitchen leapt out, clearly visible in green and black, and vanished again. And then followed such a concussion as I have never heard before or since. H.G. Wells, The War of the Worlds (1898) And meteors fright the fixed stars of heaven, Richard II, II, iv, 8. In t h e previous c h a p t e r I a r g u e d t h a t meteoritic a i r b u r s t s with explosive yields of > 100 t o n s of TNT occur over t h e British Isles on average a t intervals of 5-10 y e a r s , a n d t h a t t h e s e a i r b u r s t s m a y c a u s e d a m a g e on t h e ground. The p r e s e n t c h a p t e r will describe a s e a r c h for records of British a n d Irish fireballs a n d meteorite falls d u r i n g t h e p a s t 2,000 years; t h e a c t u a l fireballs are t a b u l a t e d in Appendix 1. It should be m a d e clear from t h e s t a r t t h a t some of t h e reports are described only vaguely, a n d t h a t it is n o t always possible to
Part II: Impacts in History identify these events with certainty; for this reason I have labelled the events with an indication of their reliability, from a for authentic well-observed fireballs to e for events whose description is too vague for reliable identification a n d / f o r legendary occurrences. In particular, it is often difficult to distinguish between meteorite falls and thunderstorms, since both are accompanied by bright flashes in the sky and by thunderous noise. It is a matter of record that several meteorites (e.g. Siena and Bremervorde) were reported to have fallen during thunderstorms. Ball lightning is particularly apt to be confused with meteoritic fireballs, since the two phenomena resemble each other closely, for example in colour and duration, and in the fact that they may end either by exploding or by fading away quietly. In addition, there is evidence that large fireballs can produce electrical phenomena, leading to even more confusion with lightning. There may also be confusion between fireballs and tornadoes or waterspouts: the dust trail from a large bolide may be mistaken for the funnel cloud of a tornado (or, of course, vice versa); both tornadoes and meteorite falls can destroy trees and buildings; and tornadoes often make small 'craters' (suction marks) in soft ground, which can be mistaken for impact pits. Older reports often refer to 'dragons' in the air, which may have been either tornadoes or fireballs. As I have mentioned previously, the explosive energy of a large bolide (around 1-10 kilotons of TNT) is equivalent to the energy of an earthquake of magnitude 5-6; such bolides may give rise to ground tremors, which have sometimes been mistaken for genuine earthquakes. Conversely, large earthquakes are sometimes accompanied by earthquake lights, which could be mistaken for fireballs. Various criteria can be used to distinguish fireball and bolide events from storms, earthquakes, and other confusing phenomena. In particular, fireballs can be seen over large areas, often for distances of tens or hundreds of kilometres; they can appear in any type of weather; the distribution of audible sound and ground tremors from fireballs is usually elliptical rather than circular; and fireballs are often accompanied by a triple detonation. Unfortunately, many of the descriptions of interesting events still lack the information required to permit scientists to identify them with certainty.
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Some of the best descriptions of fireballs come from the scientific literature of the 18th and 19th centuries, when these phenomena were still considered worthy of scientific attention. Greg's (1860) catalogue of fireballs and meteorite falls is a very useful source of historical data, and the published reports of the great meteor observer W.F. Denning (1848-1931) are invaluable for the study of fireballs during the late 19th and early 20th centuries. Unfortunately since Denning's time scientific interest in meteors has declined. Astronomers have concentrated on stellar and galactic studies, and meteorologists have generally ignored meteoric and fireball phenomena in favour of weather studies. At the same time, the post-war rise in belief in UFOs and extraterrestrials has led to widespread popular misinterpretation of fireballs and other atmospheric processes, and has discouraged scientists from studying these phenomena for fear of ridicule, loss of employment, or of becoming trapped in the morass of pseudoscience and superstition that forms the foundation of UFOlogy and belief in the paranormal. This is not the place to refute the assertions of the 'UFOlogists'; I hope to devote another book to this subject. Anybody who is inclined to believe in alien visitations should ask themselves how the alien craft are propelled, what the aliens eat during their long interstellar flights and how they keep warm in the darkness and cold of interstellar space, why such supposedly advanced beings are interested in visiting the Earth in the first place, and why they crash their craft with such monotonous regularity. My own opinion, the fruit of many years of study, is that UFOlogy is a pernicious superstition, and that the only good thing that has come from it is the discovery of historical records of interesting atmospheric phenomena. The phenomena tabulated in Appendix 1 include all the British and Irish fireball records that I have found before the foundation of the Royal Society in 1663. Afterwards I have included only fireballs that were particularly bright, that are reported as having caused damage, or that yielded meteorites. The records for the 11th, 12 and 13th centuries yield on average about eight noteworthy fireballs per century, almost all of them over England. The very detailed records for the 18th
Part II: Impacts in History century suggest that there were about seven fireballs that caused damage, five of them in England; given the relative area of England, this suggests that there should have been about 12 such fireballs over the whole of the British Isles. This figure is consistent with the argument of the previous chapter that potentially damaging fireballs occur over the British Isles about once per 5-10 years. In the light of this evidence it does not appear to be necessary to invoke ball lightning or other hypothetical rare meteorological phenomena to account for luminous bodies that explode and cause damage to buildings. There are fewer records of fireballs for the 14th to 16th centuries. It is not clear, however, whether this deficiency is due to a real decrease in the number of fireballs or to a failure to report these events during the disruptive period of the Hundred Years War, the Black Death, and the Wars of the Roses. Coversely, there is an unusually large number of fireball records in the late 1 l t h century (seven in only 33 years). The contemporary chronicler Peter of Blois [ca. 1070-1117) gives a graphic description of heavenly portents during the reign of William Rufus (10871100), such as lightning and thunderbolts, terrifying thunder, whirlwinds, earthquakes, and marine inundations. Clube and Napier (1990) argue that this increased number of records corresponds to an increase in the flux of fireballs of the Taurid meteor swarm; they suggest that these celestial portents brought about an episode of religious enthusiasm that inspired the First Crusade in 1095. There appears to have been another increase in the fireball flux during the 17th century. In the short period of 23 years between 1638 and 1661 I have found seven reports of probable and possible fireballs, all of them in the southern half of England, four of which are stated to have caused damage or injury. Moreover, during the 1620s two meteorites with m > 10kg fell in Britain, at Stretchleigh (Devon) and Hatford (Berkshire). This record, of two recorded British meteorites with m > 10 kg in a single decade, has been equalled only once, by the Appley Bridge (Lancashire) and Strathmore (Tayside) falls in 1914 and 1917. Although it is often stated that no meteorite has ever started a fire, Lewis (1996) cites many examples to the contrary, and several of the fireballs listed in Appendix 1 are said to have caused fires. Among these are the Christchurch fireball of 1113, which
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burnt ships and houses, the Cambridgeshire fireball of 1646, and the Horringer 'meteor' of 1801. It is, of course, possible that these fires had another cause, such as lightning, but there is no physical reason why a large meteorite should not remain hot enough on impact to ignite inflammable material. The Juromenha (Portugal) meteorite of November 1968 is said to have been incandescent when it was found, and the Brentwood (Essex) fireball of 1890 December 14 was still incandescent at a height of only 13 km. Most authorities are equally incredulous about reports that people and animals have been killed or injured by meteorite falls, although again Lewis (1996) cites documentary evidence of such meteoritic fatalities. I have found three such fatal events that appear to be of meteoritic rather than meteorological origin: the Bungay-Blythburgh (Suffolk) fireball of August 1577, when four people were killed; the Widecombe disaster of October 1638, also with four people killed and about 60 injured; and the Athlone (Westmeath) explosion of October 1697, when eight people were killed and 36 injured. It must be emphasised that none of these is an established meteorite fall, and it is possible that these disasters were caused by lightning or tornadoes. Moreover, even if these deaths were caused by meteorite falls, one must keep the danger in proportion; 16 deaths by meteorite in the British Isles since the 16th century is a small number by comparison even with the number of people killed by lightning. However, it is possible that a meteorite fall was responsible for a more recent disaster. At 11.57 a.m. on Sunday 24 March 1968 an Aer Lingus Viscount fell into the sea off south-east Ireland, between Hook Head and Tuskar Rock, killing all 61 people on board. In the absence of any other explanation for the crash, it was suggested that the aircraft had collided with something. One of the witnesses to this disaster reported hearing loud rumbling noises and seeing a plume of water at the time of the crash; these phenomena are consistent with the explosion of a fireball (perhaps above clouds) and the fall of a meteorite into the sea. It is therefore possible that the aircraft was brought down either by the impact of a meteorite or by a nearby fireball explosion. Horrifying though this crash was, we must still keep things in proportion. I do not know of any other air crashes that have been attributed to meteorite strikes, but many aircraft have crashed
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as a result of encountering bad weather, and even more have been brought down by mechanical failure or pilot error. We have a long way to go in increasing the safety of aircraft before meteorite impacts become the main hazard to airline passengers. In a previous chapter I referred to the danger of tsunami produced by oceanic impacts. I have found three 11th century reports of marine inundations that may have been caused by impacts. According to Short (1749), in 1012 'a burning fire like a tower fell from heaven with a great noise, and an inundation of the sea overwhelmed many towns in England and Germany.' At the end of October 1097 or 1098, 'the heavens appeared in flames all night, and the Goodwin Sands were swallowed up by the sea.' Finally, on 1099 November 11 (St. Martin's Day), a 'great tide' swept up the Thames Estuary, destroying villages and drowning people and animals. None of these floods can be certainly attributed to meteorite impacts; the luminous phenomena reported in 1012 and 1097 or 1098 may have been aurorae or even lightning, and the floods themselves may have been the results of storm surges. The 'great tide' of November 1099 occurred on the first day after New Moon, and therefore during the spring tides. The fact remains that the floods of 1097/8 and 1099 occurred not only during the years of the increase in the Taurid fireball flux but at the time of year of the Taurid meteor shower. Searching farther back in time, Baillie (1998) uses tree-ring records from oak-trees to identify an 'environmental downturn' at AD 536-40, which he attributes to a close encounter between the Earth and a comet and to consequent terrestrial bombardment by fireballs and meteorites. Baillie links this 'downturn' with the legends of King Arthur and Beowulf. Keys (1999) also describes these events in great detail, although he attributes them to an eruption of Krakatau. However, some of Keys's references, for example to 'swarms of flying creatures' and 'stones of baked clay' [ibid., p. 88) are suggestive of a local meteorite fall rather than a distant volcanic eruption. The Dolorous Stroke of Arthurian legend, which reduced Britain to a Waste Land, is described as being accompanied by a great noise; it is said to have felled trees and destroyed crops and left traces of fire in many places in its wake (ibid., pp. 164-5). This event again
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sounds more like a local disaster, perhaps the explosion of a fireball, than the effect of a distant eruption. It may have occurred in South Wales or south-west England; there is a legend (mentioned by Haining (1976)) of a destructive earthquake in Somerset in 534 AD. Of course, it is not necessary to set up an opposition between a volcanic eruption and a series of cosmic impacts. A bombardment episode and an eruption of Krakatau could have occurred together, collaborating to t u r n a disaster into a catastrophe. Clube and Napier (1990) present evidence of another episode of deterioration in global climate during the 440s AD, and a contemporary disaster in East Anglia and the Midlands which led to widespread depopulation and deforestation. They link this disaster with evidence of a Tunguska-type impact in the Isle of Axholme, between Doncaster and Scunthorpe, where many felled trees were found partly burnt and lying in a northwest-southeast direction. Baillie (1998) and Ashe (1968a) describe an episode of social disruption in the late 360s AD. In 367 AD Roman rule in Britain broke down for some unexplained reason, and the province was invaded by the Saxons, Picts and Irish. At about the same time there was a severe inundation of the Scilly Isles, which flooded much of the lower parts of the islands. It is possible that the breakdown of Roman authority and the foreign invasions followed inundations of the coasts of the British Isles and Northern Europe by either a storm surge or a tsunami, itself possibly the result of an impact. At about the same time the Somerset Levels around Glastonbury were flooded, and Glastonbury Tor was left essentially an island surrounded by fenland (Rahtz, 1968); this flooding may have been caused by the same event as the inundation of the Scilly Islands, or it may have been a separate event, perhaps caused by deteriorating climate and deforestation of the lowlands. It may be significant that a destructive earthquake and tsunami occurred at about the same time in the Mediterranean Sea, and that Bede records a world-wide earthquake in AD 369. At about the same time a new temple was built to the Celtic god Nodens at Lydney, on the lower Severn; since Nodens was apparently held responsible for the Severn Bore, the implication appears to be
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that the social disruption of the 360s was in some way related to marine inundations. Hills and Goda (1993) also remark on the danger posed by impact tsunami; they specifically draw attention to the nearabsence of Atlantic coastal settlements until the arrival of the Vikings in about 800 AD, and suggest that previously existing settlements had been destroyed by an impact tsunami. It is therefore possible that the marine inundation of the 360s AD, or other inundations during the 5th and 6th centuries, overwhelmed the whole of the Atlantic seaboard of the British Isles. Baillie finds evidence of other world-wide environmental downturns at 207 BC, 1159 BC, 1628 BC, 2345-54 BC, and possibly 44 BC, which he attributes to close encounters between comets and the Earth, with consequent bombardment by fireballs and meteorites. These events are correlated with disasters recorded in the Irish king-lists. He finds other growth reduction events in tree rings during the 32nd century BC and in 2911 and 2740 BC. The earliest of these events may be correlated with a 'worldbeginning' event of Mayan chronology in 3114 BC. Steel (1995) argues that this date marks a break-up of the giant precursor comet of P/Comet Encke and the Taurid meteor swarm, and that, because the swarm had a node at the Earth's orbit in about 3000 BC and about 2700 BC, the Earth was subjected to spectacular meteor storms and frequent destructive airbursts. He suggests that these meteoric disasters inspired the construction of Stonehenge I, near the end of the 4th millennium BC. We can now summarise the conclusions of this chapter. The evidence that I have presented implies that meteoritic airbursts that cause slight or moderate damage to buildings occur more frequently over the British Isles than has been supposed, probably at an average rate of once per 5-10 years. Larger airbursts, with enough energy to kill people and to cause serious damage to buildings, have also been plausibly identified; but they happen much less often, less than once a century on average. Thus the hazard from such airbursts is similar to that from earthquakes, and much less than the danger from lightning, floods, tornadoes, and such man-made hazards as road accidents. However, meteorite impacts and fireballs differ from meteorological phenomena in that there is effectively no upper limit to
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their energy; some of the available documentary and physical evidence suggests that destructive airbursts and cratering impacts, perhaps with energies of around a megaton, have occurred in the British Isles within historical or at least post-glacial times. If so, these events have probably been part of encounters with swarms of cosmic debris rather than random impacts; such encounters have been regarded as causes of climatic change (usually for the worse!) and of declines in civilisation. It is these indirect climatic effects that have had most influence on the course of history; the direct destructive effects even of megaton impacts, although spectacular, are local (covering an area of a few counties), and are probably of secondary importance. These last two chapters have examined the evidence for historical crater-forming impacts and large airbursts ('atmospheric craters'), and have only alluded briefly to the effects of oceanic impacts. It is now time to examine these marine impacts, which t u r n out to present the greatest danger to the British Isles; such impacts may indeed have been the cause of an event that has become a dominant theme of British and European mythology.
CHAPTER
British Atlantis? 'there occurred violent earthquakes and floods; and in a single day and night of misfortune ... the island of Atlantis ... disappeared in the depths of the sea.' Plato, Timaeus. 'of the land of Westernesse that foundered, and of the great dark wave climbing over the green lands and above the hills, and coming on, darkness unescapable.' J.R.R. Tolkien, The Lord of the Rings 'but thus to destroy animals ... we must shake the entire framework of the globe.' C. Darwin, The Voyage of the 'Beagle' The story of Atlantis is derived immediately from t h e Critias of Plato (ca. 4 2 9 - 3 4 7 BC), a n d before t h a t from a story told to Solon t h e Wise (ca. 6 4 0 - 5 6 0 BC) in Egypt; it r e c o u n t s t h e history of a n island civilisation beyond t h e Straits of Gibraltar (i.e. in t h e North Atlantic Ocean) t h a t ruled m u c h of n o r t h e r n E u r o p e a n d p a r t of north-west Africa a n d w a s supposedly destroyed by t h e gods 9,000 years before Solon's time. The island is described a s m o u n t a i n o u s a n d forested, a s being rich in m e t a l s (including tin), a s a b o u n d i n g in all m a n n e r of fruit a n d vegetables, a n d a s
British Atlantis?
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possessing an advanced civilisation, with temples, palaces, harbours, and wharves. Numerous authors have set out to identify the lost Atlantis, and a history of 'Atlantology' would be a very long book in itself. Atlantis has been identified in the Azores Islands, in Morocco, Spain, the Canary Islands, Nigeria, the Hoggar (Ahaggar) mountains of the Sahara Desert, South Africa, Heligoland, Sweden, Spitzbergen, the Antilles, the Bahama Plateau, the Mid-Atlantic Ridge, Crete and the Aegean, Troy, Armenia, Sri Lanka, Brazil, Bolivia, and even Antarctica. Clube and Napier (1990), in The Cosmic Winter, appear to identify Atlantis as an interplanetary cloud that was destroyed at a time of encounters between the Earth and the debris of a giant comet. Their arguments are impressive, but Plato's description of a terrestrial empire seems to me to be too detailed to be derived from observations of a celestial phenomenon. 1 Much of the problem, as it seems to me, stems from a failure to take Plato's account literally, as describing an island civilisation in the Atlantic Ocean. One clue to the location of Atlantis comes from Plato's statement that the island was larger than Libya and Asia put together. Asia here refers to the ancient province of Asia in what is now western Turkey; it had an area of about 110,000 km 2 , or about 42,000 square miles. The North African territory that was called Libya in Plato's time was larger than 'Asia', but its boundaries and its area are uncertain. If Libya extended westwards from the Nile Delta to longitude 25°E, its area was about 60,000 km 2 (about 23,000 square miles). If, on the other hand, Plato's 'Libya' included the territory of Cyrenaica, which extended westwards to longitude 20°E, its area was about 2 x l 0 5 k m 2 (80,000 square miles). Thus the area that Plato assigned to Atlantis was probably between 170,000 and 300,000 km 2 , or between 65,000 and 120,000 square miles. This area is comparable to that of Great Britain (229,988 km2) or even to the whole of the British Isles (315,173 km 2 ). If Plato's account can be trusted, one fact may be deduced from this, namely that Atlantis was not in the Mediterranean Sea. The lr
This disagreement with Clube and Napier's interpretation of Atlantis does not imply a rejection of the whole of the thesis of The Cosmic Winter.
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largest of the Mediterranean islands is Sicily, with an area of 25,709 km 2 , less than a quarter of the size of the province of Asia, and less than one-sixth of the smallest area attributed to the island of Atlantis. Thus we must look for Atlantis outside the Mediterranean Sea. Also, although a civilisation may be destroyed, for example by a natural disaster or as a result of barbarian invasions, it is geophysically impossible for a large, mountainous island to disappear or to be swallowed up by the sea; if Atlantis ever existed as a geographical entity it must still be there as an island. It seems strange, even perverse, that students of this question have, almost without exception, disregarded the possibility that the largest island in the Atlantic Ocean, namely Great Britain, is the true site of Atlantis, and that Plato was describing the Wessex culture. There are several elements of the story that point towards Britain. In particular, Plato gives the dimensions of the rectangular central plain of Atlantis, where the capital was built, as 3,000 stades long by 2,000 stades wide, that is about 540km by 360km (or 340x220 miles).2 These dimensions are in fair agreement with those of the English Lowlands, which are about 420 km from north to south (from the mountains of the Lake District to the south coast of England) and about 320 km from west to east (from the Welsh Mountains to the coast of Suffolk). The implied area of the plain of Atlantis is about 2xl0 5 km 2 , or 75,000 square miles; this area is in fair agreement with the area derived above from the comparison with Asia and Libya, and is also close to the total area of Great Britain (230,000 km 2 ). It is also larger than the area of any other Atlantic island; even Cuba is only 114,500 km 2 . Collins, in Gateway to Atlantis, points out that the area of the 'plain of Atlantis' is not of continental dimensions, and that it was therefore probably located on an island. Another point in favour of the identification of Atlantis with Britain is that there are legends of lost lands around the British coast, such as the drowned country of Lyonesse between Land's End and the Scilly Isles, and Cantref-y-Gwaelod in Cardigan Bay. Across the Channel, Brittany has the lost country of Ker-Is. These stories go back at least to the 1st century BC; Ashe 2
According to this measure, 1 stade ~180m -200 yards.
British Atlantis?
IEM
(1968b) indeed hints that legends of these drowned lands may have inspired the myth of Atlantis. Plato says that the civilisation of Atlantis was founded by the god Poseidon, and that his temple contained a statue of the god driving a chariot with winged horses. Poseidon was the god of earthquakes and the sea; his name has been interpreted as meaning something like 'master of horses' [hippos adonai?). Since Plato describes earthquakes and floods as the cause of the destruction of Atlantis, this destruction may have been attributed to Poseidon. In at least one respect Plato's account is contradictory. He states that Atlantis was destroyed about 9,000 years before the time of Solon (i.e. in about 9600 BC, near the end of Upper Palaeolithic time), but his Atlantis is clearly a Bronze Age society, using metals (gold, silver, copper, tin and bronze, but not iron) and possessing a large fleet of ships. This patent contradiction has led many students to argue that Plato overstated the age of Atlantis by a factor of ten, and that the events that he described occurred about 1500 BC, a date consistent with the identification of Atlantis as Minoan Crete. Other students have pointed to the abrupt climatic variations of Late Glacial time (about 15-11.5 kyr BP, or 13000-9500 BC) as proof of the correctness of Plato's date and as confirmation of the truth of his story. It has only recently become apparent that both schools of though may be correct, and that Plato may have conflated two separate events, about 900 and 9,000 years before his time, into a single story of natural disaster. If the identification of Plato's Atlantis with Bronze Age Britain is accepted, it is clear that Plato was describing the archaeological Wessex Culture of about 2000-1500 BC, and the components of his account can be used to shed light on that culture. For example, the Wessex Culture was strongly centralised, and its people were able to construct great monuments; but no settlements belonging to the culture have ever been identified, although logic would demand that such a civilisation must have had a 'capital city'. According to Plato such a capital city did exist; it was built on a plain near to a mountain that sloped downwards on all sides, and was connected to the sea by canals. The outer wall of the capital of Atlantis was coated with bronze,
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and the inner wall was coated with tin. In addition, great importance is attached to a mysterious reddish metal called orichalcum (meaning something like 'gold-copper'), which is said to have been mined in many places on the island. The capital of Atlantis possessed hot and cold springs, and these were used for public and private baths. The city was also a port, about 9 km from the sea, and its wharves were crowded with ships. It seems reasonable to suppose that the capital of the Wessex Culture would be between the main copper and tin mining areas of Cornwall and Devon to the west, and Salisbury Plain, where the megalithic monuments were built, to the east, that is to say, in the plains of Somerset. This location is in good agreement with Plato's description. Otto Muck, in The Secret of Atlantis, also points to this locality when he says of orichalcum that 'these were natural deposits of very rich mixed ores (copper, tin, arsenic, antimony, etc.), which also occurred in Cornwall.' It is a matter of history that Cornwall was once the main source of tin in Europe, and that Great Britain was the world's chief producer of copper as late as 1800. Atlantis is also said to have been rich in gold and silver. Gold could have been obtained from the Wicklow Mountains in Ireland; indeed, during Bronze Age times Britain exported Irish gold and Cornish tin to Greece. The tin deposits of Cornwall were formerly associated with lead-silver ores (argentiferous galena); and silver could also have been obtained from the Lake District, where some of the lead veins were formerly rich in silver, from the Harlech Dome, or from the Sttvermine Mountains in Tipperary. There were also formerly silver mines in North Devon, much nearer to the centre of the Wessex Culture. Finally, Pliny the Elder, in his Natural History, refers to aurichalcwn as an important ore of copper. This mineral is identified as copper pyrites, or chalcopyrite (CuFeS2), the most widespread copper mineral. It is found in Cornwall and South Devon, and in the Lake District, particularly in the Caldbeck Fells. The hot and cold springs of Atlantis and the public and private baths are still there, a fact that can be confirmed by any visitor to the city of Bath! It is also plausible that the Icknield Way, originally dating from between 1800 and 3000 BC, formerly continued from its present
British Atlantis?
WSk
ending in the Marlborough Downs to the Bronze Age cities of Somerset. It is obvious that a road of such importance cannot have been intended to die out in the open country; it must have gone somewhere, and the absence of any western terminus is a long-standing mystery. It may even be possible to identify the mountain, 'gently sloping in all directions' that was the site of the capital of Atlantis. This circular mountain rising out of the lowlands of Somerset may be the historic city of Glastonbury, the famed 'Isle of Avalon'. This suggestion is also consistent with Plato's description of canals; the low-lying country of Somerset would require artificial drainage channels in order to be inhabitable at all, and these drainage channels would serve the additional function of being usable by ships. The capital of Atlantis was connected to the sea by a canal, and was therefore a port as well as the capital city. This is a reasonable state of affairs for an island nation that lives by trade, and is consistent with the identification of the cities of Somerset as the capital of Atlantis. It may also be noted that the city of Bristol is about 9 km from the sea, just as the capital of Atlantis was said to be. Somerset in the Bronze Age would, indeed, have been in much the same position as the present Netherlands; and, like the Netherlands, it would have been vulnerable to flooding as a result of storm surges or tsunami. The advocates of the 'Minoan Atlantis' hypothesis attribute the destruction of Minoan Crete to the eruption of the Aegean volcano Thera (or Santorini), probably in 1628 BC; they suppose that the effects of this eruption were restricted to the eastern Mediterranean Basin. However, evidence from dendrochronology (Baillie, 1998) shows that there was a world-wide deterioration in climate around 1628 BC. In Britain, submerged forests around the coast (particularly in Wales and the west of England) have been dated at around 1500-1800 BC; this time also marks the end of the Early Bronze Age and the decline of the Wessex Culture. Moreover, around the middle of the second millennium BC, at the end of the Early Bronze Age, there was an eastward shift in the centres of civilisation from Wessex and Brittany to the valleys of the River Thames and the River Seine. At the same time the
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religious rituals based on great stone circles lapsed and were replaced by offerings of weapons to bodies of water, such as rivers and lakes. This change has been interpreted as a change from a religion directed to the sky or the heavenly bodies to one focused on the deities of earth and water, such as the EarthShaker and Sea-God Poseidon. The eastward movement of the main centres of civilisation suggests that the cause of this change was something emanating from the Atlantic Ocean. All this evidence indicates that the Early Bronze Age civilisations of the British Isles and Western Europe declined at about the same time as Minoan Crete collapsed. The decline of these West European civilisations can hardly be attributed to the eruption of Thera, and the presence of submerged forests around the British coasts implies that this decline was associated with coastal flooding. This flooding may have been the result of a deterioration in climate and of an increase in the frequency and the intensity of Atlantic storms; but the arguments of Chapter 14 suggest that a tsunami produced by the impact of a large meteorite or small asteroid could have the same effect. The impact of an asteroid with a diameter d ~ 1 0 0 m in the north-east Atlantic Ocean could create a tsunami with a run-up height of 4-10 metres, sufficient to devastate low-lying country along to west coasts of Great Britain and Ireland. The distribution of the submerged forests of the 2nd millennium BC, from the Solway Firth to Cornwall, suggests that the flood waves came from the south-west and entered the Irish Sea and Cardigan Bay through St. George's Channel. Farther south the wave was compressed into the narrowing gulf of the Bristol Channel, its height increasing like an enlarged version of the Severn Bore, until it over-ran the Somerset Levels, drowning all that stood in its path. Probably only the summit of Glastonbury Tor stood above the flood, and the capital of Atlantis perished 'in a single dreadful night.' However, as was explained above, there are reasons to think that the story of Atlantis also includes a memory of an earlier tsunami than the Bronze Age disaster of about 1628 BC. I have already referred to the myths of the lost lands of Lyonesse between Land's End and the Scilly Isles and of Cantrefy-Gwaelod in Cardigan Bay, and to the lost country of Ker-Is off Brittany. It should be emphasised that these lost lands are not
British Atlantis?
IB
mere parishes; measurement of the submarine contours suggests that the area of Lyonesse was about 11,000 km 2 (equivalent to Devon and Cornwall combined), and that the area of Cantrefy-Gwaelod was about 4000 km 2 , equivalent to nearly a fifth of the present area of Wales. These lost counties are not likely to have been drowned as late as the second millennium BC, since sea level has been at essentially its present level for about the last 6,000 years, that is, since about 4000 BC. At present the region between Land's End and the Scilly Isles is between 50 and 100 m below sea level, and this area has probably been submerged since at least the end of the Pleistocene epoch, about 11,500 years ago. This time for the submergence takes u s back to about the traditional date for the drowning of Atlantis, although I am not suggesting that anything like Plato's wonderful Atlantean empire existed during Pleistocene time; there is not a scintilla of archaeological evidence for any metal-using civilisation at such an early date. However, it is a curious fact that the Upper Palaeolithic cultures of Western Europe appear to have come to an abrupt end at about the end of the Pleistocene epoch. The Magdalenian culture of western France, famous for its cave paintings, lasted from about 17kyr to about 12kyr BP; and the contemporary Creswellian culture in Britain has been dated to the Allerad interval of relatively mild climate between about 15 and 13kyr BP. Moreover, these cultures are best known from highland areas. The remains of the Magdalenian culture have been described from the hill country of the upper Dordogne River, rather than from the lowlands of the Biscay coast and the lower Garonne, which are now the more densely populated areas. Were these lowland areas unpopulated during the Late Palaeolithic, or have all Magdalenian remains in the lowlands been destroyed? The same is true in Britain; the type area of the Creswellian culture is at Creswell Crags, on the Derbyshire-Nottinghamshire border, not in the warmer south of England. The anomalies of the archaeological record are confirmed by geological evidence. Around the time given by Plato for the destruction of Atlantis the climatic warming at the end of the last glaciation was suddenly interrupted by the severe cooling of the Younger Dryas in Europe and the Valders glacial advance in
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North America. It is a measure of the importance of this climatic cooling that sea level fell by about 15 metres, equivalent to extraction of about 5.4 x 10 6 km 3 of water from the oceans to feed the resurgent glaciers. This volume is in turn about a fifth of the present total volume of glacial ice. At present rates of precipitation (99,000 km 3 /yr for the whole land surface of the Earth) it would take more than 50 years to accumulate so much ice, and more like 180 years for precipitation on the ice-covered area of the Earth. The Younger Dryas glacial stage began about 12.9-13 kyr BP (10,900-11,000 BC) and ended about 11.64kyr BP (9640 BC). The date of the end of the Younger Dryas is almost exactly the same as Plato's date for the destruction of Atlantis, whereas the date of its beginning is very close to Hancock's (1996) astronomically deduced 'disaster date' of 10,500 BC. At about the same time as the Younger Dryas or Valders climatic cooling there was a mass extinction of the Pleistocene fauna. This extinction was particularly severe in the Americas, where >70% of the large mammal species died out. In Europe, the extinction of the mammoths now appears to have occurred at the same time as the American extinctions, and to have been contemporary with the Younger Dryas cooling.3 The stratigraphic evidence and 14C ages suggest that the mass extinction in North America coincided with the beginning of the Valders glacial advance. Most scientists have attributed these extinctions to over-hunting by human beings, 4 but a small number have presented evidence for regarding them as a consequence of climatic variations. In particular, the 'muck beds' of Alaska, which contain splintered trees and the carcasses of large animals that have literally been torn limb from limb, testify to the occurrence of a natural disaster; these beds appear to date from near the PleistoceneHolocene boundary. 5 As additional evidence of a natural disaster, 3
Is Plato's statement that there were elephants on Atlantis based on a memory of the European mammoth?
4
In spite of the fact that the extinctions appear to have destroyed human civilisations as well as animals. 5 It may be significant that the Lake Sythylemenkat meteorite crater in Alaska is about 10 kyr old; perhaps it was this impact that caused the destruction.
British Atlantis?
m
Collins (2000) cites an unpublished paper by Kloosterman that describes a 'charcoal-rich layer' (the Usselo Horizon) of Altered age (i.e. shortly before the onset of the Younger Dryas) that has been found in Europe (including Great Britain), Africa, India, and Australia. Such a layer can only have been produced by extensive fires, and its great extent implies that these fires had a natural origin. The smoke from such fires would itself contribute to climatic cooling by blocking sunlight from the Earth's surface. The memory of these world-wide fires may be preserved in the myth of Phaethon and in other myths that describe fires that burnt entire countries. The Valders climatic cooling and the end-Pleistocene extinctions may be linked with another long-standing mystery of American geomorphology, the origin of the Carolina Bays. These 'bays' are shallow elliptical depressions found in the coastal plain of the south-eastern United States. As their name implies, they occur particularly in North and South Carolina, but they are found as far north as New Jersey and as far south as Florida. There are about 500,000 'bays,' ranging in size from about 90 metres to as much as 6 km. The average size of the 'bays' increases towards the south-east, and the larger 'bays' have narrow raised rims on their south-eastern boundary. The age of the 'bays' is uncertain. Melton and Schriever (1932) assigned a date between 50 kyr and 1 Myr. Radiocarbon measurements have yielded ages between 6 kyr and 70 kyr BP. Collins (2000) cites Savage (1982) as quoting an average age of 10.5 kyr BP (8500 BC). Sedimentary and pollen evidence suggests that before the formation of the 'bays' the Carolinas were occupied by boreal forests, and that the formation of the 'bays' was followed by deforestation and the advent of a drier climate and then by the final deglaciation that ended the Pleistocene epoch (about 11.6kyr BP). This evidence is consistent with the Carolina Bays having been formed near to the beginning of the Younger Dryas (~ 13kyr), and at least not impossibly long before the traditional date for the destruction of Atlantis. 6 6
Collins (2000) explicitly links the Younger Dryas (Valders) cooling with the destruction of Atlantis.
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The Carolina Bays do not resemble meteorite craters in their morphology; no meteorites have been found associated with any of the bays; and there is no evidence of shock effects. These facts led to the rejection of the impact hypothesis in favour of the impact hypothesis in favour of terrestrial interpretations based on coastal hydrological processes. However, the recent discovery of felled trees all lying in the same direction beneath a 'bay' in South Carolina has revived the catastrophic interpretation, and analogies have been drawn between the 'bays' and the shallow surface depressions found at the site of the Tunguska explosion of 1908. It may be noticed that the mere presence of trees is consistent with the bays having been formed during the mild Allerod interstadial rather than earlier in the glacial period, when the Carolinas were covered by tundra rather than forest. The modern impact interpretation then sees the Carolina Bays as having been formed by blast waves from the explosion in the atmosphere of fragments of a disintegrating comet. The path of this comet was from north-west to south-east, and the final impact occurred in the Atlantic Ocean to the south-east of the Carolinas. In confirmation of this interpretation, Collins (2000) cites Native American myths of fire and cloud of different colours coming from the north and west and causing destruction. Although there are myths that associate astronomical catastrophes with the constellations Orion and Taurus, it is clear that the object that made the Carolina Bays cannot have had a radiant in these constellations. At the time Orion and Taurus lay south of the celestial equator and cannot possibly have been seen to set in the north-west from any location in the Northern Hemisphere. The radiant must have been in a northern constellation; mythological associations suggest that the source of this comet was in Draco (the Dragon). Otto Muck argued that the Carolina Bays were formed by an asteroid rather than a comet, and that its diameter (d) must have been about 10 km; this would make it essentially the same size as the K-T boundary impactor. Later work indicates that the object may have been much smaller. Eyton and Parkhurst (1975) use energy considerations to conclude that a cometary impactor would have had a diameter between about 1.0 and 2.6km, depending on the collision speed. The smaller diameter is perhaps
British Atlantis?
lit!
more probable. The energy of such an impact would be about 120,000 Mtons (5xl0 2 0 J), about 10 4 times the energy of the Tunguska impact or of a full-blown Atlantic hurricane. This diameter is consistent with the Carolina Bays impact having been responsible for the Younger Dryas/Valders climatic cooling. Hills and Goda (1993) argue that a comet or asteroid must have d> 600-1000 metres for the atmospheric dust layer produced by its impact to obstruct enough sunlight to cause a significant deterioration in climate. This diameter is in good agreement with that obtained independently by Eyton and Parkhurst (1975). An impact on the scale proposed here must have created a tsunami that would have swept all the coasts of the North Atlantic Ocean, either by direct impact into the sea or as a result of the more than hurricane-force winds caused by the explosion of the comet or asteroid. In confirmation of this conclusion, Collins (2000) cites mythological evidence that the low-lying Bahama Islands (800-900 km from the Carolinas) were once drowned by a great flood. Since the height of a tsunami varies only inversely with the distance from its source, proof of such an event anywhere in the North Atlantic Ocean implies that evidence for it will be found all round the ocean basin. Hills and Goda (1993) conclude that at 1000 km from the impact point the deep-water wave produced by a cometary nucleus with d = l k m is h ^ = 30-40 m. The run-up height of the tsunami on land is h^-40 x h ^ . Since Land's End is about 6000 km from the Carolina coast, the tsunami from the Carolina Bays impact was about (30x40)/6 = 200 metres high when it struck Cornwall and the land of Lyonesse. It will be seen that once the impact origin of the Carolina Bays is accepted, everything else, the Younger Dryas climatic cooling, the mass extinction of animals in North America, and the great tsunami sweeping all the coasts of the North Atlantic Ocean, necessarily follows. An impact of the size required to create the Carolina Bays cannot have avoided bringing about a catastrophe on the scale of the end-Pleistocene event; and the terrified human survivors would remember the fire from the sky, the earthquakes, and 'the great dark wave climbing over the green lands and above the hills,' and would pass the story on from generation to
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generation, until, with the coming of uniformity, memory was lost in amnesia and the destruction of Atlantis was regarded as myth. And so the wheel comes full circle; the Palaeolithic civilisation destroyed by the tsunami is rediscovered, and its descendants slowly and painstakingly assemble the evidence that enables them to recognise the part played by cosmic catastrophes in the history of the Earth and of the human species.
Epilogue: The Silverpit Crater As I was finishing this book (on 1 August 2002), news came in of the discovery of the first authenticated impact crater in the British Isles (Stewart and Allen, 2002). This structure, called the Silverpit crater, has a unique multi-ringed morphology, which has not previously been recognised in terrestrial impact structures. The crater is in the North Sea, at 54°14'N, 1°51'E, below the south-west flank of the Dogger Bank and about 130 km east of Scarborough; it has been named after a nearby channel in the sea floor. The crater is concealed beneath about 1,000 m of Tertiary and Quaternary sediments, and was discovered in the course of a seismic reflection survey of the Trent gas field. The crater is about 2.96km in diameter, and one would normally expect an impact structure of this size to be a deep bowlshaped crater like New Quebec or the Barringer Crater, with a depth-diameter ratio of about 1:8. However, in the case of Silverpit our expectations are exceeded. The depth of the crater is 300 m, yielding a depth-diameter ratio of 1:10, but it also has a conical central peak in the Cretaceous rocks beneath the crater floor; this central peak is about 250 metres high. Outside the crater is a zone, with an external radius of 4km, occupied by
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Epilogue: The Silverpit Crater
extensional fault scarps and tilted fault blocks; the seismically detected stratigraphy in this zone is cut up by the faults but is not completely destroyed. This zone might be identified with the peripheral depression and the ring of rim terraces found in terrestrial complex impact structures and large lunar craters, whereas the 3-km inner crater, where the Cretaceous stratigraphy has been obliterated, would correspond to the central uplift and transient crater. According to this interpretation, the full diameter of the Silverpit structure is 8 km, rather than the 3 km given by Stewart and Allen (2002). However, in view of the unique morphology of the basin, this interpretation of its structure and dimensions can only be tentative. The most striking feature of the Silverpit is that it is surrounded by about 10 concentric ring graben, with a spacing of about 800 metres and an outermost diameter of 19-20 km. This morphology is unique among known terrestrial impact structures; it is, however, similar to the multi-ring basins observed on the Jovian satellites Callisto and Europa, such as Valhalla and Asgard on Callisto and Tyre and Callanish on Europa. These multi-ring basins were formed by impacts on an icy crust on top of a global ocean, and it is probable that the Silverpit crater was similarly formed by an impact on a Chalk interbedded with clays in its lower levels. As the rim of the transient crater collapsed during the modification stage, the incompetent clay flowed towards the crater, causing subsidence and concentric fracturing of the overlying competent rock. It is even possible that the stratigraphy beneath the crater floor is similar to that in the Broomhills inlier in the Rochford Basin, with a melange of interstratified Chalk and clay. There must have been similar impacts into soft sedimentary rocks elsewhere on the Earth, and it is likely that more will be discovered during seismic exploration for oil and gas in deep sedimentary basins. However, for the time being the Silverpit crater is unique, and its discovery shows that there are new impact morphologies to be identified and analysed. Silverpit crater is thought to be 60-65 Myr old, probably belonging to the Danian (Early Paleocene) age and contemporary with the volcanic eruptions of the North Atlantic igneous province. It is of similar age to the Chicxulub structure, but the
Epilogue: The Silverpit Crater
WBMBB&BB
uncertainties in the age are too large for one to say whether it was formed as part of a bombardment episode at the end of the Cretaceous period. The Palaeocene volcanic eruptions in the Hebrides and Northern Ireland were accompanied by tectonic subsidence in the North Sea basin, and at the time of the impact the Dogger Bank was on the western flank of a 50-300 m deep marine trough that accumulated turbidites and shales transported from the English mountains to the west. The great thickness of Tertiary sediments that has concealed and preserved the Silverpit crater testifies to the extent of the erosion on land that has largely destroyed the impact craters that must have existed on the British and Irish mainlands. The asteroid or comet that formed the crater is thought to have had a diameter d = 120-200 m and an impact energy of 100-500 Mtons. An impact of this energy would produce an earthquake with a Richter magnitude MR = 7-8, and a tsunami with a deepwater height of 1-5 metres at r = 1,000 km. In the confined waters of the North Sea, the tsunami may have had a run-up height h> 100m when it struck the surrounding coasts. 1 Although no Danian sedimentary rocks are known from the British mainland, either because they were never deposited or because they were eroded away by this very tsunami, some record of this devastating impact event should be traceable in the Lower Paleocene sediments of the North Sea. Stratigraphic identification of the 'impact level' would make it possible to set more restrictive limits on the age of Silverpit, and to explore the long-range effects of the formation of this unique crater.
:
If the diameter of the crater is D = 8 k m the diameter of the impactor was d~300m, the energy was E~ 2000Mtons, and the tsunami was » 1 0 0 m high.
Appendix 1. Historical Fireballs in Britain Description
Evaluation
Mont. St-Michel
'Giant' hurls rocks in sea & sinks ships
/
Helston (Cornwall)
Dragon/devil drops rock on town
/
Wilmington (E. Sussex)
Giants hurl rocks at each other
/
St. Albans (Herts.)
Dragon lives in 'cave' near town
/
Place
Date
Penrith (Cumbria)
1000 AD?
Giant lived in 'caves,' killed men & cattle
/
Snowdonia
5 t h cy. AD?
Giants hurled steel
/
England & Wales
AD 457
Blazing globe, like dragon
b
Kent?
ca. 525-50
Dragons, lions, etc. fighting in air
f, c (volcanic aerosols & clouds?)
Appendix 1 Place
Date
Description
SW England?
-540 AD
Dolorous Stroke. Great noise, trees felled, people killed, fires, Waste Land.
Barking (Essex)
664
Huge light from sky, like daylight
b
Coldingham (Berwlcks)
679
Monastery destroyed by 'fire from Heaven'
c (fire?)
Sockburn (Durham)
'Saxon times'
'Fiery dragon'
/
England? 1
776
Two 'flying luminous shields'
b
England
788
Fiery meteors fell to Earth
b
Northumbria
793
Fiery dragons in sky
b
England
1012
Burning tower fell from heaven. Destructive marine inundation (tsunami?)
c (storm surge?)
Worcester, Derby
1048.5
'Earthquake'; damage by 'wild fire' in Derbyshire.
e
1077.4.16
Rotating sphere of fire fell in sea Comet or blazing star
c (tornado, ball lightning) c (comet?)
England
1087-88
Fiery dragon in air
b
Winchcomb (Gloucs)
1090/1.10.1 Tempest of thunder & lightning damaged church
England
1092.10
'Comet'. Salisbury Church damaged by thunder
c (thunder storm?)
England
1094
Flashes of fire fell from sky, etc.
c (aurora, mock Suns?)
Northumbria
1067
Another reference locates this event in Germany.
Evaluation '/
(earthquake?)
c (thunderstorm?)
Appendix 1 Place
Date
Description
Evaluation
England
1095.12.7
Stars fell from heaven all night
b
England
1097/8.10
Heavens aflame all night. Goodwyn Sands swallowed up by sea.
e (aurora, storm surge?)
Christchurch (Hants.)
1113
Fiery dragon destroyed ship and houses
b
England
1115
Blazing star. Earth 'cast forth flames'
e
England
1122.8.26
Great fire in NE, divided into four parts
c (aurora?)
Scotland
1165.8.
Comet with two long tails
c (comet?)
St. Osyth (Essex)
1170.3.9
Large dragon in air, burnt house to ashes
c (lightning?)
England
1177.11.30
Fiery dragon flying east to west
b
England
1178
Shower of blood, and great star
c (aurora?)
Darlington (Durham)
1178.12.25
Great tower fell from sky, with much noise. Formed pit of Hell's Kettles.
c (ground subsidence?)
London
1193.6.20
Bright white ball with thick black cloud
c (storm cloud, tornado?)
England
1208, 1224
Birds carried live coals in bills and set fire to houses
e
Gravesend (Kent)2
1211,1270? Ship's anchor descended from sky into churchyard
e
England
1219
Great comet, and 2 fiery dragons
e
South coast of England
1233.6
Two great dragons seen fighting in air
c (water spouts?)
2
Other versions of this tale locate it in 'Cloera' (Clogher?), Ireland and in Bristol in 1270.
Appendix 1
BO
Place
Date
Description
Evaluation
Hereford and Worcester
1239.7.24, sunset
Great star like torch, travelling S to N. Gave off smoke and flashes
b
St. Albans (Herts.)
1254.1.1
Large 'ship' appeared in clear night sky
c (aurora, moonlit cloud?)
Bristol
1270
Strange flying burning craft
e (c/Gravesend event)
Newcastle
12745.12.7?
Earthquake, thunder & lightning, blazing star, comet like dragon. Great terror.
c (storm, earthquake?)
Byland Abbey (North Yorks)
1290.10.20?
Shining object flew over abbey, caused terror
e (modern hoax?)
Ireland
1294
'lightning and meteors destroyed corn'
c (thunderstorms?)
England
1320
General earthquake; much noise
c (earthquake?)
Uxbridge (Middx.)
1322.11.4
Pillar of fire, moving from S to N.
b
Leics. -Northants.
1387/8.11/
Fiery objects in sky, like wheels and beams
e (aurorae, comet?)
12
Leicester
1389.4.
Flying dragon seen in many places
c (aurora?)
Scotland
1402.1-3
'Blazing star' or 'comet'
c (comet?)
Weymouth
1456
'Great cock' emerged from sea, crowed three times, and vanished
e
Ludham (Norfolk)
16th cy.?
'Roaring dragon,' lair blocked by stone (meteorite?)
/
Blythburgh to Bungay (Suffolk)
1577.8.4 (9-10 am)
Fiery monster ('Black Shuck') appeared during thunderstorm. 4 dead; much damage
c (thunderstorm, tornado)
London
1593.5
Flying dragon, surrounded by flames
e (thunderstorm?)
Appendix 1 Place
Date
Description
Evaluation
Wells (Somerset)
1596.12.5
Exploding fireball, noise like cannon fire
c(ball lighting?)
Wldecombe (Devon) 1638.10.21 p.m.?
c (tornado, ball Great darkness, with thunder & lightning. lightning?) 4 killed; church damaged; many stones fell
Antony (Cornwall)
1640.5.24
'Fiery ball' struck church. c(ball 14 injured lightning?)
S England
1643.5. 10-11
Luminous cloud, like sword, point to N.
c (aurora?)
East Anglia
1646.5.21, afternoon
Luminous phenomena and thunder-like noise. Meteorite fall near Soham, Cambs?
c (tornado, lightning?)
Standish (Gloucs.)
1650.11.30
'Blue-hilted fiery sword, with long flame
Markfield (Leics.)
1659.9.7.
Thunder and lightning with thick clouds. Fire in air, destroyed houses and uprooted trees
c (tornado, thunderstorm?)
Worcester
1661.10
'Monstrous flaming things' seen in sky. Earth tremors.
c (aurora, earthquake?)
London
1668.5.16
Very bright rocket-like fireball. Meteorite at Henham (Essex)?
Athlone (Ireland)
1697.10.27 Night
Fireball blew up magazine of Athlone Castle. 3 terrifying claps of thunder. 8 killed
Devon
1704.1.8?
V-shaped formation of dark objects with trails seen in daylight. Disintegrating fireball?
England
1716.3.6
'Great meteor'
England and N Europe
1719.3.19
Brilliant meteor (M« -12.5), with detonations. Meteorite fall near Tiverton?
c (lightning?)
e (aurora?)
Appendix 1
im
Place
Date
Description
Aynho (Northants.)
1725.7.3
Disintegrating fireball during 'thunder-storm,' Meteorite fall at Mixbury (Oxon.)
Halstead (Essex)
1731.3.12
Fireball & stone-fall reported.
Springfield (Essex)3
1732/3.8.15 Fireball, E to W, fell in English Channel. Possible meteorite fall.
Fleet (Dorset)
1733.12.8
Daylight fireball with train, heading E to N
Kilkenny (Ireland)
1737.12.5
Exploding bolide. Whole sky seemed in flames; earth tremors.
Dorset to Derbys.
1738.8.29
Daylight fireball. Explosions heard at Reading
Southern England
1741.12.11? Conical fireball with smoke trail & explosions
London
1750.3.19
Fireball and earthquake. Houses wrecked. Westminster Abbey damaged; stones fell.
Evaluation
c (whirlwind?)
a
a
(earthquake?)
Edinburgh
1750.6
Vast ball of fire, moving slowly
b
Northampton
1750.10.11
Ball of fire & earthquake. Four shocks and loud noise; MMI V-VI.
d (earthquake?)
Glasgow
1752.12.25
Exploding ball of fire, followed by great shower of hail
b
Dublin
1754.2.26
Fireball as bright as Moon
a
3
There appears to be some confusion here. Hey (1966) puts the Springfield event 3 days after the Halstead meteorite, so this may be another observation of the same event with an error in the date. However, Greg puts the Springfield fireball at least a year after the Halstead fall.
Appendix 1 Place
Date
Description
Evaluation
Cambridge to Ross
1759. 11.25/6
Dazzling meteor, as light as day.
a
Coupar Angus to Blairgowrie
1767.9
Luminous body with thick dark smoke, caused much damage
d (tornado?)
Coventry
1769.1.27
Ball of fire seen by daylight
Steeple Ashton (Wilts.)
1772. 6.20/22
Exploding fireball with black smoke and sulphurous smell.
Northallerton
1773.8.8
Large fireball, W-E; houses shook
Beeston (Notts.)
1780.4.11?
Prominent meteor. Meteorite fall?
Shetland Islands to Burgundy
1783.8.18 2114
Great fireball, whole sky illuminated. Explosions heard from York to Bath.
S. England
1786.9.2
Bright fireball in hurricane
c (tornado?)
Edinburgh
1787.9.1/11
Exploding fireball
a
Scotland
1792.11
'Fiery red dragons,' from N to E
b
Derby
1795.11.18 (2310 hrs)
'Meteor' accompanying earthquake. Felt Bristol., London, York.
d (earthquake?)
Hereford
1799.11.12
Red pillar of fire, N-S, caused alarm
b
Steeple Bumpstead (Essex)
1800.4.1.
Detonating fireball, SW-NE. Hissing sound
a
Horringer Mill, Ickworth (Suffolk)
1801. 10.23/30
House set on fire by 'meteor.' Meteorite fall?
c (lightning?)
Cambridge, Swaffham (Lines.)
1803.5.9 (1400 hrs)
Daylight fireball with train, SW-NE. Loud noise & hissing sound.
a
d(ball lightning?)
Appendix 1 Place
Date
Description
Evaluation
East Norton (Leics.)
1803.7.4
Detonating fireball & meteorite fall. Much damage to building.
c (ball lighting?)
London
1803.11.13
Dazzling fireball, noise like distant thunder
a
Swansea, Shrewsbury
1805/ 6.10.14
a Great fireball, lit up large area of countryside. Fell NW of Shrewsbury
Basingstoke
1806.5.17
Detonating meteor. Meteorite fall?
Surrey & London
1814.12.2
Great meteoric light, equal daylight
Surrey & Oxford
1816.3.23
Fireball, S-N, rumbling noise, 5 mins.
a
Edinburgh, Comrie
1816.8.13?
Crescent-shaped luminous body
c (aurora?)
Lincolnshire
1818.2.6
Daylight fireball, NW-SE? Rumbling & hissing sounds.
a
Inverness
1818.2.20
Meteor with streamer. Seismic effects
c (earthquake?)
Bushey-Stanmore (Herts.-Middx.)
1818.4.26 (1230 hrs)
Tornado and 'meteorite fall'. Trees & buildings damaged
d (tornado)
Southampton & N France
1822.8.6
Large fireball with serpentine trail
a
Gloucs., Wilts.
1826.5.12
Blue fireball. Meteorite fall?
a
Allport (Derbys.)
1827/8.8/9. Exploding fireball. Carbonaceous material reported to have fallen
Horton (Lanes.)
1828.9.7
Fireball, bright as sunlight, travelling eastwards
a
Glasgow & Stirling
1846.12.21
Exploding fireball with tail, S-N. 'As light as day.'
a
a
b
Appendix 1 Place
Date
Description
Evaluation
Buckinghamshire
1848.1.27
Silver-white daylight fireball with tail
a
Dover Straits
1852.12.17
Dull red fireball (SE-NW) exploded; object fell into sea off Dover.
c (ball lightning?)
Hereford
1858.12.2
Afternoon daylight fireball
a
Carlisle
1869
Pillar of fire, E-W. Explosion, sound like cannon. Great heat felt
b
Nairn
1872.11.3 (1730hrs)
Brilliant fireball, like daylight, ESE-WNW. Sound like 3-4 distant cannon
a
Bristol, Portsmouth 1872.11.3 (2115 hrs)
Very bright meteor, heading E, followed by explosion
a
Seven Stones (Cornwall)
1872.11.13 (0200 hrs)
Fireball exploded over lightship; deck covered with cinders
b
Banbury, Slough
1872.11.30 (1200-1400)
'Luminous tornado' caused damage at Banbury; daylight meteor seen at Slough.
c (tornado, ball lightning)
Liverpool, Chester
1873.2.3 (2158 hrs)
Large blue detonating a meteor, E-W. Sound like thunder or distant gunfire.
Holyhead
1874.5.19 (0050 hrs)
Brilliant oval meteor, throwing off sparks, moved N'wards. Crackling sound heard.
Hexham (Northumberland)
1874.8.1. (2300 hrs)
Massive ball of intense d (a hoax) light & pear-shaped fireballs. 137-kg meteorite fall near Hexham
Waterford to Hampshire
1877.1.7
Brilliant slow-moving double fireball, W-E? Lit up landscape; meteorite fall?
a
a
WSL
Appendix 1 Place
Date
Description
Evaluation
Chester
1877.11.23
Fireball, explosions like artillery fire; doors & windows shaken.
S England, NW Europe
1882.11.17 (1805 hrs)
Green torpedo-shaped object, travelled ENE-WSW.
c (auroral beam?)
Norfolk to Berkshire
1887.11.20 (0820 hrs)
Daylight fireball. Explosions & earth tremors (MMI V-VI). Damage to buildings
a
Twickenham (Middx.)
1889.8.5 (-1230)
Yellow rod-like object exploded, damaged house. Meteorite fall?
c(ball lightning?)
Southern England
1890.12.14 (09/21.42)
Dazzling fireball, with thunderous report. End-point at h - 13 km over Brentwood.
a
Worcester
1894.1.25
Brilliant fireball, 'like daylight.' Sound heard to 60 km. Earth tremors.
a
Culdaff (Donegal)
1895.8.24
Daylight fireball, W-E. Explosion, boy injured.
c (ball lightning)
Oxford to Scarborough
1895.8.31 (-2000 hrs)
Fireball, brighter than Venus, moved slowly eastwards
a
Hereford
1896.12.17 (0535 hrs)
Great 'meteor', 'brighter than daylight.' Earth tremors, severe damage. Sound heard to 180 km (Liverpool, Lincoln, Exeter).
c (earthquake?)
Essex
1904.4.13 (a.m.)
Fireball exploded during 'thunderstorm.' Meteorite fall? Impact pits found.
c (ball lightning?)
Street (Somerset)
1904/5.11.18 Triple detonation (1500 hrs)
b
Appendix 1 Place
Date
Description
Evaluation
Barsham (Suffolk)
1906.2.8 (sunset)
Yellow-white fireball, loud explosion. Church damaged.
c(ball lightning?
Morchard Bishop (Devon)
1906.5.13
Dark fireball exploded into 'thousands' of red-hot sparks.
c(ball lightning?)
Peterborough
1909.3.23 (0510 hrs)
Oblong body with light passed overhead. Buzzing sound
b
Pontypool to Norfolk
1909.5.19 (-2300 hrs)
Very bright fireball crossed country, west to east.
b
Bath to Gloucester
1912.3.6
Triple-headed fireball, S-N
a
South Wales
1913.1.17 (-1700 hrs)
Huge 'airship' leaving dense smoke trail
b
Appley Bridge (Lancashire)
1914.10.13 (2045 hrs)
Large detonating meteor. Explosion; earth tremors, windows shook. Meteorite fall.
a
Cambridgeshire
1916.1.31 (2045 hrs)
Fireball & explosion. Meteorites reported over radius of 30 miles.
b
Strathmore, Perthshire
1917.12.3 (1318 hrs)
Brilliant exploding fireball, SE-NW; deto-
a
Aberystwyth
1924.12.31
Fireball scattering fire, c (ball loud noise. Houses shook, lightning?) window pane cracked.
Tetbury (Gloucs.)
1929.12.12
Meteorite fall kills three sheep.
b
Yorkshire & North Wales
1931.4.14 (1153 hrs)
Daylight fireball & explosions. General alarm. Pontlyfni meteorite fall.
a
Coleford (Gloucs.)
1946. 11.21/28
Flash & explosion. Buildings shaken, windows shattered, lights put out.
b
Appendix 1 Description
Evaluation
Place
Date
Nailsea (Avon)
1954.7.6? Exploding fireball, (1600-30 hrs) windows splintered, ovens burnt out, boy knocked off bicycle
Irish Sea
1955.3.24 (-1915 hrs)
Brilliant meteor (S-N), with smoke trail. Seen over Wales & England. Sounds?
Charlton (Wilts.)
1963.7.10 (0600 hrs)
Luminous object; explosion; 2.4-m crater. No meteorite found.
Warminster (Wilts.)
1964.12.25 (0612 hrs)
Loud noise from above, like falling chimney stack.
Midlands
1965.12.24 (1620 hrs)
Disintegrating fireball. Explosions; slight damage. Barwell meteorite fall.
Stevington (Beds.)
1966.07
Flash & explosion; tiles shaken off roofs. Crater 6 m long formed.
Crail (Fife)
1966/8.8 (p.m.)
Daylight fireball with c(ball trail. Loud explosion. lightning?) Alarm; building damaged.
c (lightning?)
c (lightning?)
a
c (tornado?)
Hook Head (Ireland) 1968.3.24 (1157)
Loud rumbling sounds heard & large object fell into sea at time of air crash.
e
Bovedy (Northern Ireland)
1969.4.25 (2122 hrs)
Disintegrating fireball with long tail. Sound heard from Bristol Channel to Limavady. Meteorite falls at Sprucefield & Bovedy.
a
Sidmouth (Devon)
1970.8.12 (evening)
Red fireball explodes; many televisions damaged.
c(ball lightning?)
Scotland
1973.12.27
Bright fireball, with sonic boom. Meteorite fall in N. Ireland?
Appendix 1 Place
Date
Description
Berwyn M o u n t a i n s (Clwyd)
1974.1.23 (2038 hrs)
Fireball, N to S. Explosion c (earthquake?) h e a r d to 100 k m E a r t h t r e m o r s (M-3.3). Some damage.
Bala Lake (Gwynedd)
1974.10.4
Fireball like 'blinding sun'; grey powder (meteorite dust) fell.
b
Berwyn M o u n t a i n s
1976.8.6
Sky lit u p ; muffled explosion; e a r t h s h o o k
e
Ford (Wilts.)
1977.2.23 (0345 hrs)
Blue-green flash & loud explosion. General a l a r m .
c (ball lightning?)
W. Midlands, Anglesey?
1977.10.2 (after 0000)
T o n g u e of flame' or silvery disc seen. ' H u m m i n g roar' h e a r d .
b
B a c u p to O r m s k i r k (Lanes.)
1979.2.24 ( - 0 2 4 0 hrs)
Orange fireball moving W. Roaring s o u n d a t Ormskirk; d a m a g e a t Scarisbrick
c (aircraft)
Aberdeen to Hampshire
1979.3.18 (1845 hrs)
Brilliant fireball with tail, N-S. Meteorite fall a t Marlborough?
a
Rickmansworth (Herts.), H a r s t o n (Cambs.)
1979.11.29 (0325 hrs)
Red fireball with tail
b
Chester to Sheffield
1980.4.9 2 3 3 5 UT
Brilliant fireball (m= - 1 7 ) . a T h u n d e r o u s noise in E Cheshire & N Staffs.
S o u t h Wales to Hertfordshire
1981.5.25 (2217 UT)
Bright fireball (m= - 1 0 ) . Fragmentation & sound effects a t Bristol
a
Cwm-y-glo (Gwynedd)
1987.6.11 (1315 hrs)
Red fireball, explosion, cottage d a m a g e d
c (ball lightning?)
Kirkby-in-Ashfield (Notts.)
1987.11.12 (0130 hrs)
Fireball(?), bright flash, c (lightning?) explosion h e a r d for 16 k m , block of flats wrecked, wood set on fire(?).
Aberdeen
1991.3.26 ( - 1 1 2 0 hrs)
Atmospheric explosion, buildings rocked. S o u n d h e a r d 2 6 k m away.
Evaluation
b
Appendix 1 Place
Date
Description
Grampians to Lincolnshire
1995.7.28?
Brilliant fireball 'like floodlights.' Meteorite fall?
Boyle, (co. Roscommon)
1996.5
Object fell in Lough Key
c (helicopter crash?)
Lewis, Outer Hebrides
1996.10.27 (1600-1700)
Luminous object with smoke trail. Explosion; object fell in sea.
c (re-entering satellite?)
Peak District (S. Yorks.-Derbys.;
1997.3.24 (2206 hrs)
Fireball, explosion, & earth tremors. Orange glow, smoke plumes; houses shook
c (low-flying aircraft?)
Scotland & Northumberland
1997.9.23 (0758 UTC)
Flashes in sky, loud bang, b windows shook. Meteorite over N. Scotland
Belleek, Fermanagh 1997.12.13 (0500 hrs)
Fireball, explosion, hissing sounds 1-2 m crater
Evaluation
d (terrorist explosion)
Cornwall to Somerset
1998.3.15 (evening)
Brilliant fireball, lit up a sky. Sound effects. Meteorite fall near Yeovil?
Leighlinbridge, Co. Carlow
1999.11.28 (2210 hrs)
Fireball,'bright as full Moon', & explosions. Meteorite fall.
a
Appleby to Newcastle
2000.1.9 (0156 UT)
Fireball with tail, m= - 1 5 to - 2 0 . Explosions heard at Morpeth.
a
Haarlem to Lines. (Mablethorpe)
2001.10.27 (1920 hrs)
Brilliant, green fireball, m ~ - 1 8 . Flares, audible explosion? meteorites Humberside?
a
rag
Appendix 1
Evaluations: a — Authentic fireball, reported in astronomical literature. b — Description matches fireball, but slight doubts of authenticity, e.g. a single unconfirmed report, or event not mentioned in astronomical literature. c — Probably a fireball, but could be a meteorological event or earthquake. Alternative interpretations are given in brackets. d — Probably a terrestrial event rather than a fireball. e — Description too vague for certain identification. Possible causes given in brackets. / — Legendary event. Possibly genuine, but description of little scientific value.
Appendix 2. Possible Impact-Induced Disasters Place
Date
Evidence
'Lyonesse' & - 1 0 , 5 0 0 BC N Atlantic b a s i n
Great wave d r o w n s l a n d W of Cornwall. Climatic cooling, extinctions, end of Palaeolithic, Carolina Bays, Atlantis?
World-wide?
- 3 1 1 4 BC
T-R. M a y a n world beginning. Stonehenge I. Encke-Taurid comet b r e a k u p (Steel, 1995)
Ireland, etc.
2 3 4 5 - 5 4 BC
T-R. IK-L. 1 Collapse of Early Bronze Age civilisations.
W.England, Wales
- 1 6 2 8 BC
T-R. IK-L. S u b m e r g e d forests, coastal i n u n d a t i o n s .
1
Volcanoes, e t c ?
Greenland ice acid layer.
Thera/ Santorini
T-R. Tree ring evidence of environmental deterioration (Baillie, 1999). IK-L. Disaster mentioned in Irish king-lists (Baillie, 1999).
Appendix 2 Place
Date
Evidence
Volcanoes, etc?
World-wide?
-1159 BC
Decline in Wessex Culture T-R. IK-L. Collapse of Bronze Age civilisations.
Hekla
Ireland, etc.
-207 BC
T-R. I-KL (cattle plague).
Britain, NW Europe
367 AD
Breakdown of Roman government. Inundation of Scilly Isles (and Somerset?). Mediterranean 'earthquakes' and tsunami.
East Anglia, Midlands, Isle of Axholme
440s AD
Depopulation & deforestation. Global climate cooling.
SW England?
-540 AD
Dolorous Stroke. Great noise, trees felled, people killed, fires, Waste Land.
Thames Estuary 1099.11.1/2 'Great tide' destroyed villages, drowned people & Netherlands & animals. Thousands killed? During peak in Taurid fireball flux.
Krakatau?
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m
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Index Names of meteorite craters and impact structures are given in italic; the names of authors are given in capitals. Sections indexed in bold deal with the subject in detail. Abberley Hills 47-9, 66, 70, 78, 82-3 Airbursts 14, 139-44, 150, 153, 160-1 Acraman 26, 27 ALBRITTON, C.C. 6 ALLEN, P.J. 100, 175-6, 200 Ames structure 23 ANDERSON, J.G.C. 91, 128, 133, 195 ANDERTON, R. 107-8, 195 Ankerite 41 Aouelloul crater 3 Aorounga. 9, 10, 15 Aptian age 107-10 Araguainha Dome 9 Arthurian legend 148-9, 158 Ashby Anticline 34-5, 38, 46, 51, 53-9, 64, 69, 73, 78 ASHE, G. 159, 164 Asteroids viii, 8, 15, 28, 43, 79-81, 104, 122, 135, 140-1, 168, 172-3, 177 Astroblemes 13, 30, 92, 104 Atlantic Ocean 36, 108, 133, 144, 160, 162-4, 168, 172-3, 193 Atlantis ix, 144, 162-74 Australia 3-4, 9-10, 12, 17, 26, 29, 33, 89, 149, 171
Austria 5, 19, 104, 149 Avon (Missouri) 15, 23, 25, 42 Axholme, Isle of 159, 194 Azuara 19, 26 Bagshot Beds 97-8, 102-3, 106, 109 BAILEY, M.E. 79 BAILLIE, M. 158-60, 167 BALDWIN, R.B. 5, 7, 19, 22-3, 40, 87, 89, 100-1 ball lightning 143, 154, 156, 179, 182, 184, 186-90 Bangweuhi Basin 16-17, 25 Barringer Crater 4, 11, 13-14, 28, 147-9, 175 BARRINGER, D.M. 4 Barytes (barite) 41-2, 62-3 Basalt 51, 64, 110, 115, 133 BEAUMONT, W.C. 5 Bee Bluff 23 BENNISON, G.M. 74, 128 Beowulf 158 Berwyn Mountains 128, 190 BEVAN, A.W.R. 142 BLANC, P. 64 Bodmin Moor 32, 145-7, 149 Bohemia 16, 19, 26-7, 85
M Bolide 154, 183 BOON, J.D. 6 Bosumtwi crater 4 Boxhole crater 3, 146 BRANCA, W. 5, 7 Breccia 5, 11, 12, 16, 20, 33, 41-2, 51, 57-64, 68-73, 75, 78-9, 81, 91, 100-1, 117, 120, 149 Brent crater 7, 11 Bressay 41-2 BRONSHTEN, V.A. 140 BROOKESMITH, P. 127 Broomhills inlier (Essex) 99-101, 176 BROWN, G.C. 120 BUCHER, W.H. 5-7, 23-4 Bunte Breccia 70, 72 Bunter (Permo-Triassic) 53-4, 57-8, 70, 73, 76-7, 79 Bushveld 16, 77, 116-18 Calcite 41, 62-3 Calvin 23 Cambrian 24, 48-51, 56, 58-9, 61-2, 67-70, 72-3, 122, 128 Cambrian-Ordovician boundary 122 Campo del Cielo 3, 13 Canada 7, 10-12, 17-18, 25, 33, 104 Carboniferous Limestone 38, 47, 51, 54-7, 61-2, 68, 70, 93-4, 132, 134 Carboniferous system 34, 46-7, 49-51, 53, 58-9, 61-2, 65, 68-74, 78, 81-2, 91, 93-5, 115, 132 Carolina Bays ix, 171-3, 193 Carswell 110 CARTER, N.L. 19, 22, 24 Chalk 35, 91, 97, 99-101, 105, 125-6, 129, 176 CHAO, E.C.T. 8 Charlevoix 25, 63 Charnwood Forest 34-5, 38, 45-6, 50-1, 54-9, 60-4, 67, 69, 73, 78, 129 Chesapeake Bay 9 Chicxulub 9, 27, 176 CINTALA, M.J. 117, 119, 122 CLASSEN, J. 9, 16, 24, 33 Clay 19, 63, 70, 97-100, 102-3, 107-8, 116, 131, 158, 176 Clearwater Lakes 7, 12, 22, 25, 43, 119
Index Clent Breccia 58, 70, 72 CLUBE, S.V.M. 122, 156, 159, 163 Coal Measures 34, 47, 50-1, 53-8, 61-2, 65, 68, 73-6, 79, 93-7, 118, 180 Coesite 7, 43 COLLINS, A. 164, 171-3 Comets viii-ix, 15, 65, 79-81, 122, 135, 140, 158, 160, 163, 172-3, 177, 179-81, 193 Complex impact structures 6, 11-15, 54, 69, 126-7, 129-30, 139, 176 Central uplifts 5, 6, 9, 11-12, 23, 34-5, 40, 54, 59, 63-4, 67, 80, 86, 88-9, 101-2, 105-6, 123, 129-30, 133, 176 Peak-ring structures 11, 44, 67-8, 73, 78-9, 120, 133 Ring anticlines 6, 23, 73, 78, 101, 105, 129, 133 Ring synclines 6, 40, 89, 92, 101, 120, 127, 133 Continental shelf 10, 20, 27 Cornwall 77, 102, 124, 145, 147, 166, 168-9, 173, 178, 182, 186, 191, 193 Craters viii, 3-11, 13-20, 31-3, 35-6, 38-40, 42-3, 67, 70, 72, 77-81, 90, 92, 100-1, 104-6, 109-11, 119-20, 135, 139-52, 154, 161, 172, 175-7, 189, 191 chains & groups 13-20, 23, 25, 32, 126, 148, 150 depth/diameter ratios 20-1, 79-80, 104, 147, 149-50, 175 surface densities 17-18, 21-30 Cretaceous 27, 29, 36, 46, 49, 100, 105, 107, 109-10, 130, 175-7 Cretaceous-Tertiary boundary viii, 9 Crooked Creek 5, 7, 15, 23, 63, 90 CROSSEY, L.J. 63 Crypto-explosion structures 7, 8, 20, 22, 33, 40, 42, 72, 90, 95 Crypto-volcanic structures 5-8, 23, 31, 90 Dalgaranga 3, 9, 149 DALY, R.A. 7 Darlington 149, 180 Dartmoor granite 37, 126 DAVISON, C. 91
Index Dean, Forest of 34, 46, 48, 82-3, 92-5 Decatwrville 5, 7, 15, 23, 100-1, 104 Deep Bay 3, 7, 11, 31, 33, 40, 110 Dellen 4 Dellenite 4 Des Plaines 10, 23 DEUTSCH, A. 117 Devonian 34, 36, 40, 48-9, 51, 65-6, 73, 82, 85, 91, 94, 114, 125, 132 DIETZ, R.S. 7, 117 Dlorite 34, 51, 61-2, 73 Dolerite 51, 64, 77-8, 132 Dolorous Stroke 158, 179, 194 Dozmary Pool 32, 145-9, 152 Dragons 151, 154, 172, 178-81, 184 DUFF, P. McL. D. 117 Duolun 26 Durham 76-7, 179-80 Dycus 22 EARP, J.R. 86-7 Earthquakes vlii, 90-1, 142-4, 150-1, 154, 156, 159-60, 162, 165, 173, 177, 179, 181-5, 187, 190, 192-3 East Grampian Basin 35-6, 112-23 Eddystone Bay 124-6 Etgygytgyn 10 Eltanin structure 10 EMANUEL, K.A. 110 EMEUYANENKO, V.V. 79 Von ENGELHARDT 4, 19, 94 English Channel 27, 108, 183 Enville Beds 50, 58, 70-1, 73, 75-6, 78-9 Eocene 25, 36, 37, 97, 102-3, 109 Epldiorite 116 Essex 35, 37, 46, 96-105, 157, 179-80, 182-4, 187 Estonia 4, 5, 10, 11, 18, 24, 140, 149 Evaporites 45, 49, 76, 92, 149 Everglades 9, 25 EYTON, J.R. 172-3 FERGUSON, J. 90 Finland 4, 18, 21, 24, 26 Fireballs viii, 28, 140, 142-4, 150, 152, 153-61, 178-92 F1RSOFF, V.A. 63 Firth, The 33, 39-44
ESH FLINN, D. 33, 39 Flint 99 Floods 110, 158-60, 162-74 Flynn Creek 7, 15, 22, 92, 94 Fohn structure 10 FOOTE, A.E. 4 FORD 56, 62, 64 FORTES, A.D. 9-10, 18, 22, 29 FRAAS, E. 5, 7 France 19, 21, 26-7, 107, 149, 169 FRENCH, B.M. 72, 118-19 Frombork 18 FUDALI, R.F. 63 Fuller's earth 106-10 Furnace Creek 15, 23, 25 Gabbro see Younger Gabbros GALLANT, R. 18, 90, 125 GERDEMANN, P.E. 15, 100 Germany 4, 5, 10, 16, 18, 21, 26, 43, 70, 104, 158 GILBERT, G.K. 4 Glasford 23 Glover Bluff 23 GODA. M.P. 14, 28, 43, 79-81, 104, 142, 150, 160, 173 Gosses Bluff 9, 10, 12, 40, 89 GRAHAM, A.L. 19, 24, 39, 140 (Lower) Greensand 107-8 GREG, R.P. 155 GRIEVE, R.A.F. 7-9, 17, 22-3, 29, 89, 94, 117-19, 122 gypsum 150 Haffield Breccia 70-2 HAINING, P. 159 HAINS, B.A. 58, 61, 69-70, 74-5, 78, 86-7 HAMILTON, W. 117 Hampshire Basin 37, 102-3, 108-9, 125-6 HANCOCK, G. 170 HARLAND, W.B. 76, 85, 107 HARTUNG, J. 9 HATCH, F.H. 61, 116 Houghton Dome 10, 12, 104 HAUGHTON, S. 74 Haviland 3, 9, 23, 149 Hazel Green 15, 23 Heft's Kettles 149-51, 180 Henbury 3, 13, 31, 146, 148-9 HENSHAW, C. 140
E2 Herau.lt craters 19 Herefordshire 34, 48, 81-3, 88, 91-2, 95 HEY, M.H. 3, 22, 24 Hicks Dome 5, 15, 23, 63 Hico 23 HILLS, J.G. 14, 28, 43, 79-81, 104, 142, 150, 160, 173 Hltchln 129-30, 134 Holleford 7, 11 Holocene 5, 14, 18-19, 170 Honduras crater 140 Hope Mansell Dome 34, 36, 83, 9 2 - 5 HORTON, A. 58, 61, 69-70, 74-5, 78 H6RZ, F. 70, 72 Howell 22 HUGHES, D.W. 28, 39, 79 Hydrothermal alteration 62-4 Hypercanes 110 'hypo-astrobleme' 13, 30, 92 Rumetsy 18 Impact melt 8, 11, 63-4, 72-3, 77-8, 117-23 Impactite 78 'infra-impact structure' 13, 30 Inundations see Floods Irish Sea 27, 168, 189 'Isis Parlis' 152 Janisjarvi 4 Jeptha Knob 5, 22-3, 43 JOHNSON, M.R.W. 117-18 Jurassic 16, 27, 36, 46-7, 49, 65-6, 131 Kaalyarv 4, 5, 11, 18, 20, 31, 149 Kara-Kul 17, 121 KEARSLEY, G. 152 Kentland 5, 7, 23 Kilmichael 23 KOEBERL, C. 9 KEYS, D. 158 Kqfels Hollow 5, 6, 19, 92, 104 Lac Couture 7 Logo di Tremorgio 19 Lake Hummeln 11 Lake Mien 4, 12, 31 LANGE, J.-M. 16 Lappqjdrvi 4, 11, 31 LEWIS, J.S. 142, 156-7
Index LIANZA, R.E. 15 Lickey Hills 66, 68-71 Lightning 143, 148, 154, 156-8, 160, 179-90 Limestone Mountain 24 Logoisk 18, 26 Longford 134 Lough Ree 134 Lukanga Swamps 16 Lyme Bay 125-6 Lyonesse 164, 168-9, 173, 193 MacLAREN, M. 4 Magnesian Limestone 76-7, 149-50 Manicouagan 10, 12, 25, 118 Manson 23, 25, 63 MarquezDome 23 Mars viii, 8 MASAITIS, V.L. 63 May Hill 84, 88, 90 McCARVILLE, P. 63 megabreccias 12, 41, 91 melange 100, 176 MELOSH, H.J. 11, 14-15, 80, 90, 104, 109, 133 MENEISY, M.Y. 78 Mercury viii, 8, 79 MERCY, E.L.P. 115, 118 Merewether 31 Meteorites 4-7, 13-15, 27-8, 32-3, 41, 43, 80, 130, 139-44, 148-58, 160, 168, 172, 181-91 Meteoroids 14, 80, 142, 148, 150 Atmospheric breakup viii, 13-15, 28, 79-80, 104, 142-4, 148 Mexico 9, 25, 140 Middiesbqro 22 Midlands Basin 38, 59, 6 5 - 8 1 MILLER, J.A. 78 Millstone Grit 47, 54-6, 74, 76, 128, 132 Miocene 37, 43, 102-5, 130 Mistastin 11 Mj0lnir 10, 20 Moira Breccia 57-9, 62, 73 Montagnais 10, 20 Montmorillonite 19, 107 Moon viii, 4, 7, 8, 11, 16, 79, 143, 152, 158 craters viii, 3, 4, 7, 11, 29, 79, 141, 176 Morasko 18, 20, 31
Index Morokweng 16-17, 25, 27 MOSSMAN, D.I. 140 Mount Darwin 4, 10 MUCK, O. 166, 172 MUSSET.A.E. 120 MYKURA, W. 33, 39, 40, 42
EE9 Quartzite 48, 50, 58-9, 62, 68, 69 Breccia 58-9, 62, 68-70, 73 Quillagua 13, 148
OCAMPO, A.C. 15 Odessa craters 4, 13, 18, 23, 146 ODIN 85 OFFICER, C.B. 19, 22, 24 Okavango 17, 140 Oligocene 25, 37, 102-5, 130 OPIK, E. 4 Ordovlcian 23, 36, 48-9, 51, 121-2, 128, 133 OWEN, T.R. 91, 128, 133
RAHTZ, P. 159 RAJLICH, P. 20 Ramghar 67 RAMPINO, M.R. 15, 23 Randecker Maar 18 RAYNER, D.H. 74-5, 77 Red Wing 23 REIDEL, S.P. 87 RHODES, R.C. 16, 117 Richat 10, 12-13, 25, 31, 40, 42-3, 63-4, 67, 88, 129 RIDPATH, I. 152 Ries 4, 6, 8, 10-11, 16-17, 26, 31, 43, 63, 70, 72, 104, 109 Rio Cuarta 15, 104 Rio Curuca 140 Roach, River 35, 96, 98-9, 101-2, 105 Rochechouart 19, 26, 107 Rochford Basin 35-8, 96-105, 130, 141, 176 Rock Elm 24 Rock flour 70, 72 ROHLEDER, H.P.T. 6 Rose Dome 15, 23 Russia 18, 24, 26
Palaeocene 97, 177 PALMER 122 PARKHURST J.I. 172-3 Penrith 'caves' 152, 178 Permian 34, 39, 49, 51, 53, 58, 68-9, 75-9, 81, 147 Rotliegendes 77 Zechstein 76, 77 Permo-Carboniferous 50, 71-3, 81 Permo-Triassic 45, 49-51, 58, 65, 68, 70, 76, 81, 91, 133 Pickering, Vale of 130-1, 134 Planar deformation lamellae 8, 121 Pleistocene 5, 33, 37-8, 40, 97-8, 100, 102, 130-1, 141, 145, 148, 169-71, 173 POAG, C.W. 9-10 POMEROL, C. 64 POPE, K.O. 15 Popigai 10, 27, 118-20 Powys, Vale of 128-9 Puchezh-Katunki 26
SAGE, R.P. 42 St. Albans 151, 178, 181 St. Bride's Bay 134 St-Imier 19, 146 St Magnus Bay 33, 3 9 - 4 4 SAVAGE, H. 171 SCHULTZ, P.H. 15 Scilly Islands 126-7, 159, 164, 168-9, 193 SEEGER, C.R. 23 Serpent Mound 5, 7, 23, 31, 63, 87 Serra da Cangalha 9, 10 Semsiyat 13, 43 SHARP, A.W. 33, 39, 133 Shatter cones 7, 13, 19, 22, 34, 43, 91, 100, 121 Shetland Islands 5, 33, 36, 39-44, 117, 184 Shock deformation 7-13, 20, 29, 34, 43, 72-3, 79, 91-2, 101, 104, 121, 172 Shoemaker structure 26
NAPIER, W.M. 122, 156, 159, 163 Neugrund 10, 20, 24 Newporte 23 New Quebec crater 11, 14, 22, 28-9, 92, 141, 175 NICOLAYSEN, L.O. 90 North Pennine arcs 131-2, 134 North Sea 27, 44-6, 100, 107-8, 110, 114, 131, 175-7 Norway 18, 29 Nyika Plateau crater 140
m SHOEMAKER, E.M. 4, 8, 79, 81, 142 SHORT, T. 150-2, 158 Sierra Madera 7, 23 Sikhote-Alin 7, 10, 13, 28, 139-40, 149 Siljan 12, 63, 133-4 Silurian 23, 34, 36, 48-9, 68-72, 83-8, 91, 128, 133 Llandovery 85-8, 91, 133 Ludlow 83-8 Wenlock 48, 84-8 Silverpit Crater 100, 175-7 Slate Islands 42 SNYDER, F.G. 15, 100 Solution pits 150 Southend-on-Sea 97 STAMP, L.D. 70 STEEL, D. 140, 160, 193 Steen River 10 Steinheim Basin 5-7, 16, 18, 26, 31, 43, 92, 94, 104 STEWART, S.A. 100, 175-6 Stishovite 8, 43 STOFFLER, D. 117 Stopfenheim Kuppel 16 STORR, M. 16 STORZER, D. 19 Strangford Lough 132-3 STUTZER, O. 5 Sudbury 9, 22, 25, 77, 116-21 SUESS, F.E. 5 Suevite 72-3, 78 Surrey crater 35-6, 106-11 Sweden 4, 6, 9, 11, 18-19, 21, 24, 26, 29, 63, 163 Switzerland 19, 146, 149 TaihuLake 26 Tannas 9, 24 Teague see Shoemaker structure Tektites 9, 16, 109 Tertiary 18, 36-9, 43, 91, 95, 99, 103, 105, 115, 125, 133-4, 175, 177 Thames, River 37, 96-7, 101, 158, 167, 194 Thringstone Fault 51, 55, 60-2 Toms Canyon 10, 20 Tonle Sap 9
Index Tornadoes viii, 142-3, 154, 157, 160, 179-82, 184-6, 189 Triassic 34, 39, 45-7, 49-51, 53-9, 61-2, 64-6, 68, 70, 73-4, 76, 77, 79, 81, 9 1 , 133 Keuper 53-4, 56-8, 61-2, 68, 70 Tsunami ix, 110, 139, 144, 158-60, 167-8, 173-4, 177, 179, 193 Tunguska explosion 4, 14, 28, 139-40, 142, 144, 148, 159, 172-3 Tycho (crater) 110 TYRRELL, G.W. 4 Ukraine 18, 24, 26 United States ix, 5, 7, 15, 17, 22-5, 33, 40, 171 Upheaval Dome 5 Uppland 18, 42 Urach 16, 42 Uriconian 48, 69-71, 73 Versailles 22-3, 43 Vredefort 7, 9-10, 16, 120 VOLK, T. 15, 23 Wabar 4 Weaubleau 15, 23, 25 WEGENER, A. 4 WELLS, A.K. 61, 116 WELLS, M.K. 61, 116 Wells Creek 5-7, 10, 12, 22, 40, 89, 91, 102, 129 WERNER, E. 4 West Hawk Lake 7 Westmeath 134, 157 Wetumpka 23 Whin Sill 78, 132 WHITTOW, J. 100 Whitwick Dolerite 51, 64, 78 WILSON, G.V. 117 Woodleigh 26 Woolhope Dome 34-6, 48, 82-95 WOOLLEY, A.R. 63 'Wormenhout' 151-2 WRIGHT, A.E. 74, 128 Wye, River 82-5, 88-9, 92-3 Younger Gabbros 35, 114-22
This book describes a search for geological evidence of meteorite impact structures in BritaJn.The statistics of impact structures indicate that Britain should have Phanerozoic impact structures up to tens of kilometres in diameter. A constant theme is the importance of atmospheric break-up of small asteroids and comets. These fragmenting bodies produce anomalously shallow craters with low rims and central peaks; three British structures of this type are identified.
BOMBARDED BRITAIN A Search for B r i t i s h Impact Structures
Analysis of fireball statistics implies that damaging fireball explosions occur over the British Isles on a time-scale of decades. On a timescale of millennia, however, more damage is done by Atlantic impact tsunami.
Imperial College Press www.icpress.co.uk
ISBN 1-86094-356-X
9 "781860"943560"
