New Forest E-Book

James Barnet

• Herausgeber: Crowood
• Kategorie: Lebensstil
• Sprache: Englisch

Seitenzahl: 234

Veröffentlichungsjahr: 2021

Beschreibung

This book is intended to be the most complete and up-to-date guide to the geology and fossils of the New Forest, providing a wealth of information of interest to both the amateur fossil collector and the professional geologist. It includes some 200 field photographs, palaeogeographic maps, digitised borehole/outcrop logs, and geological cross sections. Also included is a tour of the regional geological evolution of southern England since the Permian Period (-280 million years ago), based on deep boreholes and coastal exposures, including the world-famous Jurassic coast of Dorset and east Devon. The author discusses the petroleum geology of southern England and the New Forest and gives a detailed overview of the stratigraphy of the Hampshire Basin, followed by related aspects of economic geology within this area, including ironstones, freshwater aquifers, geothermal energy, sand, clay and peat resources. Finally, there is an up-to-date and complete account of the principal fossil localities, together with a comprehensive gallery of photographs with accompanying descriptions of the most abundant fossils within the New Forest National Park.

Leseprobe

THE NEW FOREST

GEOLOGYAND FOSSILS

JAMES BARNET

First published in 2021 byThe Crowood Press LtdRamsbury, MarlboroughWiltshire SN8 2HR

[email protected]

www.crowood.com

This e-book first published in 2021

© James Barnet 2021

All rights reserved. This e-book is copyright material and must not be copied, reproduced, transferred, distributed, leased, licensed or publicly performed or used in any way except as specifically permitted in writing by the publishers, as allowed under the terms and conditions under which it was purchased or as strictly permitted by applicable copyright law. Any unauthorised distribution or use of this text may be a direct infringement of the author’s and publisher’s rights, and those responsible may be liable in law accordingly.

British Library Cataloguing-in-Publication DataA catalogue record for this book is available from the British Library.

ISBN 978 1 78500 817 7

Contents

Dedication and About the Author

Introduction and Aims of this Book

1.

Geological History of the New Forest National Park

2.

Petroleum Geology of the Wessex and Weald Basins

3.

Palaeogene Stratigraphy of the Central and Eastern Hampshire Basin, including the New Forest National Park

4.

Natural Resources within the New Forest National Park and Surrounding Area

5.

Fossil Hunting in the New Forest National Park

6.

Fossiliferous Sites of the Hampshire Basin outside of the New Forest

7.

The Fossils of the New Forest National Park

8.

Useful Resources for Keen Amateur Geologists

Glossary of Geological Words and Terms

Bibliography

Index

Dedication

This book is dedicated to my father (Douglas Barnet), mother (Shirley Barnet), brother (Peter Barnet) and grandmother (Daphne Pratley), who accompanied me on many of my early fossil hunting expeditions in the New Forest area and along the Jurassic coast.

About the Author

James Barnet was born on 9 August 1986 and grew up in Bramshaw in the New Forest. He became interested in geology and fossil hunting from an early age after discovering fossil seashells and shark’s teeth in a local riverbed with his parents at the age of four, just a ten-minute walk from his home. The intrigue of how marine fossils were now found on land at a considerable distance from the coastline would lead to a long interest and ultimately a career in geology. He learned about fossiliferous strata of a similar age exposed at Lee-on-the-Solent and Stokes Bay, frequently searching in these localities during visits to see his grandmother in Gosport. As his interest grew, so his fossil hunting travels took him further afield to the cliffs between Highcliffe and Barton-on-Sea, as well as the Jurassic coast. James became a particularly frequent visitor to the fossiliferous cliff sections between Lyme Regis and Seatown, as well as at Kimmeridge Bay and Osmington Mills.

In 2008, James graduated from the University of Southampton with a First Class Master’s Degree in Geology. After a six-year stint in the petroleum industry, he then graduated from the University of Exeter in December 2018 with a PhD in Geology. His PhD focused on the evolution of climate and the carbon cycle during a period of major long-term global warming from the end of the Cretaceous to the early Eocene (~67–52 million years ago), leading up to the time when the fossils that initially sparked his interest were deposited.

Since November 2019, James has been working as a postdoctoral researcher at the University of St Andrews, where he is constructing a 100-million-year long record of atmospheric carbon dioxide (CO2), spanning the Late Cretaceous to modern times.

This book represents the culmination of almost thirty years of fossil hunting and geological study in and around the New Forest, as well as along the Jurassic coast.

Dr James S.K. Barnet (PhD, MCSM, MSci, FGS)

Introduction and Aims of this Book

The New Forest is predominantly located in south-western Hampshire, although its northern margin crosses over the county boundary into south-eastern Wiltshire (Fig. 1). Designated a national park in 2005, it has long been a popular and unique area for visitors to walk and explore in the company of animals such as New Forest ponies, deer, donkeys, pigs and cows. Whilst appearing to roam wild across the area, these animals (with the exception of the deer) are owned by local people who have grazing rights within the national park, known as ‘commoners’.

Fig. 1. Location map of the New Forest National Park. The two principal fossil localities within the national park are also indicated.

Fig. 2. The Knightwood Oak is over 500 years old and as the oldest tree in the New Forest has probably experienced a lot of change in its time. However, geological time is vast and the local fossils are over 40 million years old, providing evidence for a very different climate and environment within this area.

The native ancient forests of the New Forest are predominantly composed of oak and beech, with holly as the most abundant form of scrub vegetation. Indeed, the Knightwood Oak to the south-west of Lyndhurst is believed to be the oldest tree in the New Forest at over 500 years old (Fig. 2). However, younger almost monospecific inclosures composed of pine, sequoia, silver birch and fir have also been planted. Scattered sweet chestnut, horse chestnut, rowan, ash, yew, alder, maple and crab apple also occur. These forests are managed by the Forestry Commission, the governmental department responsible for managing forested areas across England.

Although known as the New Forest, the approximately 565sq km of the national park are also home to significant areas of open heathery or grassy moorland and farmland. The New Forest boasts more valley mires than the rest of England and western Europe put together, and is home to many rare and endangered wetland species, along with carnivorous plants such as the sundew (Drosera sp.). The underlying geology also plays an important role for the plant species that thrive in the national park, with much of the area characterized by acidic, nutrient-poor, clayey soils with low permeability that is determined by the predominantly clayey nature of Eocene sediments deposited in the central part of the Hampshire Basin. However, where sandier formations outcrop at the surface, such as in the Lyndhurst area and to the west of the national park around Bournemouth and Ferndown, the soils are sandy and free-draining, with pine trees forming a more significant component of the forest cover. The national park also includes 42km of coastline, comprising flint shingle beaches such as Lepe, along with extensive salt marshes and mudflats around Lymington that are home to many wading bird species.

Fig. 3. The highest point in the New Forest, Pipers Wait (129m), during a hard freeze in December 2009.

Fig. 4. Fleeting snowfalls, such as this one in January 2013, can transform the New Forest into a transient winter wonderland.

Fig. 5. A heavy snowfall in January 2013 coats every branch in this small pine forest near Telegraph Hill in a wintry white.

Fig. 6. Winter can be a challenging time for the resident New Forest ponies.

The colours and moods in the New Forest change spectacularly through the seasons. The winter is characterized by cold, bare, hard grey lines, occasionally transiently transformed by a thick blanket of white snow (Figs 3–6). The growing warmth of the strengthening sun through spring and summer produces an emergence of lush vivid greens, as crisp new leaves burst out on the trees and new bracken shoots unfurl and grow tall after their winter slumber, whilst an abundance of yellow and then pink graces the open moorland as the gorse and heather come into flower (Figs 7–10).

Fig. 7. Spring time on Hampton Ridge, between Fritham and Frogham, bathed in yellow as the gorse comes into flower.

Fig. 8. The growing warmth of spring encourages lush vivid greens in the New Forest.

Fig. 9. New bracken shoots unfurl and grow tall after their winter slumber.

Fig. 10. A pair of New Forest ponies graze a quiet glassy glade on a lazy hot summer’s day in July 2012.

The cooling temperatures and shortening days of autumn encourage russet browns and oranges, as the lush summer foliage dies back in one final burst of colour in preparation for the cold, dark winter months, whilst armies of fungi appear from nowhere to colonize and decompose the damp organic matter. Sweet chestnuts and beech nuts fall in prolific numbers from the trees during October in certain parts of the forest, gathered eagerly as a source of winter fuel by squirrels and other forest inhabitants (Fig. 11).

Fig. 11. Sweet chestnuts are prolific in certain parts of the New Forest in October.

The topography of the national park broadly rises to the north and west, before dropping into the valley of the River Avon along its western margin. The highest topography of the northern New Forest is predominantly occupied by heathery moorland plateau, peaking at Pipers Wait (129m [423ft]) and Telegraph Hill (128m [420 ft]), close to the northern boundary of the national park. From certain high points, fine views can be gained southward to the chalk spines running along the central axes of the Isle of Wight and Purbeck, as well as northward to the rolling chalk hills of Wiltshire. The chalk hills around the margins of the Hampshire Basin are easily identifiable by their predominantly grassy nature, due to the alkaline soils weathering from the chalk bedrock, along with the absence of heather that is so characteristic of the acidic nutrient-poor soils of the New Forest. The national park is dissected by a complex network of footpaths and gravel cycle tracks, offering easy access to quieter sections of peaceful countryside by foot or by bike.

However, whilst visitors enjoy the landscape of the New Forest that is so familiar today, few appreciate that in the past the landscape and climate of the area were very different. Not far below the surface, blue, green and grey clays yield prolific fossil evidence of a time over 40 million years ago (Ma) when a shallow subtropical sea home to sharks and eagle rays covered almost the entire area now occupied by the New Forest National Park. Extensive mangrove forests fringed this marine embayment, whilst a burning hot sun beat down on the gently undulating chalk landscape of Wiltshire and rising chalk hills of the Isle of Wight and Purbeck, where palm and sequoia trees grew in abundance and crocodiles roamed the watercourses. Whilst the coastal fossiliferous sites such as Barton-on-Sea and Bracklesham Bay are well studied, the fossiliferous sites inland within the New Forest National Park are generally poorly known, except to local inhabitants and professional geologists. Shepherd’s Gutter near Bramshaw is a popular tourist spot in summer, where the trees frequently echo to the sounds of excited children on a warm summer’s day. Yet most will visit here and play in the stream bed for hours, blissfully unaware of the richly fossiliferous clays exposed in the stream banks.

This book aims to be the most complete and up to date guide to the geology and fossils of the New Forest National Park and surrounding area, providing information of interest to both the amateur fossil collector and professional geologist alike. First, Chapter 1 will take the reader on a tour of the regional geological evolution of southern England since the Permian Period (~280Ma), based on rock sequences recovered by deep wells drilled within the national park and surrounding area, as well as coastal exposures across southern England and south Wales, including the world-famous Jurassic coast of Dorset and east Devon.

This is followed by a detailed discussion of the petroleum geology of southern England and the New Forest in Chapter 2. Chapter 3 then presents a detailed overview of the stratigraphy of the Hampshire Basin, followed by related aspects of economic geology within this area, including ironstones, freshwater aquifers, geothermal energy, sand, clay and peat resources (Chapter 4).

Chapter 5 presents an up to date and complete account of the principal fossil localities within the New Forest National Park, to serve as a geological reference guide and encourage visitors to explore these poorly known but extremely fossiliferous sites. This is followed by brief accounts of the more famous fossiliferous coastal exposures on the mainland bordering the New Forest in Chapter 6. A comprehensive suite of photographs and descriptions of the most abundant fossils to be found in the New Forest National Park is provided in Chapter 7, to serve as an identification checklist for fossil finds made from these localities.

Finally, useful resources for young and amateur geologists in this area are provided in Chapter 8. There then follows a Glossary of the specific geological terms, words and names used in this book and a Bibliography.

Disclaimer

Fossil hunting at any geological site carries its own inherent risks. Inland in the New Forest area, these risks typically take the form of becoming trapped in sinking clay or deep bogs, slipping on steep, muddy river bank slopes, losing your footing in fast-flowing river water, or being struck by falling trees and branches. Coastal sites carry the additional risks of becoming trapped by a rising tide, high waves and storm surges. All of these risks are significantly greater during and immediately after inclement (wet and windy) weather. It is therefore strongly advised from a safety perspective that fossil hunting in these localities is only carried out in groups and during a fine, settled spell of weather.

Fossil hunting is carried out at your own risk and the author accepts no responsibility for any injuries or fatalities sustained when exploring the fossil localities detailed in this book.

CHAPTER ONE

Geological History of the New Forest National Park

Our knowledge of the ancient geological history of the New Forest National Park is derived from deep wells (>1,000m) drilled for either petroleum or geothermal energy exploration within the national park and in the Southampton area, along with correlation to cliff exposures along the coastlines of southern England and the Isle of Wight. Only a couple of petroleum exploration wells have been drilled within the New Forest National Park to date (Fordingbridge-1 and Bransgore-1), although there have been unsuccessful applications to drill others. Now that the area has become a national park, it is unlikely that planning permission will be granted for any future petroleum exploration.

Fig. 12. Geological timescale of the past 300 million years of Earth’s history, illustrating the eras, periods, epochs and stages referred to in this book. The key geological events are also shown. The stages and geological events of the Cenozoic are expanded upon in Fig. 67.

Fig. 13. Composite legend for all of the lithological logs presented in this book.

Fig. 14. Summary lithological log of the Fordingbridge-1 well, based on text descriptions by Falcon & Kent (1960). A legend for all lithological logs is provided in Fig. 13.

Fig. 15a. Detailed lithological log of the Middle Jurassic–Late Cretaceous stratigraphy penetrated by the Southampton-1 well between 500 and 1,150m depth, based on text descriptions by Thomas & Holliday (1982) and Edwards & Freshney (1987a). A legend for all lithological logs is provided in Fig. 13.

Fordingbridge-1 was drilled in 1959 by British Petroleum (BP) on the western slopes of Hasley Hill near Fordingbridge (GR 50.906°N, 1.735°W; TD: 1,367.6m), close to the western national park boundary (Falcon & Kent, 1960). The well did not find any oil, but as it penetrated into Late Triassic rocks, does provide a useful control point on the stratigraphy at depth within the western part of the national park (Fig. 14). The Southampton-1 well was drilled for geothermal energy exploration near Southampton Civic Centre to the east of the New Forest during 1979–80 (GR 50.907°N, 1.409°W; TD: 1,827m), and since it penetrated into Devonian rocks, provides an important stratigraphic control point to the east of the New Forest (Fig. 15a, b).

Fig. 16. Stratigraphy of the Wessex Basin, exposed along the coastline of eastern Devon, Dorset and the Isle of Wight. Groups and formations are colour coded by depositional environment and dominant lithology, including: continental alluvial, fluvial and lacustrine environments (orange); shallow marine sandstones (yellow); marine siltstones, shales and clays (grey); and limestones (blue). The age of the continental Permian–Triassic formations is poorly constrained. The complete Early Cretaceous sequence is only preserved and exposed along the coastline of the southern Isle of Wight, whilst the Gault and Upper Greensand formations unconformably overlie the Wealden Group or older Jurassic/Triassic rocks in Dorset and east Devon. Formations that cannot be displayed in the main column have been indicated on the right, along with additional important formation members mentioned in this book (in smaller font).

During the late Palaeozoic and Mesozoic eras, the New Forest was located between two sedimentary basins across a broad, relatively uplifted region known as the Hampshire–Dieppe High. To the south-west lay the Wessex Basin, an east–west oriented elongated basin that stretched along the Dorset and east Devon coastlines, with greatest thicknesses of sedimentary rocks deposited across Purbeck, Portland and the southern half of the Isle of Wight, and offshore in the adjacent part of the English Channel, south of major basin-bounding extensional faults (Hawkes et al., 1998). Within this basin and along its northern margin, the rock sequence now comprising the spectacular Jurassic coast was deposited from Permian through to Early Cretaceous times, spanning a time period of ~200 million years (Fig. 16).

To the north-east lay a younger Jurassic–Early Cretaceous basin, known as the Weald Basin. Knowledge of the stratigraphy of the Weald Basin is predominantly derived from deep wells drilled for petroleum exploration (Hawkes et al., 1998). However, Fordingbridge-1 and Southampton-1 penetrated a similar rock sequence to that exposed along the Jurassic coast, albeit generally thinner and shallower marine, suggesting that the rock sequence of the Wessex Basin represents a more distal and deeper marine analogue for the more proximal shallower marine subsurface stratigraphy of the New Forest.

By contrast, the onset of a new crustal stress regime resulting from closure of the Tethys Ocean and collision between Africa and Europe, compounded by opening of the North Atlantic, resulted in structural inversion during the early Palaeogene Period. Basin-bounding extensional faults that controlled subsidence within the Wessex and Weald basins were reactivated as reverse faults with an opposite sense of displacement. Sedimentary rocks of the two Mesozoic basins were uplifted, while the former structural Hampshire–Dieppe High between these basins gently subsided to form a new basin – the Hampshire Basin (southern England) and contiguous Dieppe Basin (north-east France) were born. The Hampshire Basin was much shallower and shorter-lived than the Wessex Basin, spanning a time period of around 20 million years. However, it covered the entire area of the New Forest National Park and within this basin the marine fossiliferous clays of the New Forest were deposited.

Study of sedimentary sequences

The Permian–Cretaceous sedimentary sequence is exposed along the world-famous Jurassic coast of Dorset and east Devon, therefore a correlation can be made between deep wells drilled in the New Forest area and nearby coastal cliff sections, to build up a picture of the geological evolution of the New Forest and surrounding area during the Late Palaeozoic and Mesozoic eras. The Palaeogene (predominantly Eocene–early Oligocene) sedimentary rocks can be studied in the cliff exposures of east Dorset and west Hampshire from Studland to Milford-on-Sea, and across the northern half of the Isle of Wight from Alum Bay to Whitecliff Bay, as well as from boreholes and stream-bank exposures within the New Forest itself.

END OF THE PALAEOZOIC AND THE MESOZOIC ERA: THE FORMATION AND INFILLING OF THE WESSEX AND WEALD BASINS

At the end of the Carboniferous Period (~300Ma), closure of the Rheic Ocean resulted in collision between the Gondwana and Laurussia continents, forming the Variscan Orogeny. All of Earth’s continents amalgamated to form one supercontinent called Pangaea, surrounded by a vast ocean called Panthalassa (Fig. 17). A broad arm of Panthalassa, the Tethys Ocean, punched towards the heart of Pangaea. The Variscan mountain range was extensive and impressive, spreading across a large part of Pangaea from the Americas, through north-west Africa, north-west France, and into central and eastern Europe. The principal geological structures and major east–west trending fault systems of southern England and south Wales were formed during this time, as well as metamorphism of the Cornish slates, shortly followed by emplacement of granite plutons forming the high moorland topography of Devon and Cornwall (Fig. 18). These same fault systems were subsequently reactivated multiple times to form the Wessex Basin.

Fig. 17. Global palaeogeography at the end of the Carboniferous and start of the Permian (~300Ma), illustrating the formation of Pangaea and development of the Variscan Orogeny.

Fig. 18. The granite complexes of Devon and Cornwall, such as Bodmin Moor pictured here, were intruded shortly after the Variscan Orogeny. This region remained an erosional land area (Cornubian Massif) throughout much of Earth’s subsequent geological history, representing an important sediment provenance area for the Wessex and Hampshire basins.

The first Permian phase of extension may have been related to post-orogenic collapse, when declining compressional tectonic forces could no longer sustain growth of the Variscan Orogeny and the high topography collapsed and spread laterally under the force of gravity. Later extensional phases are likely to be related to various stages in the opening of the North Atlantic and form part of the break-up of Pangaea to a global geography with which we are more familiar today.

Fig. 19. The hot desert of Oman (Middle East) represents a modern analogue for the landscape across much of Western Europe at times during the Permian and Triassic periods.

Fig. 20. The spectacular red cliffs of the Early–Middle Triassic Sherwood Sandstone Group at Budleigh Salterton (Devon), with a close-up of fluvial cross-bedding in lenticular channels of the Otter Sandstone Formation at nearby Ladram Bay. The white scale bar on inset image represents 1m.

Fig. 21. A desert deflation surface (yellow horizon) at the contact between the Budleigh Salterton Pebble Beds Formation and the overlying Otter Sandstone Formation. Pebbles characterized by a sheen of ‘desert varnish’, created by sand blasting and typical of modern deserts, can be picked out from this horizon. The white scale bar represents approximately 2m.

Fig. 22. Red-coloured fluvial sandstones of the Buntsandstein (‘coloured sandstone’) at Porte de Pierre (858m) in the northern Vosges mountains, along the western shoulder of the Rhine Graben in north-east France. Similar sandstones also outcrop in the Black Forest on the eastern shoulder of the Rhine Graben in south-west Germany.

During the Permian–Middle Triassic (~280–242Ma), southern England was located in the centre of Pangaea at a low latitude of ~15–20°N (Torsvik et al., 2012; Van Hinsbergen et al., 2015). The climate was hot and semi-arid (Fig. 19). There is evidence that desert environments developed at times across southern England, whilst at other times, rivers formed by monsoon rains over the Variscan mountains deposited coarse-grained sediment. The distinctive red-coloured Permian–Middle Triassic conglomerates and sandstones are well exposed in the spectacular red cliffs of east Devon and Somerset, such as on the coast around Budleigh Salterton (Figs 20–1) and Minehead, and in the road-cutting around the southernmost junction of the M5 motorway. The red colour is derived from the oxidation of iron under the arid climate and it weathers to give the distinctive red soils of these two counties. Indeed, a similar climatic regime resulted in deposition of distinctive red coarse-grained sediments across much of Western Europe during this time (Fig. 22).

Fig. 23. Depositional environments and palaeogeography during deposition of the Sherwood Sandstone Group (Early–Middle Triassic; ~252–242Ma) across southern Britain and north-west Europe, modelled after Butler (1998), Hawkes et al. (1998), McKie & Williams (2009) and the author’s personal geological observations. The principal cliff exposures are indicated.

The predominant quartzite composition of conglomerate clasts, along with evidence of past current directions, suggests sediments were sourced from erosion of the Variscan highlands across north-west France (Armorican Massif). The subsiding Wessex Basin formed a focus for the accumulation of sandy material eroded from Armorica (Fig. 23). However, it appears that a major river system, the ‘Budleighensis River’ (Wills, 1970), was still able to punch through the northern basin margin and deposit sandy sediment of a similar composition within the Worcester Graben during this time. Although Fordingbridge-1 did not penetrate deeply enough to reach Permian–Middle Triassic sediments, Southampton-1 cored through 20m of the Triassic Sherwood Sandstone Group before encountering Devonian sandstone, confirming the presence of at least thin Early–Middle Triassic sandstones in this area (Fig. 15b).

Fig. 24. Depositional environments and palaeogeography during deposition of the Mercia Mudstone Group (Middle–Late Triassic; ~242–206Ma) across southern Britain and north-west Europe, modelled after Butler (1998), McKie & Williams (2009) and the author’s personal geological observations. The principal cliff exposures are indicated.

By the Middle–Late Triassic (~242–206Ma), the Armorican highlands of north-west France were becoming more denuded, supplying increasingly fine-grained sediment to the basins of southern England. An extensive network of shallow playa lakes developed through the sedimentary basins of the time, including the Wessex Basin, Worcester Graben, English Channel, Bristol Channel and Irish Sea (Fig. 24). Within these lakes, muds of the Mercia Mudstone Group were deposited and are exposed in the coastal cliffs of east Devon (Fig. 25), Somerset and Gloucestershire. The presence of evaporitic minerals such as anhydrite and halite at certain stratigraphic levels indicates that the climate was still hot and fairly dry (Fig. 15b). The shallow lake basins were surrounded by long-lived uplifted landmasses where no Permian–Triassic sediments were deposited, including the Cornubian Massif of Cornwall and west Devon and the Irish, Welsh and London–Brabant massifs.

Fig. 25. Red mudstone cliffs of the Branscombe Mudstone Formation (Mercia Mudstone Group) at Seaton (east Devon). The white scale bar represents approximately 2m.

Fig. 26. Rhaetian limestones of the Langport Member of the Lilstock Formation (Penarth Group; ‘White Lias’) overlain by interbedded latest Rhaetian–early Sinemurian organic-rich shales and limestones of the Blue Lias Formation (Lower Lias Group) in Pinhay Bay, west of Lyme Regis.

Fig. 27. The sharp contact between Rhaetian limestones of the Langport Member of the Lilstock Formation (Penarth Group) and latest Rhaetian basal shales of the Blue Lias Formation (Lower Lias Group). Vertical burrows penetrating down from the contact into the upper limestone bed of the Lilstock Formation can be observed.

At the end of the Triassic, significant subsidence started to take place along the major basin-bounding faults of the Wessex Basin, in response to the onset of rifting that would culminate in the opening of the North Atlantic during the Jurassic. Rocks of the latest Triassic Penarth Group (‘White Lias’; ~206–201Ma), exposed in the cliffs of Pinhay Bay to the west of Lyme Regis, preserve evidence of major submarine slumps and debris flows, which may have resulted from large magnitude earthquakes (Figs 26–7). Rapid subsidence allowed a marine incursion from the Tethys Ocean to the south-east to flood into the shallow lake basins of the Late Triassic, and a relatively deep marine basin had been established in the centre of the Wessex Basin by the start of the Jurassic Period (~201Ma).

Fig. 28. Stratigraphic scheme for the Early Jurassic fossiliferous rocks of the Lyme Regis–Charmouth area, along with their diagnostic ammonites and other fossil assemblages (based on Lang, 1924; House, 1985; Weedon et al., 1999; Gallois, 2008; Ewin, 2018 and the author’s personal observations).

The world-famous fossiliferous Lower Lias Group shales and limestones of the Lyme Regis area were deposited during the latest Triassic and Hettangian–early Pliensbachian stages of the Early Jurassic (~201–187Ma), yielding some of the most spectacular marine fossils in southern England (Fig. 28