Geology of London : Special Memoir for 1:50 000 Geological sheets 256 (North London), 257 (Romford), 270 (South London) and 271 (Dartford) (England and Wales)

R A Ellison Contributing authors M A Woods D J Allen A Forster T C Pharoah C King

Bibliographical reference: Ellison, R A, Woods, M A, Allen, D J, Forster, A, Pharoah, T C, And King, C. 2004. Geology of London. Memoir of the British Geological Survey, Sheets 256 (North London), 257 (Romford), 270 (South London) and 271 (Dartford) (England and Wales).

British Geological Survey

Geology of London : Special Memoir for 1:50 000 Geological sheets 256 (North London), 257 (Romford), 270 (South London) and 271 (Dartford) (England and Wales)

R A Ellison Contributing authors: M A Woods D J Allen A Forster T C Pharoah C King

Keyworth, Nottingham: British Geological Survey 2004

© NERC copyright 2004 First published 2004

The grid used on the figures is the National Grid taken from the Ordnance Survey maps. (Figure 2) is based on material from Ordnance Survey 1:50 000 scale maps, numbers 162 and 163.

© Crown copyright reserved. Ordnance Survey Licence 100017897/2004 ISBN 0 85272 478 0

Authors

R A Ellison, BSc, FGS, CGeol M A Woods, BSc, FGS, CGeol D J Allen, BSc, FGS, CGeol A Forster, BSc, FGS, CGeol T C Pharoah, BSc, PhD, FGS, CGeol British Geological Survey, Keyworth

C King, BSc, PhD 16A Park Road, Bridport, Dorset DT6 5DA Printed in the UK for the British Geological Survey by Halstan & Co Ltd., Amersham.

Acknowledgements

R A Ellison has compiled this memoir. The authorship of each chapter is as follows:

• One – Introduction – R A Ellison
• Two – Concealed strata – T C Pharaoh and R A Ellison
• Three – Cretaceous: Upper Chalk – M A Woods
• Four – Palaeogene: – R A Ellison and C King
• Five – Structure – R A Ellison and T C Pharaoh
• Six – Quaternary – R A Ellison
• Seven – Applied Geology – A Forster, D J Allen and R A Ellison

The authors thank the many people who have assisted in some way towards the completion of this memoir. R D Lake and F G Berry, both now retired from the BGS, oversaw the start of work. C J Wood, formerly with BGS contributed a great deal of information on exposures of Chalk in Kent and Essex and R N Mortimer (Brighton University) allowed access to his interpretations of sections in Chalk boreholes. D W Jolley (Sheffield University) examined palyno-morphs from boreholes in the Lambeth Group and advised on their correlation. P L Gibbard (Cambridge University) advised on the mapping of River Terrace Deposits in south Essex. Members of the Tertiary Research Group, notably D J Ward, J Cooper and J J Hooker, S Tracey and G Ward, provided general advice on the local Palaeogene geology and invaluable information on temporary exposures. Figures were drawn by R J Demain, P Lappage and G Tuggey, BGS Cartography, Keyworth. The memoir was edited by A A Jackson and pageset by J Norman.

Staff at the Unitary London authorities, Kent County Council and Essex County Council are thanked for their co-operation in providing borehole and other data. M Dixon at the London Research Centre made available facilities for examining aerial photographs. London Underground Ltd kindly allowed us to publish information in (Figure 14) and (Figure 15) from the Crossrail and Jubilee Line Extension site investigation boreholes; Union Rail Ltd similarly permitted use of borehole information from the Channel Tunnel Rail Link. A Howland and the former London Docklands Development Corporation provided detailed information on the geology of the docklands area. Staff from the Environment Agency, particularly V Robinson, and K Baxter of Thames Water Utilities, and R Sage, formerly with the company, are thanked for support and advice on matters concerning the main aquifer and its properties. Thames Water Utilities also allowed the use of data from the Trafalgar Square well in (Figure 51) and the Environment Agency provided information concerning changes in groundwater levels in London in (Figure 46).

Notes

The word ‘district’ used in this memoir means the area included in the geological 1:50 000 Series Sheet 256 North London, Sheet 257 Romford, Sheet 270 South London and Sheet 271 Dartford.

Figures in square brackets are National Grid references; those not preceded by two letters lie within the 100 km grid square TQ.

Enquiries concerning geological data for the district should be addressed to the Manager, National Geological Information Centre, BGS, Keyworth. Information is available on-line at the BGS web site: www.bgs.ac.uk

This memoir provides an overview of the geology of the district. For fuller details of sections and exposures the reader should consult relevant BGS maps, databases and internal reports.

Preface

Geology has played a significant role in the development and prosperity of the London area. The original Roman settlement of Londinium was sited on dry sand and gravel deposits close to the River Thames, with readily available water supplies from riverside springs. As the city grew, the abundant water supplies from the major Chalk groundwater aquifer supported the rapid population growth. A ready supply of locally worked aggregate and brick clay deposits aided the infrastructure development. The construction, and rapid expansion of the world’s first major underground railway system was made possible by the presence of the excellent tunnelling medium of the London Clay, and even today, the pattern of the London Underground network closely reflects the bedrock geology of the London Clay Formation.

Study of urban geology and the interplay between geology and the environment has become increasingly important as the world’s urban populations continue to increase. BGS has carried out detailed studies of the major urban conurbations of Britain for many years, and this study of the capital city is of particular importance.

A variety of geohazards can affect development and it is important that consideration be given to these prior to construction work taking place. Careful planning, linked to an understanding of the engineering characteristics of the geological strata and its three-dimensional model, has provided early warning of many potentially difficult ground conditions, and thus assisted in finding solutions to specific hazards in advance of the development taking place. Major infrastructure projects which have benefited recently from the improved geological understanding of the London region include London Underground’s Jubilee Line extension and the Channel Tunnel Rail link Project.

David A Falvey Director Kingsley Dunham Centre Keyworth Nottingham

Geology of London

This book brings together information that results from research on the ground beneath the streets of London. It describes the geological strata, how they came to be there, and how they impinge on the life of those who live and work in the city, in its maintenance and sustainable development.

The development of London is intimately tied to the ground conditions. The original settlement was originally located at a crossing point on the River Thames in an area of dry land where sand and gravel banks were surrounded by rather boggy marshland. A ready supply of gravel and brick clay helped with the early infrastructure development, and much later the extensive underground tunnel network grew because of the ease of excavating the London Clay. Water was always readily available, initially from riverside springs and later, in larger volumes, from underground Chalk.

This explanation of the strata that underlie London gives an insight into the geological history of the last 500 million years. Over the past 200 years, boreholes have explored the deeper layers and countless geologists have systematically recorded the near-surface strata in quarries and excavations. The geological history includes periods of earth movement, inundations by the sea, the development of coastal mudflats and the effects of great ice ages.

Exploration for oil and gas in the North Sea has benefited from an understanding of the rocks beneath London, and their geological history. Effective use of water resources, efficient ground investigation for new buildings and infrastructure, and sustainable planning and development are all founded on the use of information about the condition and structure of the ground. This book provides the background information for the maintenance of good practice in these activities, and illustrates some aspects of the ground that have in the past caused difficulties.

Environmental concerns such as the legacy of contamination, sea level rise as a consequence of global climate change, and the recycling of water are issues that will increase in importance in this century. Basic data, fundamental for dealing with these concerns are presented in this book, and on the associated 1:50 000 scale geological maps for London.

Summary of the geological strata of the London district. (Inside front cover)

(Front cover)  Two of the massive steel-plated gates that form part of the Thames Barrier at Greenwich, completed in 1982 and designed to protect London from tidal surge floods. (Photograph supplied by www.libraryphotos.com.) Frontispiece Generalised geology of the London district.

Chapter 1 Introduction

This memoir is a review of the current knowledge on the geology of London written to describe the four 1:50 000 scale geological maps that cover the district. It presents a regional overview with some specific examples, and for more detailed accounts the reader is referred to the earlier memoirs. A concise account of the geology of the wider region is given in Sumbler (1996), which includes a bibliography of key recent papers on the principal lithostratigraphical units and a section on applied geological issues in the London district.

The district is dominated by the greater London conurbation, reaching the outskirts of Watford in the north-west, and the expanding towns of Brentwood and Billericay in the north-east. The land alongside the Thames in the east of the district from Woolwich to Gravesend, known as the Thames Gateway, has been designated for major development in the 21st century. It has a legacy of past industrial use and some of the largest brown-field sites in Britain.

The geology and topography of the district are shown on the Frontispiece and in (Figure 1) respectively. The topography is dominated by Chalk downland in the south, and by a ridge that forms the edge of Epping Forest around Chigwell and Loughton in the north; rural Essex in the east is gently undulating and supports mixed farmland. The River Thames and its tributaries form the main drainage network. The river, tidal as far as Teddington Lock [TQ 168 715] in the west of the district, is associated with an alluvial tract at about 10 m above OD in the west and around sea level on the marshlands in the east. River terrace gravels rising to about 30 m OD underlie the gently sloping valley sides. Interfluves in the north-west consist of dissected London Clay, and some hills (up to 150 m OD) are capped by the Bagshot Formation and Quaternary gravels. In the north-east, the ground rises to a dissected plateau of till at about 100 m; other hills of similar height are capped by Bagshot Formation and Quaternary gravels. South of the Thames the ground rises gently across the London Clay on to the Chalk that forms the dip slope of the North Downs escarpment. The dip slope is covered, in part, by Clay-with-flints and is interrupted by subsidiary hills formed by outliers of London Clay and Thanet Sand. The crest of the escarpment reaches 200 m above OD in the south-east, the highest point in the district.

History of survey

The first geological survey of the district was carried out by W Whitaker and others, on a scale of one inch to one mile (1:63 360). The maps, covering a larger area than in this account, were published on [TQ Old Series] sheets 1, 6, 7 and 8 between 1861 and 1868. A wealth of local detail on exposures in the area was published in the first memoir of the district (Whitaker, 1872). Drift deposits were mapped later and shown on new editions of the maps, published between 1869 and 1889. Information about the superficial deposits was incorporated in a revised memoir (Whitaker, 1889). Four special 1:63 360 scale maps covering London were published in 1903, and these made use of the first mapping at the scale of six inches to one mile (1:10 560) in part of the present district by T I Pocock and J A Howe. A memoir (Woodward, 1909) was also produced, and subsequently revised by C E N Bromehead and C P Chatwin for a second edition published in 1922.

The first systematic large-scale mapping (1:10 560) of the district on the County Series maps, was completed in 1922; C E N Bromehead, H G Dines, F H Edmunds and H Dewey were the principal geological surveyors. The mapping was compiled and published on the new component one-inch (1:63 360) map sheets for the district: North London (256), Romford (257), South London (270) and Dartford (271). The memoirs for the sheets, respectively, by Bromehead (1925), Dines and Edmunds (1925), Dewey and Bromehead (1921) and Dewey et al. (1924) describe information obtained from the detailed survey and supplement the data from the earlier memoirs. Minor amendments to the 1:10 560 scale maps, as a consequence of temporary exposures, were incorporated in later editions of the one-inch maps. A partial resurvey at 1:10 560 scale was made by F G Berry, T E Lawson and B S P Moorlock in south London from 1973 to 1980, as part of an environmental geological study funded by the former Department of the Environment, which also funded similar work in south-west Essex that entailed resurvey at 1:10 000 scale and the publication of a set of thematic applied geology maps (Moorlock and Smith, 1991). In 1992, a new project, known as LOCUS (London Computerised Underground and Surface) was initiated to produce digital 1:10 000 scale maps for the district, a comprehensive borehole database and three-dimensional models of the geology of London (Ellison et al., 1993). Between 1992 and 1996 Artificial deposits were added to all the maps, the North London and South London sheets were revised, and large areas of the Romford and Dartford sheets were resurveyed, largely by A J M Barron, R A Ellison, D H Jeffery, R T Mogdridge, B S P Moorlock, A Smith, P J Strange and I T Williamson. This work led to the publication between 1993 and 1998 of new 1:50 000 scale geological maps, available also in digital format. Further details on survey dates of specific sheets can be obtained on the BGS web site: www.bgs.ac.uk.

Internal BGS Reports on various aspects of the geology have contributed to this memoir: notably by C Hallsworth on heavy minerals in the Palaeogene, J M Pearse, S J Kemp and V L Hards on the mineralogy and petrography of the Lambeth Group, G E Strong on the petrography of the Thanet Sand, R Harland on the Palaeogene dinoflagellates and M A Woods on the Chalk palaeontology.

Geological history

In early Palaeozoic times, most of the district lay within the tectonic province known as the Midlands Microcraton, which consists of crystalline basement rocks of Neoproterozoic age overlain by a relatively thin cover (3000 m maximum) of early Proterozoic strata. The crystalline substrate comprises island-arc volcanic rocks which were erupted about 600 Ma and associated marginal basin, sedimentary rocks comparable to the Neoproterozoic strata of Charnwood Forest. The nearest proving of these rocks to the London district is in the Withycombe Farm Borehole about 75 km to north-west.

The oldest cratonic cover rocks are clastic sedimentary rocks of Cambrian and Tremadoc age. These are overlain by Silurian, shallow-water, shelf sediments and Devonian fluvial deposits derived from newly emergent Caledonide massifs of Wales and northern England. Ordovician strata are probably absent beneath the district.

The edge of the Midlands Microcraton lay in the north-east of the district. Beyond it, Silurian strata of turbiditic facies were deposited in relatively deep water, as proved in deep boreholes in East Anglia and north-east Kent (Molyneux, 1991; Sumbler, 1996). The Palaeozoic cratonic cover rocks were deformed in Early Devonian (Emsian) times by the Acadian phase of the Caledonide orogeny, which was caused by the collision of the Armorican Microcontinent with the Caledonian Terrane (Soper et al., 1987). North-east of the Midlands Microcraton, in the area now known as the Eastern England Caledonides, this deformation was intense, as shown by Silurian strata proved in boreholes in East Anglia that are folded and show a penetrative cleavage (Bullard, 1940; Pharaoh et al., 1987; Woodcock, 1991).

The succeeding middle and late Devonian strata rest unconformably on strata of the Acadian fold-belt. They consist mainly of nonmarine clastic sedimentary rocks intercalated with marine deposits laid down during incursions from the south by the Rheic Ocean. During Carboniferous times the uplifted Devonian succession became part of a new stable high, the Anglo-Brabant Massif that stretched from the Welsh Borders to Belgium. Foreland extensional basins developed within the massif as a consequence of the northward advance of the Variscan orogenic front. These basins received clastic sediment eroded from the Massif (Besly and Kelling, 1988) which accumulated in large deltas to form coal measures and red beds of Upper Carboniferous age. The nearest coal measures to the London district are in the Oxfordshire–Berkshire area and east Kent. Illite crystallinity values for the Lower Palaeozoic strata beneath London indicate former burial beneath 4000 to 5000 m of sediment, possibly including Coal Measures, which has been subsequently removed by erosion.

The main east–west orientated Variscan orogenic front lay to the south of the district, although subparallel linear gravity anomalies in the district may be due to peripheral Variscan thrust faults in the Palaeozoic basement rocks (Figure 42) and (Figure 43) (Keary and Rabae, 1996).

Throughout much of the Mesozoic, the district continued to be a stable upland, now known as the London Platform. North–south extension during Permian and Triassic times resulted in the formation of the Weald Basin to the south, which received sediment derived from the London Platform. The bounding structures of this basin had a dominant influence on all subsequent events in the district. From Early Jurassic to Early Cretaceous times, there was further extensional subsidence in the Weald Basin, on reactivated Variscan thrust structures (Chadwick, 1986), including the Addington Thrust (Figure 2). North of this structure, the London district remained as a stable area covered by shallow shelf seas, but lying quite close to land. Jurassic to Early Cretaceous strata record this shallow marine environment but also indicate periods of emergence from time to time with the deposition of nonmarine sediments.

The latest part of the Jurassic Period and the early part of the Cretaceous, sometimes referred to as ‘Late Cimmerian’, was a time of intense tectonic activity in Europe. Late Cimmerian earth movements were associated with the northward extension of a zone of ocean-floor spreading that caused the gradual opening of the north Atlantic, and led to uplift of the London district, retreat of the sea and extensive erosion of the Jurassic strata, uncovering once more the Palaeozoic basement rocks. Subsidence of the fault-bounded Weald Basin to the south continued, and sediments derived form the London Platform accumulated in terrestrial environments to form the Wealden Group. In the early Aptian, the sea once again flooded the London Platform initiating deposition of a post-rift sequence of shallow marine sediments, beginning with the Lower Greensand Group and culminating with the deposition of the Gault and Chalk during a prolonged period of high sea level.

In late Cretaceous times, this region was affected to some extent by the tectonic events that produced the Alpine mountain chain in southern Europe. An early phase of late Cretaceous to early Palaeogene inversion caused gentle folding and erosion of the Chalk. The overlying Palaeogene sediments were laid down in shallow marine and coastal environments. The main Alpine compressional event occurred in late Oligocene to mid-Miocene times, when the London Basin, a broad synclinorium, was developed. Smaller folds, notably the Greenwich Anticline (Figure 42), were caused by inversion of the long-established, deep-seated faults at the southern edge of the London Platform.

For most of the period between the late Eocene and Quaternary, about 40 million years, the district was land, and pre-existing deposits were weathered and dissected. The sea may have covered the district temporarily in Pliocene times, and remnants of coastal gravel deposits now cap the highest ground. A new drainage pattern was established in the early Quaternary. At this time, rivers flowed across the district from the south and south-west towards a major river, the precursor of the River Thames that flowed from Wales, across the Midlands and East Anglia to the North Sea.

When great ice sheets advanced to cover much of Britain as far south as the outskirts of the London district, about 500 000 years ago, the river system was irreversibly changed and the Thames was diverted to its present-day valley. The succeeding river terrace deposits laid down in the valley are well preserved. The main periods of gravel deposition and intervening phases of downcutting took place during cold episodes when rivers were swollen with glacial meltwaters and erosion was more intense. Finally, the most recent deposits, river alluvium and tidal river sediments, have been deposited in the last 8000 years or so, during an interval of relatively low river discharge and periodic flooding.

Chapter 2 Concealed strata

Palaeozoic

Cambrian and Tremadoc clastic deposits, proved in boreholes in Buckinghamshire and Hertfordshire (Molyneux, 1991), probably extend beneath the north-west of the district, where they are overstepped by Upper Devonian strata (Figure 2). Rocks of Ordovician age, such as the Caradoc strata proved in Bobbing Borehole, 19 km to the east, and Arenig strata proved by Strat-A1 Borehole, 18 km to the south-west, are probably absent in the district (Smith, 1987).

Nonmarine Silurian strata of Přídolí age were proved in the Streatham Common Borehole (Table 1); (Table 2), and in the north-west turbidites of Silurian age may be present, similar to those proved in deep boreholes in East Anglia and north-east Kent (Molyneux, 1991; Sumbler, 1996). However, most of the Palaeozoic strata known in the district are nonmarine sandstone of Devonian age; the thickest proved succession is 509 m in Willesden No. 1 Borehole. Lower Devonian (Emsian) sedimentary rocks were proved in Beckton Gasworks Borehole, and other boreholes prove Middle to Upper Devonian rocks of Givetian or Frasnian age (Table 2).

Carboniferous Limestone is proved in the Warlingham Borehole, south of the Addington Thrust (Figure 2) identified as a Variscan structure (Keary and Rabae, 1996).

Triassic

Strata of Triassic age are probably not present in the district, but thin sequences are recorded in the Weald Basin to the south, including conglomeratic limestone and red and grey mottled mudstone proved in the Warlingham Borehole that was tentatively regarded as Triassic – Jurassic in age (Worssam and Ivimey-Cook, 1971).

Jurassic

Movement on the major growth faults that bounded the Weald Basin governed the distribution and thickness of strata of Jurassic age. They occur only in the south of the district (Table 3); (Figure 3) and consist of mudstone with some interbedded limestone and sandstone. The deposits thicken appreciably south of grid line 70; 12 m were proved in the Streatham Common Borehole compared with 1051.6 m in Warlingham Borehole, 15 km to the south (Table 3). This change in thickness is possibly due to syndepositional growth faulting. A similar pattern is illustrated by seismic reflection data to the south of the region (Whittaker, 1985). In general terms, successive Jurassic formations overlap farther onto the London Platform. Some of the younger formations may have originally extended right across it and were removed by contemporary erosion during periods of low sea level and uplift in late Jurassic or early Cretaceous times. The main variation in this pattern is due to syndepositional movement on faults bounding a small graben beneath the eastern part of the Thames estuary (Figure 3) where Oxford Clay is preserved but younger Jurassic strata have been removed by erosion (Owen, 1971).

Cretaceous

Earth movements during the latest part of the Jurassic period and the early part of the Cretaceous resulted in the elevation of the London Platform high above sea level, and further subsidence of the Weald Basin to the south. Jurassic strata were eroded from the platform, exposing the Palaeozoic basement rocks, and accumulated in the basin to form the Wealden Group (Table 4). Mud and sand were deposited in an environment of swamp with lakes and lagoons of varying salinity and crossed by meandering river channels. The Wealden Group occurs along the southern boundary of the region, and is proved in boreholes at Addington, Hartley Bottom and Russell Hill. The succession, largely of interbedded mudstone and sandstone, is 256.6 m thick in the Warlingham Borehole (Worssam et al., 1971), considerably thinner than in the central Weald Basin, although several faults noted in the borehole core may indicate that part of the succession has been faulted out.

Rising sea level in Aptian times flooded the Weald Basin and the southern margins of the London Platform, so that the Lower Greensand Group overlaps the Wealden Group to rest on Jurassic strata (sheets 270, 271). The Lower Greensand occurs in the south of the district, thickening south-west into the Weald Basin. It also occurs in a graben (Owen, 1971) beneath the Thames estuary in the east of the district. At Warlingham the Lower Greensand Group is 94 m thick and includes strata correlated with all formations recognised in the thicker successions in the Weald Basin. At the base the Atherfield Clay Formation is about 7.5 m thick; it passes up into the Hythe Formation (25.5 m thick) of Aptian age, which consists of calcareous greenish grey sand, sandstone and thin sandy mudstone with chert at the top. The succeeding Sandgate Formation rests disconformably on the Hythe Formation marking a marine transgression in the nutfieldiensis Zone. The Sandgate Formation (1.3 m thick) comprises fine-grained glauconitic sand that is characteristically more argillaceous elsewhere in the Weald. The succeeding Folkestone Formation (59.7 m thick) comprises coarse-grained sand of jacobi Zone age. This is interpreted as sandwave deposits like those exposed in the Weald between Reigate and Sevenoaks. Although only small amounts of coarse-grained sand were recovered from the Warlingham Borehole, the gamma-ray logs suggest a relatively uniform sand lithology throughout.

North of Warlingham, (for example in the Richmond, Addington, Russell Hill and Carshalton boreholes), the Lower Greensand is thinner, and the constituent formations cannot be identified with certainty. The sequences are dominated by sand that is variably glauconitic and calcareous with bioturbation in places. These lithologies resemble aspects of the Atherfield, Hythe and Sandgate formations rather than the Folkestone Formation. For example in the Richmond Borehole, 3.2 m of hard, grey, calcareous, glauconitic, shelly sandstone with granules and pebbles of chert may correlate with the Hythe Formation or possibly the Bargate Formation that occurs at the base of the Sandgate Formation in northern parts of the Weald. The Gault Formation was deposited following a marked deepening of the sea in mid-Albian times. It rests disconformably on the Lower Greensand, overstepping northwards onto Jurassic and then Palaeozoic rocks and extending across the entire London Platform, the earliest Cretaceous formation to do so. The Gault Formation is 50 to 70 m thick in most of the district, but ranges from 40 to 94 m, although there is no obvious regional trend (Figure 4). The formation consists mainly of grey mudstone, with variable amounts of silt, in which primary bedding structures are not clearly preserved because of bioturbation. Small, greyish buff concretions or nodules rich in calcium phosphate may be developed around fossils or burrows and seams of bluish black, commonly phosphatic, pebbles and reworked fossils mark non-sequences; these represent debris winnowed from the sediment during periods of erosive current action.

A distinction is made in some boreholes between dark grey mudstone of the Lower Gault, which is of mid-Albian age, and paler, more calcareous mudstone of Upper Gault, of late Albian age (Figure 4). The Lower Gault (less than 10 m thick) is known in detail only in boreholes in the Thames estuary (Owen, 1971) and is absent in the north where it was removed by erosion in late Albian times. In the Thames estuary boreholes, the basal 2.5 m or so consist of glauconitic grey mudstone, gritty in places, with a pebbly and indurated basal bed, and black phosphatic nodules throughout. They are succeeded by about 6 m of medium to dark grey and fawn grey mudstone, which is burrowed and shelly in places and contains seams of phosphatic nodules. A short period of tectonic activity during the earliest part of the late Albian gave rise to slight uplift of the London Platform and movement on the faults that trend east– west across the district (Figure 3). This led to the removal of the Lower Gault from the northern part of the district (Figure 4) The succeeding Upper Gault is between 48 and 52 m thick. It consists of mid to pale grey shelly mudstone, glauconitic in the top 2.5 m, and with a bed (2.44 m thick) containing ‘small black phosphatic nodules’ in the middle of the succession (Owen, 1971).

The Upper Greensand Formation consists of glauconitic, calcareous fine-grained sandstone and siltstone that was deposited during the late Albian. It is present in the central part of the district, but elsewhere it passes laterally into the Upper Gault or is absent having been removed by erosion prior to deposition of the Chalk Group (Figure 4). The maximum thickness is 16.7 m in the Warlingham Borehole. In some borehole records the Glauconitic Marl, which marks the base of the succeeding Chalk Group, may have been mistakenly classified as Upper Greensand.

Chapter 3 Upper Cretaceous: Chalk Group

The main outcrop of the Chalk Group is on the scarp and dip slopes of the North Downs in the south of the district. A small outcrop around Watford, in the north-west of the district, lies on the dip slope of the Chiltern Hills. The typical thickness of Chalk Group is between about 175 and 200 m, with a general thinning of about 20 m from west to east (Figure 5). This is a relatively thin succession compared to over 400 m in the Hampshire Basin and East Anglia.

The BGS borehole at Fetcham Mill, near Leatherhead [TQ 1581 5650] provides the most useful information on the full chalk succession in the London region. (Wood in Murray, 1986; Mortimore and Pomerol, 1987). It is reasonably typical of the Chalk Group of the North Downs, although compared with other locations in the district the Lower Chalk is anomalously thin. The Fetcham Mill succession varies from that of the Chiltern Hills in several important aspects that are influenced by the depositional history of the chalk. Deeper water in the south-east led to more continuous sedimentation than in the north-west, where there were more breaks in sedimentation and development of more prominent indurated and mineralised chalk horizons, known as hardgrounds (Figure 5). This is particularly apparent in the Lewes Chalk in the north-west of the region, where the ‘Chalk Rock’ and ‘Top Rock’ occur, but south-eastwards these pass laterally into a thicker succession containing marls (calcareous mudstones), thin hardgrounds and nodular chalks.

There is an imbalance of information on the Chalk Group in the London district. The Upper Chalk is well documented at outcrop and in exposures in north Kent and south Essex, but the Middle and Lower Chalk stratigraphy is largely inferred from published information on the North Downs (see Robinson, 1986), including sections in quarries in the River Medway valley. In the majority of the London district, where the Chalk is concealed beneath Palaeogene deposits, information is available only from site investigation boreholes that penetrate the topmost 20 m or so.

Correlation of the subsurface chalk succession has been achieved using electrical resistivity logs (Murray 1986). In general terms these logs discriminate well between clay-rich intervals (marls; see below) and hard beds (nodular chalk and hardgrounds), which give high and low resistivity values respectively. However, the presence of numerous, closely spaced, flint-bearing horizons may give an irregular and unpredicatable signature that can be interpreted as either marl or hardground. Wood (in Murray, 1986), aware of this potential problem, cautioned against over interpretation of downhole logs and placing reliance on detailed correlations using them without good borehole core control such as is provided by the Fetcham Mill Borehole. Using this log as a standard, correlation of other selected resistivity logs is shown in (Figure 6).

Nomenclature

Shortly after the completion of mapping in the London district, Bristow et al. (1997) formally subdivided the Chalk Group, recognising up to ten members in the stratigraphically more complete succession in the Wessex Basin. Subsequently, Rawson et al. (2001) re-assigned these members to formational status, and divided the Chalk Group into two subgroups: the Grey Chalk Subgroup and White Chalk Subgroup. The boundary between these is defined at the base of a widespread clay-rich horizon, the Plenus Marls Member (Table 5). This represents the latest step in the progressive refinement of Chalk stratigraphy that has occurred over the last 20 years. Correlation formerly relied on traditional biozones that may include up to 80 m of strata, whereas current resolution, based on a combination of detailed biostratigraphy and lithostratigraphy, defines units of less than 5 m in much of the Chalk. Integration of the new detailed lithostratigraphy with the established biozonation (Table 5) means that former exposures from which fossils were collected can now be referenced to a lithostratigraphical framework. Many of the formations now defined correspond closely with the stratigraphical subdivisions recognised by Mortimore (1986) in the Chalk of the South Downs, and largely supersede the separate stratigraphy erected by Robinson (1986) for the Chalk of the North Downs.

In the London district, formations and their component members are mapped using a combination of topographical features, field brash and biostratigraphical data. However, because of local problems in mapping some of the subdivisions originally defined by Bristow et al. (1997), and partly because the revisions of Rawson et al. (2001) postdate work in the London district, a slightly modified succession is described herein and depicted on the published maps (sheets 256, 257, 270, 271).

The generalised Chalk Group succession, shown in (Figure 7), includes several distinctive beds recognised in exposures and boreholes that are valuable for correlation. The locations of key successions, and their stratigraphical range, is also shown on (Figure 7), and includes some that are outside the district but which are essential for correlation or as good reference exposures.

Lithology

Chalk is typically a very fine-grained white limestone. It consists predominantly of the disaggregated skeletal remains (coccoliths) of tiny planktonic algae, composed of almost pure calcium carbonate in the form of low magnesian calcite. The lower part contains up to 30 per cent clay and includes intercalated clay-rich horizons (marls) and spongiferous limestones.

Layers of flint generally provide the most prominent indication of bedding, and are a conspicuous feature of the higher part of the Chalk Group where they occur at regular intervals. Flint layers are most abundant in the upper part of the Lewes Chalk where they average one every metre. Flint is composed of silica, in the form of microscopic quartz amorphous, derived from the dissolved skeletons of siliceous sponges and microfossils (radiolarians and diatoms) that inhabited the Chalk sea. Silica preferentially replaced the sediment infill of animal burrows some distance below the sea bed while the chalk was still being deposited (Clayton, 1986).

Several types of flint are recognised. The most common is a branching, nodular horn-flint with shapes reflecting the shape of original burrow structures produced by soft-bodied organisms and hard-bodied organisms such as crustaceans (e.g. Thalassinoides-type burrows) (Clayton, 1986). These are up to 0.2 m thick and 1 m in length. Other beds contain elongated nodular flints 40 to 50 mm in diameter, some forming an interconnected branching network through a thickness of 3 to 4 m of chalk (for example the Lewes Tubular Flints in the Lewes Chalk). Laterally continuous or semi-continuous tabular flints, 10 to 100 mm thick (for example Whitaker’s 3-inch Flint in the Upper Chalk), are typical in homogeneous, well-bedded chalk in which burrows are either absent or poorly defined. Layers such as this, which have a distinctive appearance and are geographically extensive, are valuable for correlation. Thin sheets of flint a few tens of millimetres or less in thickness, commonly with a medial cavity, are discordant or subparallel to bedding. They are inferred to have formed along fracture and shear surfaces developed by compressive stress in the Late Cretaceous (Mortimer and Pomerol, 1997).

Clay-rich, calcareous beds, generally described as ‘marls’, are a conspicuous feature particularly of the Lower Chalk. The marls are typically medium to pale grey in colour and occur either as discrete beds or more diffuse concentrations of anastamosing thin marl wisps. Marls in the Lower Chalk are typically up to 0.6 m thick (Ditchfield and Marshall, 1989); those higher in the succession are generally 0.1 to 0.15 m thick. In exposures, the marls are darker in colour than the adjacent chalk beds and where weathered have a rather flaky texture imparted by primary lamination. At outcrop they are preferentially vegetated on account of their relatively high moisture content, and in the subcrop give sharp responses in borehole resistivity and gamma-ray profiles, making them important for the long distance correlation of chalk successions (Murray, 1986; Mortimer and Pomerol, 1987). Some marls show a distinct trace element geochemistry (Wray and Gale, 1993), which is also potentially valuable for correlation. Relatively thick beds of marl in the lower part of the Chalk were probably deposited when carbonate sedimentation was suppressed (Ditchfield and Marshall, 1989). The origin of the thinner marls in the higher part of the Chalk is unresolved, but some are interpreted as decomposed volcanic ash (Wray, 1995; Wray and Wood, 1995) while others are thought to be the result of an enhanced influx of land-derived sediment.

At some levels in the Chalk, hard nodular beds that commonly contain fossil sponges are characteristic (Figure 7). The beds have a lumpy texture and may be stained red or brown due to penecontemporaneous oxidation on the sea floor, rather than subaerial weathering. Nodular beds are thought to have formed in response to lower rates of sedimentation associated with a eustatic fall in sea level, tectonic uplift, or proximity to sea floor highs or basin margins (Hancock, 1989).

Horizons of highly indurated nodular chalk, which are locally iron stained, glauconitised and phosphatised, are referred to as hardgrounds. These formed by enhanced sea-floor cementation associated with periods of nondeposition (Hancock, 1989). Hardgrounds are well developed at or close to basin margins, but in progressively deeper water they pass into nodular beds and eventually become almost imperceptible ‘omission surfaces’, recognisable only by weakly developed iron staining.

Macrofossils are rather rare in the Chalk. Ammonites, used as the basis of standard zonations elsewhere in the Mesozoic, are particularly uncommon and poorly preserved, except in the lower beds. The standard biozonation of the Chalk has developed, therefore, using a combination of ammonites, brachiopods, bivalves, crinoids and echinoids. Inoceramid bivalves have proved to be highly effective in Chalk biostratigraphy, and form the basis for several newly proposed zonal schemes (Jarvis and Woodroof, 1984; Gale, 1995, 1996). Microfossil biozonations for the Chalk (Carter and Hart, 1977; Hart et al., 1989) can be indirectly related to the macrofaunal zones, and are valuable where other faunal data is absent or ambiguous.

Concentrations of the trace fossil Zoophycos are observed at several horizons, and are locally useful for correlation. In unweathered exposures, they appear as dark grey lenses of marl a few millimetres thick, and represent a two-dimensional section through the spirally developed detrital feeding trace of a soft-bodied animal.

Lower Chalk

The Lower Chalk, 65 to 70 m thick, crops out in a tract of gently rising ground less than a kilometre wide at the foot of the North Downs escarpment where it overlies the Gault. In the Warlingham [TQ 3476 5719] and Fetcham Mill boreholes it is about 60 m thick (and overlies Upper Greensand), and in boreholes at Luddesdown over 70 m [TQ 6615 6588]. About 75 m of Lower Chalk are interpreted from the records of a borehole near Watford [TQ 1195 9577].

The lowest bed, the Glauconitic Marl, is soft grey marl rich in glauconite and phosphatic pebbles, typically less than 1 m thick. It is overlain by rhythmic alternations of marl and hard chalk that comprise the Chalk Marl (Jukes-Browne and Hill, 1903). These beds are approximately equivalent to the West Melbury Marly Chalk of Bristow et al. (1997). They were formerly exposed in the large quarry at Halling [TQ 694 645] in the Medway valley. Isotopic studies suggest that the rhythms in the Chalk Marl are due to the effect of regular climatic oscillations on patterns of sedimentation (Ditchfield and Marshall, 1989), probably induced by cyclical changes in the Earth’s orbital parameters (Leary et al., 1989; Gale, 1995).

The higher part of the Lower Chalk succession in the North Downs corresponds to the Zig Zag Chalk of Bristow et al. (1997). It consists of slightly marly, massively bedded chalk, locally known as the Grey Chalk, overlain by soft, white chalk (the ‘White Bed’ of Jukes-Browne and Hill, 1903). The Grey Chalk and ‘White Bed’ are poorly fossiliferous, but close to their junction in the North Downs a 2 m-thick unit of bioclastic chalk contains large specimens of the zonal ammonite Acanthoceras jukesbrownei. The highest beds of the Lower Chalk are alternating greenish marls and marly chalk, typically 3 m thick (Wood in Sumbler, 1996), named the Plenus Marls after the belemnite Actinocamax plenus. In the North Downs, the Plenus Marls normally overlie a burrowed erosion surface. They form the lower part of the exposed succession in Halling quarry [TQ 694 645] where they contain large specimens of the oyster Pycnodonte vesiculare. Up to eight beds, some separated by erosion surfaces have been recognised (Jefferies, 1961, 1963). Jeans et al. (1991) have suggested that they were laid down during a period of enhanced erosion at a time of falling sea level. The Plenus Marls form a widespread marker in southern England and northern France and give a sharply defined, low-value spike in resistivity logs (Figure 6) and a peak in gamma-ray logs.

Middle Chalk

The Middle Chalk is subdivided into the Holywell Chalk overlain by the New Pit Chalk. Traditionally it has been defined as the strata between the top of the Plenus Marls and the base of the Chalk Rock. As the Chalk Rock is not present in the London district, the top of the Middle Chalk is defined at the appearance of the feature-forming hard, nodular chalk at the base of the Lewes Chalk (Figure 7); (Table 5).

Holywell Chalk Member

The Holywell Chalk crops out at the base of the North Downs escarpment in the south-east of the district. It is between 13 and 18 m thick at Halling, and about 14 m are proved in boreholes in the southern and eastern parts of the district, including those at Fetcham Mill and Warlingham.

As a general rule flint does not occur in the Holywell Chalk. Anastomosing marl wisps, locally concentrated into horizons several tens of millimetres thick, are common. The basal few metres are typically indurated and nodular, and give rise to a conspicuous topographical feature above the much softer Plenus Marls. They contain the bivalve Inoceramus ex gr. pictus and acmes of the straight-shelled ammonite Sciponoceras (Mortimore, 1986; Gale, 1996). In previous accounts these basal beds were known as the Melbourn Rock (Jukes-Browne and Hill, 1903; Dewey et al., 1924). The remainder of the Holywell Chalk is less indurated, nodular and has a gritty texture caused by locally abundant, broken shells of the inoceramid bivalve Mytiloides. In borehole resistivity logs, the hard basal beds form a sharp spike and the entire unit gives relatively high values (Figure 6).

The best exposures in Holywell Chalk are in the pit at Halling [TQ 694 645] and the track at Court Farm [TQ 6891 6425].

New Pit Chalk

The New Pit Chalk forms much of the North Downs scarp face. It also crops out in the upper reaches of valleys flowing north on the dip slope of the North Downs. The thickness around Luddesdown [TQ 67 66] and Stansted [TQ 61 62] is 25 to 40 m and at Shoreham [TQ 52 62] 35 to 50 m. Boreholes at Winchester House in central London [TQ 3302 8142] and at Chingford [TQ 3740 9212] prove 46 m and 41 m of New Pit Chalk, respectively.

In contrast to the Holywell Chalk, the New Pit Chalk is softer, smooth textured and more massively bedded. The basal beds contain thin-shelled Mytiloides fragments that are usually poorly preserved chalky moulds. Common fossils in the remainder of the New Pit Chalk are T. lata, the bivalves Inoceramus cuvieri and I. lamarcki, and the echinoid Conulus subrotundus. Small flints occur near the base (Glyndebourne Flints of Mortimore and Pomerol, 1986) and top of the succession, but the greater part is flint free. Marl seams and marly chalk horizons, up to 0.1 m thick, are common throughout. The most widely correlated are the two New Pit Marls (Mortimore, 1986) and the Glynde Marls (Figure 7). The New Pit Marl 2 and Glynde Marls are exposed in the quarry at Dean Farm [TQ 696 657].

In borehole geophysical logs there is a sharp drop in resistivity values at the base of the New Pit Chalk. A further decline generally occurs for some metres above this horizon (Figure 6), punctuated by frequent sharp, low value resistivity-spikes representing marl beds. Similar profiles are identified in numerous boreholes across southern England (Mortimore and Pomerol, 1987).

The lower beds of the New Pit Chalk are exposed in the large quarry at Halling [TQ 694 645] and probably form the greater part of the mostly inaccessible faces in the abandoned quarries at Whorn’s Place North Halling [TQ 7050 6610] and Wingate Wood [TQ 693 651] (Dibley, 1918). A track ascending the main escarpment above Upper Halling [TQ 6895 6448] to [TQ 6895 6460] shows sporadic exposures, but the best currently available section is in a newly opened quarry at Dean Farm [TQ 696 657] near Cuxton, just east of the district, where the top 22 m of New Pit Chalk and the contact with the overlying Lewes Chalk were exposed.

Upper Chalk

In the London district, the lower part of the Upper Chalk is mapped as the Lewes Chalk. The remainder is undivided, but includes strata equivalent to the Seaford Chalk, overlain by beds equivalent to the Margate Chalk and Newhaven Chalk (Robinson, 1986; Bristow et al., 1997; (Table 5)). The maximum total thickness is in the order of 100 m.

Dewey et al. (1924) appear to have partly relied on a distinctive molluscan fauna, known as the Reussianum Fauna (named after the ammonite Hyphantoceras reussianum) to infer the base of the Upper Chalk. However, this fauna apparently occurs below the Bridgewick Marl 1 (Figure 7) in railway cuttings between Coulsdon and Purley, and possibly also in the basal M. cortestudinarium Zone (Davis, 1926, 1929). It is therefore concluded now that the presence of the fauna is an unreliable marker for indicating the base of the Upper Chalk.

Lewes Chalk

The Lewes Chalk crops out mainly in the south-east of the district, at or near the top of the North Downs escarpment, and in the upper reaches of northward-trending valleys around Luddesdown [TQ 67 66], Meopham Green [TQ 64 65], South Street [TQ 640 635], Stansted [TQ 61 62] West Kingsdown [TQ 570 625] and Shoreham [TQ 52 62]. It forms a positive feature on the sides of many of these valleys; the best example of this is in the Darent valley near Eynsford [TQ 540 655] where the higher beds of the Lewes Chalk also give rise to a narrow bench feature. Farther west, it occurs in valley floors between Pratt’s Bottom [TQ 47 62] and west of Biggin Hill [TQ 40 61], and in an inlier in the floor of Swanscombe Western Quarry [TQ 5808 7344]. It may outcrop also in the north-west around Watford and Bushey (Jukes-Browne and Hill, 1904) but has not been mapped.

At outcrop, the Lewes Chalk is generally 25 to 35 m thick, but thins to 20 m locally around Luddesdown and Meopham Green, and is 35 to 40 m in the Darent valley. Geophysical logs indicate that the thickness ranges from 27 to 40 m.

The Lewes Chalk is characteristically hard, nodular, locally iron stained and flinty. Marl seams, up to 0.1 m thick, occur throughout, but are especially conspicuous in the lower beds. Hardgrounds occur locally, and at least some of the thickness variation within the Lewes Chalk may be caused by condensed sequences or depositional breaks at these horizons. Layers of nodular flints are regularly spaced throughout the succession, becoming distinctly carious (i.e. with abundant cavities) in the higher part. At some horizons these flints almost interlock to produce laterally continuous bands. Conspicuously large nodular flints, with dimensions exceeding 0.2 m are developed locally. Thin sheets of flint subparallel or strongly discordant to bedding also occur.

The Lewes Chalk is richly fossiliferous with sponges, brachiopods, bivalves and echinoids. Inoceramid bivalves and the echinoid Micraster are especially important for biostratigraphical correlation. In the higher part of the Lewes Chalk, the occurrence of the inoceramid Cremnoceramus near the junction of the S. plana and M. cortestudinarium zones can be used to identify the base of the Coniacian Stage (Kauffman et al., 1996).

Several of the marls and hardgrounds are useful for correlating the Lewes Chalk. The Southerham Marl 1 is 0.1 m thick with a plastic texture and is characterised by the acme occurrence of the foraminifer Labyrinthidoma (= Coskinophragma of Mortimore, 1986). It is underlain by the Southerham Flints (Mortimore, 1986) a bed characterised by a mixture of relatively large burrow-form nodular flints and smaller finger-like flints. They are exposed in the higher part of the Dean Farm Quarry [TQ 696 657].

Although not all are exposed in the district, the Southerham, Caburn, Bridgewick and Lewes marls of Sussex (Mortimore, 1986) have been traced across the region using geophysical logs (Figure 6); the Lewes Marl is known particularly for the occurrence of the large morphotype of Micraster leskei.

The ‘Lewes Tubular Flints’ (Mortimore, 1986) are vertically elongated burrow-form flints, typically with a central rod of silica surrounded by soft chalk. When weathered or broken open, the core is easily removed and the result is a hollow tube of flint. This is a good marker bed, exposed in Dean Farm Quarry, and identified in a borehole beneath the Thames [TQ 5911 7639] between Thurrock and Swanscombe.

The apparently localised and weakly developed Swanscombe Hardground occurs about 6.5 m below the top of the Lewes Chalk, at possibly the same stratigraphical position as the thicker Cornhill Hardgrounds on the Kent coast (Robinson, 1986).

In the top 5 m of the Lewes Chalk, two thin marl horizons about 2 m apart are underlain by chalk with Zoophycos, tubular flints and orange-stained spongebearing beds. The succession is correlated with the Shoreham Marls, Beachy Head Zoophycos and Beachy Head Sponge Beds (Mortimore, 1986). It is exposed in Swanscombe Western Quarry, and identified in boreholes in the Thurrock–Swanscombe area, at Swanscombe Marshes [TQ 6040 7534]; [TQ 5997 7593] and Islington [TQ 3376 8495].

The beds containing the Shoreham Marls appear to pass laterally into the Rochester Hardground (Robinson, 1986) that has been recorded at the Rose and Crown Pit [TQ 337 594] near Croydon and Martin Earle’s Pit in the Medway valley [TQ 722 679].

Upper Chalk undivided

The youngest Chalk in the region, including most of the chalk outcrops and all the chalk directly beneath the Palaeogene deposits, is mapped as Upper Chalk undivided. The most extensive outcrops are in the north-west and south-east of the region. There are also smaller outcrops in the cores of shallow anticlines at Lewisham [TQ 375 760], Chiselhurst [TQ 43 70], on the south bank of the Thames between Greenwich and Erith, and in the Grays–Purfleet area [TQ 55 78] to [TQ 61 79].

The succession in the Upper Chalk is particularly well known in the south-east of the district because of the extensive exposures in numerous large quarries. These are situated along the length of the Purfleet–Grays outcrop, in the Cray valley and along the south bank of the Thames and between Dartford and Gravesend. Many of the quarries are now abandoned but some still provide extensive sections, the best of them at Swanscombe (Blue Water Park) [TQ 5808 7344], Northfleet [TQ 6207 7394] and [TQ 6321 7410], Grays [TQ 605 795] and Purfleet [TQ 573 786]. Smaller exposures can be examined at Knockholt [TQ 4830 6300] and Farningham Road Station [TQ 5545 6920]. At Chiselhurst Caves [TQ 4322 6957], Camden Park, near Bromley [TQ 4275 7015], and Upper Ifield [TQ 6834 7124], the chalk was formerly worked in underground excavations; those at Chiselhurst are open to the public. Knowledge of the succession at depth is based on cored boreholes, principally between Stratford and Barking, and at London Docklands and the Thames Barrage.The thickness is 59 m in the Grays–Purfleet area (Figure 8), an estimated 50 m in the vicinity of Hook Green [TQ 614 705], 60 to 70 m between Orpington [TQ 45 65], Swanley [TQ 51 68], Hartley [TQ 61 68] and Cobham [TQ 67 69], and 50 m in the Fetcham Mill Borehole.

Firm to soft, non-nodular chalk with common large nodular, tabular and semitabular flints is the typical lithology, but the uppermost beds locally include soft, poorly flinty chalk. Beds rich in the sheet-like shell fragments of the inoceramid bivalve Platyceramus occur at intervals throughout most of the succession. In the lower beds, conspicuous concentrations of thick shell fragments also include the inoceramid bivalve Volviceramus involutus. Thin marls occur in the lower part of the succession, but are absent at higher levels. At least one hardground occurs locally near the top of the succession (see below).

The succession as a whole has relatively low resistivity values on geophysical logs; the profiles are typically spiky, mainly caused by flints, but too variable to allow detailed correlation between boreholes. A drop in average resistivity, accompanied by a reduction in the amplitude and frequency of spikes, occurs towards the top of the succession (Figure 6). It corresponds to the appearance of softer chalk, which in the Fetcham Mill Borehole has a higher water content than the underlying beds (Gray, 1965).

The Belle Tout Marls (Mortimore, 1986) are a series of thin marl seams in brittle chalk in the bottom 8 m of the succession, equating with Robinson’s (1986) Hope Point Marls and Otty Bottom Marl. They have been seen at Swanscombe, Knockholt and in two boreholes at Swanscombe Marshes.

Three distinctive flint beds have proved useful for correlation between exposures and in shallow boreholes in the south-east of the region. None has been identified in the north and west. The lowest is the Seven Sisters Flint, a key marker identified in the North and South Downs and northern France (Mortimore, 1986; Mortimore and Pomerol, 1987). In the North Downs, Robinson (1986) named this flint the Oldstairs Bay Flint, and Bailey et al. (1983) referred to it in east Kent as the East Cliff Semitabular Flint. It crops out about 12 m above the top of the Lewes Chalk at Swanscombe Western Quarry [TQ 5808 7344], occurs in the cutting near Knockholt Station [TQ 4830 6300], and was formerly exposed in the deepest levels of the Pinden landfill site [TQ 5935 6970] (Figure 8). It is a massive rusty, carious semitabular flint about 0.2 m thick and containing inoceramid shell fragments. The chalk adjacent to, and especially below, the Seven Sisters Flint contains abundant, thick-shelled fragments of Platyceramus and Volviceramus.

The Bedwell’s Columnar Flint was identified by Robinson (1986) in sections across the North Downs, although within the London district it has only locally developed ‘columnar’ morphology, and is generally difficult to recognise in sections. It is associated with the upper of two acmes of the inoceramid bivalve Cladoceramus undulatoplicatus. The lower one marks the boundary of the Coniacian and Santonian stages, and the upper is coincident with the flint horizon (Robinson, 1986). Both horizons crop out in the Grays–Purfleet quarries and at the Swanscombe Western Quarry. At Pinden, only the upper horizon is present, and one or other of them has been identified in boreholes around Islington [TQ 3376 8495], Stratford, [TQ 4129 8525], and in a quarry at Northfleet [TQ 6207 7394].

The semi-continuous Whitaker’s 3-inch Flint forms a conspicuous marker horizon, a few metres below the Palaeogene in most of the quarries in the Grays–Purfleet area. It is tabular in form, typically about 0.07 m thick, and has a slightly irregular lower surface that distinguishes it from other (thinner) flat-sided, sheet flints that occur locally in this part of the succession. At Swanscombe Western Quarry [TQ 5808 7344] it lies 22 m above the Seven Sister’s Flint, and may be the prominent flint recorded at the base of the former Ruxley Quarry [TQ 4895 7010] by Dibley (1909). It was also proved in a borehole near Aveley [TQ 5528 7932]. The thickest successions exposed above the Whitaker’s 3-inch Flint are 13 m at Gibbs Quarry [TQ 595 787], 15 m at Swanscombe Eastern Quarry [TQ 590 735] and 20 m at Pinden [TQ 5935 6970] (Figure 8). The succession between this flint and Shoreham Marl 2 is particularly variable in thickness, being 46 m around Grays and 34 m at Swanscombe.

The Barrois Sponge Bed, recognised in east Kent, and its lateral equivalent the Clandon Hardground in Surrey, occur at the top of the Seaford Chalk in the London district; the upper surface of the sponge bed/ hardground marks the base of the overlying Margate Chalk (Robinson, 1986; Bristow et al., 1997). This marker bed is probably represented by two conspicuous horizons of hard, yellow chalk formerly exposed in the pit at St Paul’s Cray (Dewey et al., 1924), and by weakly developed spongiferous chalks at Pinden and in the lower part of the old quarry at Farningham Road Station. It also forms the hardground in the floor of the mines at Chiselhurst, and two indurated chalk horizons 3.6 m apart in the workings at Camden Park (Whitaker, 1889; Jukes-Browne and Hill, 1904). The interval of strata at outcrop between the Barrois Sponge Bed/Clandon Hardground and Whitaker’s 3-inch Flint appears to thicken to the south-east of the region, ranging from 2.5 m at Chiselhurst to 21, 15 and 13 m at Pinden, Swanscombe and Gibbs Quarry [TQ 595 787].

Strata above the Barrois Sponge Bed consist of relatively flint-free chalk in the Orpington area, for example at Cacketshill Wood [TQ 5143 6677] near Crockenhill and north-east of Holwood Farm [TQ 4272 6365] where about 5 m are present beneath the Thanet Sand. Elsewhere the chalk above this horizon contains sporadic flints throughout and locally large flattened flints, typically 0.3 m across, forming discontinuous beds. Conulus albogalerus is locally abundant in these beds which can be seen in a pit at Upper Ifield [TQ 6834 7124] and the cutting at Farningham Road station [TQ 5545 6920] (Figure 8), and were formerly exposed at Ruxley Quarry [TQ 4895 7010] (Dibley, 1909).

In areas where marker beds in the Upper Chalk have not been recognised, fossils diagnostic of the U. socialis and M. testudinarius Zones, which lie above the Barrois Sponge Bed, support determination of the lithostratigraphy. These zones have been documented widely at outcrop around Croydon and south of Bromley (Young, 1905; Dewey et al., 1924). The key fossils are the calyx plates belonging to the index crinoid species although, in the case of U. socialis, these are quite inconspicuous. Notable records of U. socialis are from the former pits at St Paul’s Cray and at Cacketshill Wood and a pit north-east of Holwood Farm [TQ 4272 6365] that apparently included the junction of the U. socialis and M. testudinarius zones (Dewey et al., 1924).

Chapter 4 Palaeogene–Paleocene

At the beginning of Palaeogene time the London district lay on the edge of a sedimentary basin that included much of the present North Sea, and extended eastwards at least as far as Poland. To the west was the proto-Atlantic Ocean, the development of which was associated with rifting and igneous activity that culminated in early Eocene time about 55 million years ago (Knox, 1994). The Palaeogene deposits were laid down during alternating transgressions and regressions driven by global sea level changes and this broad pattern was overprinted locally by tectonic influences. The general succession (Table 6) is divided into major depositional sequences (Knox, 1996) and related to the magnetic chronology and nannofossil zones.

The strategic importance of understanding the stratigraphical and palaeo-environmental history of the Palaeogene deposits of the London Basin became apparent with the discovery, in the early 1970s, of oil-bearing strata of this age beneath the North Sea. In the local context, this understanding, underpinned by information provided from key boreholes (Table 7) has also played a role in the development of better interpretations of site investigation data for the planning of major infrastructure projects in London. Some of the larger ones have been the development of the London Docklands, the building of underground services, including the London Underground, and recently the Channel Tunnel Rail Link.

Thanet Sand Formation

The oldest Palaeogene deposit in the London district is the Thanet Sand Formation. Its base is unconformable on the eroded surface of the Chalk Group. The unconformity is not caused by a single event but, based on evidence in the London Basin as a whole, is attributed to erosion and reworking during two or more depositional sequences (Knox, 1996).

The Thanet Sand Formation occurs at depth beneath much of London and the north-east of the region but is absent north-west of a line from Hillingdon [TQ 10 85] to Borehamwood [TQ 20 95]. The principal outcrops are in the south and east of the region, mainly in outliers around Dartford [TQ 52 73], Swanley [TQ 51 69], Southfleet [TQ 614 711] and Cobham [TQ 671 685]. Thanet Sand is also preserved locally in dissolution pipes and hollows in the Chalk peripheral to these outliers, in some cases beneath a cover of Head or Clay-with-flints. The maximum thickness is about 30 m in the vicinity of Stanford-le-Hope [TQ 68 82], decreasing to the north-west (Figure 9).

The lowest beds of Thanet Sand lie on a roughly planar dip slope formed by the top of the Chalk. This feature is particularly well developed in the vicinity of the A2 road and to the south of it, around Istead Rise [TQ 63 70] and Singlewell [TQ 66 71], and around Horton Kirby [TQ 57 68]. Pockets of Thanet Sand occur in dissolution cavities developed on this surface, but the majority of them do not form features. They are known only from augering, excavations and borehole records; in general they are too small to show on the geological maps. The outliers that constitute the main outcrop of the formation form positive, well-drained features with convex slopes. The basal contact with the chalk is, in many places, at a pronounced concave break of slope. The top of the formation is difficult to place on the basis of features, and is generally located by augering or placed on the evidence of borehole data.

The basal unit, known as the Bullhead Bed, is a conglomerate up to 0.5 m thick. It is variable in lithology, with sporadic rounded flint pebbles up to 50 mm in diameter, and almost unworn nodular flints (‘bullhead flints’) up to 150 mm across set in a dark greenish grey, clayey fine- to coarse-grained sand matrix containing pellets of glauconite up to 1 mm in diameter. The nodular flints are typically coated with dark green crystalline glauconite less than 0.5 mm thick. Small, pale yellow-grey slivers and flakes of unpatinated flint are also present in the matrix. These are resinous in appearance when freshly broken. Small fossils, including fish vertebrae, pelecypod shells, bryozoa and echinoid spines all derived from the underlying chalk are also present.

The bulk of the Thanet Sand is a coarsening-upward sequence, dominantly of fine-grained sand, but clayey and silty in the lower part as illustrated by a typical gamma-ray log signature (Figure 10). The proportion of fine-grained sand ranges from 10 per cent at the base to 60 per cent in the upper beds.

The unweathered sediments are pale to medium grey to brownish grey in colour. They weather at the surface to pale yellowish grey. Contemporary weathering and pedogenic processes locally give rise to a typical podsol profile, with purplish brown weak ferruginous cement developed within 0.8 m of the surface, in the lower part of the profile.

The sediments are intensely bioturbated so that primary sedimentary structures such as lamination are generally lacking. Bioturbation structures are identified as wisps of relatively dark grey clay and silty clay in hand specimen and in exposures. Dark grey to black manganese-rich silt has been observed in the linings of sinuous burrows up to 8 mm in diameter. Scattered oblique and near-vertical burrows also occur in the top 1 to 1.5 m. These are filled with glauconitic sand derived from the overlying Upnor Formation.

Faint bedding is seen in places, in weathered exposures, and some fine lamination is recorded near the top of the formation in the Crystal Palace Borehole (at 152.2 m to 145.8 m depth). Glauconite grains and flakes of white mica are sparsely distributed throughout. Beds weakly indurated by iron oxide have been described in north Kent, and irregular to oblate masses of siliceous sandstone (colloquially known as ‘doggers’) have been recorded in the vicinity of Thurrock and Grays. Irregular nodules of pyrite less than 5 mm across occur rarely, presumably replacing small wood fragments, and Prestwich (1852) described gypsum, presumably from the dissolution of pyrite, at Blackheath.

Grain size analyses of the sediments (Figure 11) reveal that the succession as a whole is remarkably uniform and well sorted; better sorted beds occur towards the top. The sand grains consist almost exclusively of quartz, and are mainly angular and subangular, with only a small proportion of subrounded grains and flint chips. Montmorillonite is generally the dominant clay mineral, thought to be, in part, a weathering product of penecontemporaneous volcanic ash falls (Knox, 1994).

Thin sections indicate the presence of corroded feldspar, minor randomly orientated white mica, chlorite and ilmenite. Authigenic pyrite and glauconite clasts are rare. Some over-sized voids may be caused by dissolution of framework grains. Apatite occurs as fine sand grade detrital grains in samples from central London, but is absent in Crystal Palace and Stanford le Hope boreholes, presumably due to dissolution by acidic groundwater (Hallsworth, 1993).

The heavy mineral assemblage of the Thanet Sand has been studied in samples from the BGS boreholes at Stanford-le-Hope and Crystal Palace (Morton, 1982), and Borehole 404T and Borehole CTRL A2 (Hallsworth, 1993; see (Table 7)). The lower half of the formation contains a rich and diverse suite of heavy minerals, 42 per cent of which are unstable, dominated by an epidote-garnet-hornblende assemblage. This is thought to be the primary heavy mineral suite. A more stable suite dominated by zircon, rutile and tourmaline and a moderately stable garnet dominates the upper part of the formation. These variations are thought to be due to weathering dissolution of the more unstable minerals by meteoric acidic groundwater during postdepositional subaerial exposure.

Representative sections

There are few natural exposures. The basal few metres of the Thanet Sand can be seen in the top part of exposures in former Chalk quarries at Grays [TQ 609 792] and Swanscombe Western Quarry [TQ 606 728]. The top part of the formation is exposed in sand pits near Orsett [TQ 673 806]. Almost complete sequences through the entire formation are held by the BGS from Borehole CTRL A2 in south-east London and Jubilee Line Extension Borehole 404T (see (Table 7) for details).

Biostratigraphy

Calcareous nannoplankton are the most important fossils for regional correlation (Aubry, 1986; Knox et al., 1994). Foraminifera (Curry, 1965) are not useful for stratigraphical purposes, but indicate that deposition occurred in a cool sea at a depth of less then 50 m. Molluscs are not well preserved; they occur sporadically throughout the formation, and include cold water genera (see Ward, 1978). Jolley (1992) has used palynofloral assemblages for regional correlation, relating them to the sequence-stratigraphy scheme of Haq et al. (1987). He recognised that the bulk of the formation in this district was laid in the seventh (T7) of nine onlapping sequences that constitute the formation and its correlatives in the London Basin as a whole. It is possible that the younger T6 and T5 sequences may be present, particularly in the east but the evidence is inconclusive.

Environment of deposition

The deposits are interpreted as inner shelf to coastal in origin. In general, they are well sorted, indicating considerable winnowing and reworking prior to deposition. Grain shape suggests a rather juvenile origin with only minor recycling from older sedimentary formations. The heavy mineral assemblage indicates an input from a single source area rich in an epidote-garnet-amphibole assemblage, and most likely to be the Moine and Dalradian metasediments in Scottish highlands (Hallsworth, 1993).

Lambeth Group

The formal term Lambeth Group has been adopted in recent years (Ellison et al., 1994) to replace the Woolwich and Reading Beds of earlier authors (see for example Whitaker, 1889; Hester, 1965). The group is divided into three formations and several informal lithological units (Table 8). The relationship between these informal units is most complex in the central part of the district, coincident with central and south-east London (Figure 12).

The Lambeth Group crops out in narrow tracts around the margins of the main mass of the Palaeogene, as outliers in the south-east of the district and as small inliers close to the margin, notably in the Lea valley [TQ 38 85]. It also occurs beneath superficial deposits principally between Camberwell [TQ 33 77] and Docklands [TQ 38 79]. No distinctive landforms are associated with the Lambeth Group, but it crops out on concavo-convex slopes or locally on the hill tops in some outliers. Minor springs occur at the top of clay-dominated units, particularly the Lower Shelly Clay.

The Lambeth Group is more than 20 m thick in the south-west, increasing to 30 m at Southwell [TQ 119 799] (Figure 13). In the south-east part of the main outcrop, and in the outliers east of Chiselhurst [TQ 46 70], the top part of the succession was removed by erosion before the deposition of the overlying Harwich Formation (see p.38). The amount of erosion is very variable (Dewey et al., 1924). Locally, for example at Plumstead Common [TQ 45 78], only the Upnor Formation is preserved, and in some of the Palaeogene outliers, for example at Kevingtown [TQ 484 675] and near Swanley [TQ 513 686], all the Lambeth Group has been removed.

The lithological variation of the Lambeth Group at outcrop is documented in considerable detail, particularly in the earlier editions of the Geological Survey memoirs (Whitaker, 1872, 1889), but also in accounts of field excursions made by the Geologists’ Association and in the publications of local societies such as the Croydon Natural History Society, and Essex Naturalist. They provide a wealth of information on sections in pits and railway cuttings that are no longer available for study. Since 1990, new information has become available in the form of complete cores of the Lambeth Group obtained during the course of site investigations in Central London. These, together with the interpretation of numerous other borehole records and the examination of contemporary open sections, have provided a regional appreciation of the variability of the group and its constituent formations, and is the basis for the informal division described below. The lithological variation in the group is illustrated by cross-sections on (Figure 14), and in graphical logs at particular localities on (Figure 15).

The best and most accessible exposure of the Lambeth Group in the London district is Charlton Sand Pit at Maryon Park [TQ 419 786] (Figure 15), now preserved as an SSSI (Daley and Balson, 1999). Other good sections, which are not permanent at the time of writing because they occur in or close to active quarries, are in the Orsett Tarmac Pit near Walton’s Hall, Orsett [TQ 673 803], Orsett Cock Pit [TQ 656 811] (Plate 1) (Ellison, 1979) and Swanscombe Eastern Quarry [TQ 605 730] (Figure 15).

Upnor Formation

The formation, present everywhere at the base of the Lambeth Group, is impractical to map in detail. It is difficult to identify the top of the formation by augering and in many places it is built on or is obscured by more recent sandy wash. The thickness is well documented in boreholes (Figure 16) and exposed sections, but in some borehole logs it is not possible to determine as the Upnor Formation cannot be separated from the Thanet Sand.

The Upnor Formation rests unconformably on Thanet Sand, overlapping onto Chalk north-west of a line from Northolt [TQ 13 84] to Borehamwood [TQ 20 95]. There may be a weak seepage at the contact due to the higher silt and clay content in the Thanet Sand. In exposures the base is generally well defined, and burrows extend up to 2 m below the contact. A basal bed of rounded flint pebbles is usually present. In the east, relatively intense bioturbation has resulted in a gradational junction. In contrast with the Thanet Sand, the Upnor Formation contains slightly coarser grade sand (Plate 2) and the lower beds may be gritty and contain fragments of subangular flint (less than 1 mm across).

The formation consists of fine- to medium-grained sand with a variable proportion of glauconite, beds and stringers of well-rounded flint pebbles, and minor amounts of clay. At outcrop the sediments are pale grey-brown to orange-brown, speckled with dark green grains of fine to medium sand grade glauconite. At depth, the sediments are mainly dark grey and dark greenish grey. In the west, the entire formation is noticeably green caused by a relatively high glauconite content. In north, west and central areas of the district, the highest part and locally the entire thickness of the Upnor Formation is oxidised to a range of brown, orange, red and purple-brown colours, as a result of emergence and pedogenesis during deposition of the overlying Reading Formation. This period of pedogenesis also gave rise to the localised development of carbonate concretions, either in the form of hard irregular masses or powdery patches up to 0.5 m in diameter, and the development of a clay matrix derived by translocation from the overlying deposits.

The sands may be completely bioturbated with no primary bedding, and perpendicular and subhorizontal burrows filled with sand of contrasting colour to the bioturbated matrix. Rare fragments of carbonaceous material occur also. A few impersistent seams of grey clay and angular clasts derived from such seams have been recorded within dominantly bioturbated beds. Other parts of the succession are well bedded and with horizontal planar lamination, ripple lamination, hummocky and planar cross-bedding, and clay drapes. Stringers of well-rounded flint pebbles occur on a few bedding surfaces and there are beds of pebbly sand, mostly less than 0.3 m thick. Thin seams of grey clay, angular clay clasts and rounded balls of clay are also present. Ophiomorpha and Macaronichnus burrows are typical in these beds (Plate 3). Clay-dominated units, up to 0.3 m thick, contain relatively small amounts of sand, arranged in flaser lamination and with lenticular cross-lamination. These strata are well exposed in quarries at Orsett Cock [TQ 657 811] and Orsett Tarmac Quarry [TQ 673 805].

The flint pebbles that occur throughout the formation are generally less than 30 mm in diameter but may exceptionally reach 200 mm, with a black to dark purple cortex and pale grey interior. They are typically well rounded, elongate, spheroidal to flattened spheroidal and in many cases with a slight concavity, presumed to be a result of pressure solution. Many have crescent-shaped percussion marks.

Pebble-dominated units occur principally at the base and top of the formation. In borehole cores, these pebble beds are almost invariably disturbed or only partly recovered, but information on bedding and other details have been recorded from exposure. The pebble bed at the base of the formation is impersistent and up to 1 m thick. A local development of pebble beds occurs in the basal part of the formation at Orsett in south Essex where the pebbles are arranged in large bedforms, in total about 9 m thick, and dipping at 21° to the east (Plate 1). Individual beds fine upwards and many are clast supported with imbrication. These pebble beds are well exposed at Orsett Cock Quarry [TQ 657 810] (Ellison, 1979). At the top of the formation in central and south-east London, there is a persistent pebble bed up to 3 m thick (see (Figure 12); (Figure 15)). Pedogenesis and the precipitation of calcrete have altered the matrix in places.

The oyster Ostrea bellovacina is the most common mollusc as in most places aragonite-shelled molluscs are not preserved. Sharks’ teeth also occur sporadically.

Woolwich Formation

The Woolwich Formation rests on the Lower Mottled Clay of the Reading Formation (Figure 12).

Lower Shelly Clay

This unit, characterised at outcrop by finely comminuted shell debris in a clay soil, is the most easily distinguished of the informal units of the Lambeth Group. It crops out principally in south-east London (Figure 17). In general, the formation thickens from central London towards the south-east, reaching a maximum of 6 m.

The Lower Shelly Clay rests disconformably on the Lower Mottled Clay of the Reading Formation. The base is sharp with burrows up to 10 mm in diameter extending to a depth of 1 m into the underlying strata. The top of the unit is generally a sharp or transitional with the Laminated beds or the Upper Mottled Clay.

The dominant lithology of this unit is dark grey to black clay that contains abundant shells. In east London, there is an increase of medium grade sand in the matrix. Some beds, up to 1 m thick, consist almost entirely of shells forming a weakly cemented coquina in places. One of these beds (0.5 m thick) was proved in a borehole near Stratford. The basal few centimetres of the unit is relatively sandy and commonly contains oyster shells. A bed dominated by oysters, encrusted with bryozoa and cemented in places, occurs locally about 1 to 2 m above the base of the Lower Shelly Clay (Dewey and Bromehead, 1921; Tracey, 1986). These shell-dominated beds indicate that sediment input was low, thus allowing the development of shell banks, and they may represent a maximum flooding surface. A few beds of brownish grey clay occur sporadically throughout, particularly in the higher parts of the unit. These are slightly cemented with siderite. Finely comminuted carbonaceous debris occurs in places; some of it is pyritised.

Lignite (usually less than 0.3 m thick) is commonly seen at the base of the Lower Shelly Clay in the south-eastern part of the outcrop (Figure 15). It consists of soft, brownish black, organic mud with small lignitic wood clasts. At Shorne [TQ 678 697], the lignite is up to 2 m thick and displays a cleat (closely spaced joints) similar to a sub-bituminous coal. It is interbedded with pale grey, leached, medium-grained sand and pale grey clay with lignitic wood fragments and small listric fractures, similar to seatearth.

Dewey and Bromehead (1921, p.20) recorded a ‘freshwater bed’ of limited distribution, within the middle of the ‘shell beds’. This bed is almost certainly the ‘Paludina Limestone’, a marker horizon that has been used to show that the Upper Shelly Clay probably rests directly on the Lower Shelly Clay in south-east London in the vicinity of Petts Wood and St Mary Cray [TQ 45 68] (Figure 14; Whitaker, 1872, p.116), and probably also at Shorne [TQ 678 697] where there is a particularly thick development of shell beds (see (Figure 15)).

Characteristically few species occur in this unit; they are mainly oysters (Ostrea tenera), corbiculid bivalves such as Corbicula cordata and the cerithiid gastropods Brotia melanoides and Tympanotonos funatus.

Laminated beds

This unit generally rests conformably on the Lower Shelly Clay and has a similar distribution (Figure 18). It is 5 m thick south of Stratford. The base may be sharp, a rapid gradation up from the Lower Shelly Clay or, locally, interfingering with Lower Shelly Clay. In some of the eastern part of its occurrence it passes up into and locally on interfingers with the Upper Mottled Clay (Figure 14); Crystal Palace Borehole, (Figure 15). A second unit of Laminated beds occurs higher in the succession around Lewisham where it was formerly known as ‘Striped loams’ (Dewey et al., 1924) (Figure 15). The stratigraphical relationships of the laminated beds are uncertain but they probably have an erosive base, cutting down through the Upper Shelly Clay.

The Laminated beds consist of thinly interbedded fine- to medium-grained sand, silt and clay with scattered intact bivalve shells. Beds are generally less than 50 mm thick and typically finely laminated. Sedimentary features include lenticular bedding, ripple lamination, burrows and some bioturbated, structureless beds. Localised bodies of sand are generally up to 1.4 m thick, but locally up to 5 m (for example Borehole 404T, (Figure 15)). They occur particularly in south-east London around Lambeth and Bermonsey and may have originated as channel fills. More extensive bodies of sands are present between Docklands and Stratford. Typically the sand is pale olive to pale brown, medium grained, well sorted and cross-laminated, with some clay drapes and scattered bivalves. Thin beds of colour mottled clay and silt (Upper Mottled Clay) occur within the Laminated beds between Docklands and Stratford (see (Figure 14)).

At Abbey Wood [TQ 484 787], shelly medium-grained sands underlying the Harwich Formation are significant for their well-preserved fauna, which includes mammals. The strata are included in the Laminated beds, although their age and precise stratigraphical relationship are uncertain (Hooker 1991).

Clay beds with leaf impressions have been recorded from the ‘Striped loams’ at Loam Pit Hill, Lewisham [TQ 375 761] (Whitaker, 1866).

Upper Shelly Clay

The main occurrence of the unit is in south London (Figure 19). South-east and north-east of this, the Upper Shelly Clay is proved only sporadically in boreholes and is inferred to be preserved in shallow depressions below an erosion surface at the base of the Harwich Formation. The unit is up to about 3 m thick.

The base of the unit is sharp and it rests disconformably on the Upper Mottled Clay (for example Borehole A4A, (Figure 15)), or where it rests on Laminated beds the contact may be a rapid gradation. It is likely that beds equivalent to the Upper Shelly Clay are present farther south-east than is shown on (Figure 19) but, in the absence of the intervening Upper Mottled Clay unit, they cannot be distinguished from the Lower Shelly Clay in sections or borehole cores. One exception to this is in the Crystal Palace Borehole where the base of the Upper Shelly Clay is taken at a thin lignite bed (see (Figure 15)). In some borehole records, the Upper Shelly Clay was formerly interpreted as the basal beds of the London Clay Formation.

This unit consists mainly of grey shelly clay with thinly interbedded grey-brown silt and very fine-grained sand with scattered glauconite grains, passing south-eastwards to mainly sand (Figure 19). Bioturbated beds, sand-filled burrows and clay rip-up clasts (less than 5 mm in diameter) are characteristic, and locally there is a weakly cemented shell bed (up to 0.43 m thick) containing Ostrea. Between Bermondsey and Lewisham, a more or less continuous bed of grey limestone with an earthy texture is known as the Paludina Limestone (see (Figure 14); (Figure 15). The bed is generally 0.1 to 0.3 m thick, up to a maximum of 1.89 m, and contains unbroken and comminuted gastropods Hydrobia, Planorbis and Viviparus, which indicate deposition in a freshwater lake. A thin bed of lignite occurs locally at the base of the unit at Crystal Palace (Figure 15).

Most of the Upper Shelly Clay contains disarticulated bivalves of more marine-tolerant species and a generally greater diversity of fauna than in the Lower Shelly Clay.

Reading Formation

The Reading Formation rests on the Upnor Formation in the centre of the district and passes laterally into the Woolwich Formation in the central and south-eastern outcrops of the Lambeth Group. In the central part of the London district the formation is divided into two leaves, the Upper and Lower Mottled Clay, separated by the Woolwich Formation (Figure 12); (Figure 14). The Reading Formation as a whole is present in most of the region, but is thin or absent in the north-east. The Lower Mottled Clay persists in the entire area of the Reading Formation. The Upper Mottled Clay occurs only between Walthamstow and Merton (Figure 20) because of nondeposition or erosion prior to the deposition of the succeeding Harwich Formation.

The formation reaches a maximum of about 20 m in the south-west of the district, thinning progressively eastwards, where it passes laterally into and interfingers with the Woolwich Formation (Figure 12).

The boundary of the Lower Mottled Clay with the underlying Upnor Formation is usually diffuse and difficult to place precisely because of clay translocation and colour mottling caused by pedogenetic processes that affected the top of the Upnor Formation. The base of the Upper Mottled Clay has, in many places, a similarly vague contact with the Laminated beds, except around Stratford where the two units interfinger.

The bulk of the formation consists of unbedded, colour mottled, silty clay and clay. This characteristic lithology was formerly called the ‘Reading Beds’ or ‘plastic clay’. Colours include pale brown and pale grey-blue, dark brown, pale green, red-brown and crimson, depending on the oxidation state of the sediments. The clays contain numerous fissures, many of them listric, which give rise to a blocky texture. Thin, black, carbonaceous clays are recorded locally in the west of the district in the middle of the sequence (see (Figure 14)). Beds of colour-mottled silt and sand constitute up to 50 per cent of the unit, particularly in the east. The colour is dominantly of brown hues, red hues being less prevalent than in the clays. These beds are thinly laminated in places with small burrows and root traces, and minor brecciation caused by soft sediment deformation. Beds of well-sorted sand, mainly in the west of the region, are recorded in borehole logs but are not known in detail. Evidence from adjacent areas to the west suggests these occupy discrete channels.

The Lower Mottled Clay typically shows purple-red hues. The top part of the unit contains irregular-shaped, hard, splintery and soft, powdery carbonate nodules up to 0.5 m in diameter. East of Stratford, the principal lithology is turquoise to dark green and brown mottled, structureless slightly clayey sand with minor amounts of irregularly iron cemented and partially cemented calcareous clayey sands; the beds as a whole become increasingly sandy in an easterly direction. (Figure 12); (Figure 20).

The Upper Mottled Clay is identified principally in cores in central and east London; it consists largely of mottled clay, silty clay and silt with colours similar to those of the Lower Mottled Clay, but the purple hues are absent.

Mineralogy of the Lambeth Group

The non-clay minerals of the Upnor Formation are dominated by quartz with variable amounts of glauconite, some alkali feldspar, chert, minor mica, traces of collophane and calcite. The quartz grains are typically subangular and well sorted (Plate 3). At outcrop glauconite grains in some places are oxidised to yellow-brown goethite and ilmenite. The clay mineral assemblage is generally dominated by smectite with subordinate illite and mixed layer illite-smectite; kaolinite is less than 10 per cent (Figure 21).

The Upnor Formation is characterised by an abundance of stable heavy mineral grains dominated by zircon, along with rutile and tourmaline. Morton (1982) inferred that they are derived from the south, probably Armorica and/or the Ardennes–Rhenish massifs. Hallsworth (1993) found compositionally diverse pyrope garnets in the Upnor Formation, chemically different from those in the Thanet Formation. They are derived either from a source area with heterogeneous rocks or by recycling of detritial minerals from several source areas.

The clay minerals of the Lower Mottled Clay (Figure 21) are dominated by well crystallised smectite that may be derived, at least in part, from the alteration of volcanic ash. The Upper Mottled Clay and clays in the Woolwich Formation are characterised by a mixed clay assemblage, with less smectite and more illite, kaolinite and chlorite. Evidence from scanning electron microscopy shows that much of the kaolinite is very fine grained and intimately mixed with other clay minerals, indicating a detrital origin. Some of the kaolinite is arranged in delicate ‘booklets’ that would not survive agitation during erosion and deposition. Therefore, it is assumed to be authigenic in origin, produced by the weathering of smectite and illite as a consequence of pedogenesis.

The characteristic colour variability of the Woolwich and Reading formations is due to the different states of oxidation and hydration of the iron minerals that constitute only a few per cent by volume of the sediment. In unoxidised grey and black deposits, pyrite is usually the main mineral. Jarosite, a yellow alteration mineral associated with pyrite, occurs in lignitic beds. In the Reading Formation, the most common form of oxidised iron mineral belongs to the goethite species and imparts yellow-brown and brown colours, and is commonly referred to as limonite. Red colours are due to extremely small quantities of hematite, which developed when the sediment was subjected to long periods of subaerial exposure that resulted in drying and dehydration. Earthy hematite nodules less than 5 mm in diameter are also present in places.

The heavy mineral composition of the Woolwich and Reading formations is similar (Hallsworth, 1993), and is dominated by a zircon-rutile-tourmaline suite. There is a low but variable proportion of the less stable minerals epidote, apatite and spessartite garnet. The variablity of the epidote and apatite was probably caused by dissolution during local emergence and pedogenesis (Morton, 1982).

The provenance of the heavy minerals is equivocal. Morton (1982) regarded it as Scottish, similar to the Thanet Sand, suggesting that the heavy minerals were carried by currents from the north and reworked in an estuarine environment. However, the spessartite garnet suggests derivation from a different source, dominated by amphibolite facies metamorphic rocks. Armorica is one such area, although the metamorphic grade is lower than is normally associated with spessartite garnet. The Cornubian granites are another potential source, although they should yield a heavy mineral suite containing a higher proportion tourmaline than is found in the Woolwich and Reading formations. Spessartite garnet occurs also in Coal Measures (Westphalian) sandstones, but there they are associated with other garnets of different composition that do not occur in the Woolwich and Reading formations.

Palaeontology of the Lambeth Group

BGS and the Natural History Museum hold large collections of macrofossils from the Lambeth Group, brought together mainly before the end of the 19th century. Rundle (1970), Hooker (1974) and Tracey (1986), among others, have documented more recent collections; Collinson and Hooker (1987) have reviewed the flora. Molluscs in particular are valuable as indicators of the environment of deposition, but there is no established biozonation of the Lambeth Group as a whole. Calcareous nannofossils in the lower part of the Upnor Formation in central London (Ellison et al., 1996) have been interpreted as nannoplankton (NP) zone NP9, enabling correlation with sequences elsewhere in northwest Europe (Aubry, 1986; see (Table 6)). Another potential tool for biostratigraphical correlation is the use of marine and terrestrial palynomorphs (acritarchs, dinoflagellate cysts, pollen and spores). A mammal fauna discovered in shelly sands at Abbey Wood probably equivalent to the Upper Shelly Clay, has been used for correlation with other European sites (Hooker, 1996).

Environment of deposition of the Lambeth Group

The regional distribution of deposits of the Lambeth Group was recognised as cyclic in nature by Stamp (1921), an idea developed further by Hester (1965) and Ellison (1983). Four depositional sequences separated by unconformities are now recognised (Knox, 1996; (Table 6)). The sediments as a whole were laid down in a coastal or possibly estuarine setting (Figure 22) in which small fluctuations in sea level led to marked changes in environment.

Following a period of regression and weathering of the top of the Thanet Sand, the lowest beds of the Upnor Formation were laid down in transgressive littoral to sublittoral marine, tidal conditions. The abundance of glauconite, most noticeable in the west of the district, indicates periods of sediment starvation, presumably in areas remote from active currents. The Upnor Formation as a whole is interpreted as highstand deposits in which marine flooding events are marked by pulses of glauconite deposition, winnowing and low sediment input (Ellison et al., 1996).

Deposition of the upper part of the Upnor Formation followed a lowering of sea level that may have led to the removal of some of the earlier deposits. Contemporary emergence is indicated by the presence of local silcretes (Kerr, 1955) and clasts of silica-cemented conglomerate in pebble beds at the top of the formation. These clasts are typical of the ‘Hertfordshire Puddingstone’ that, in association with silica cemented sandstones (sarsens), were widespread to the north and west of the district (see Potter, 1998 for a review). This period of emergence marked the beginning of deposition of the Reading Formation on an alluvial floodplain that was liable to intermittent floods and water table fluctuation. Periodic emergence led to oxidation, pedogenesis and the development of the characteristic red coloration, but the instability of this environment precluded the development of extensive colonisation by vegetation. Estuarine and fresh water palynomorphs in the sandier parts of the Lower Mottled Clay in the east are evidence of intermittent encroachment by the sea onto the alluvial floodplain. A temporary rise in sea level led to the establishment of lagoonal and estuarine conditions and the deposition of the Lower Shelly Clay and Laminated beds in the central and eastern parts of the district. Sand bodies in these units contain a brackish water palynomorph assemblage consistent with deposition in estuarine tidal channels. This sequence culminated with a return to continental conditions and deposition of the Upper Mottled Clay. A second rise in sea level and renewed flooding resulted in the deposition of brackish lagoonal and estuarine sediments (Upper Shelly Clay and ‘striped loams’). This period of deposition was terminated by uplift and erosion that removed much of the Lambeth Group sediments in the north and east of the district.

Harwich Formation

This term was introduced by Ellison et al. (1994) to include all sediments between the Lambeth Group and the London Clay Formation. They were formerly differentiated by Prestwich (1854) as the ‘Basement-bed of the London Clay’, and subsequently divided by Whitaker (1866) into the Basement-bed sensu stricto, the Oldhaven Beds and the Blackheath Beds. In the London district, strata formerly mapped as Blackheath Beds are now included in the Harwich Formation.

The Harwich Formation occurs throughout the region (Figure 23), reaching a maximum thickness of 10 to 12 m around Orpington and Chiselhurst. South and east of London, where the deposits were formerly mapped as Blackheath Beds, there are numerous descriptions of former exposures recording very variable thicknesses over quite short distances; this probably indicates an irregular base. Elsewhere in the district, most of the information on the Harwich Formation is from borehole records, which indicate that the Harwich Formation is less than 4 m thick, strata formerly referred to the Basement Bed of the London Clay. In the north-east, an incomplete thickness of 6.88 m was proved in the Stock Borehole [TQ 7054 0045], and in the south-east the Stanford-le-Hope Borehole [TQ 6965 8241] proved 4 m.

The base is sharply defined, and forms an erosive contact on the Lambeth Group. Locally, in outliers at Kevingtown [TQ 485 675] and Swanley [TQ 515 686], the Harwich Formation oversteps onto the Thanet Sand Formation.

Glauconitic fine-grained sand and pebble beds of rounded black flints are the principal lithologies with, in places, common disarticulated and broken shells of marine to brackish fauna (see Dewey et al., 1924). The proportion of pebbles varies considerably. Calcareous, ferruginous and siliceous cements occur locally in beds and masses up to several metres thick (for details see Dewey and Bromehead, 1921; Dewey et al., 1924), particularly at outcrop in the south-east of the district. In the north-east the Harwich Formation is dominated by relatively fine-grained sand and is generally less pebbly. The succession is known in detail only in Stock Borehole [TQ 7054 0045] where grey-green silty fine-grained sand with scattered broken shell fragments and stringers of black flint pebbles are recorded. At outcrop in south Essex, the Harwich Formation consists of sand and pebbly sand that is green-grey in colour, weathering to pale yellow-brown, highly glauconitic and fine to medium-grained. Calcareous mollusc fossils and scattered sharks’ teeth are typical although the shells are decalcified in places.

In areas where pebble beds dominate the sequence they consist of a series of superimposed channels (Tracey, 1986; (Figure 24)), with large foresets composed almost entirely of flint pebbles, imbricated in places, and rare pebbles of siliceous sandstone (similar to sarsen stone). The pebble beds include clasts up to 150 mm in diameter but generally less than 20 mm.

Harwich Formation pebble beds are best exposed in former quarries, now Sites of Special Scientific Interest (SSSI), at Charlton [TQ 419 786] and Elmstead Rock Pit, Chiselhurst [TQ 423 706], and in a small pit at the Inn on the Lake [TQ 675 699] near Gravesend.

Chapter 5 Palaeogene–Eocene

London Clay Formation

The London Clay Formation has had an important influence on the development of London infrastructure particularly as it is a relatively homogeneous and easy tunnelling medium. It underlies most of the district and crops out extensively, except in the south-east where it has been removed by erosion. Much of it is covered by a variable thickness of superficial deposits. It gives rise to low, subdued topography in the Thames valley, for example in the area between Romford and Basildon, and rolling topography with convex slopes generally less than 4º, where dissection has been more pronounced, for example between Stanmore and Hampstead.

As its name implies, the London Clay is predominantly argillaceous, and about 60 per cent of the formation consists of thoroughly bioturbated, slightly calcareous, silty clay to very silty clay. Beds of clayey silt grading to silty fine-grained sand increase in number and thickness from east to west. The sand and silt grains are of subangular quartz, generally less than 125 microns in diameter. Glauconite grains, up to fine to medium sand grade are dispersed through most of the sandy beds. Glauconite grains are concentrated also in some of the more clayey beds, forming marker horizons.

The London Clay at outcrop is oxidised to a brown colour, and may contain secondary carbonate nodules, known as ‘race’. The thickness of the oxidised zone is generally 3 to 6 m but this depends on the lithology. The more permeable sandy beds are more deeply weathered, in places to more than 10 m. Beneath superficial deposits this oxidised zone is generally less than 1 m, but increases under successively older superficial deposits (Chandler, 1999). Below the oxidised layer the unweathered London Clay is grey to blue-grey and characteristically fissured. The top few metres of unweathered London Clay, and the bottom part of the weathered profile (see (Figure 25)), contains selenite (gypsum), the result of pyrite oxidation and mobilisation of carbonate, as crystals up to several centimetres in length. Pyrite is common throughout, and occurs mainly as millimetre-thick sticks (pyritised algal tubes) along with irregular millimetre-sized lumps or flakes and irregular concretions up to 50 mm in diameter. Plant fragments are generally also impregnated with pyrite. White mica flakes are most common in the coarser grained lithologies.

Carbonate concretions of varying size occur throughout the London Clay. The main horizons are shown in (Figure 26). The concretions are mainly flattened spheroids of ferroan calcite (Huggett, 1994; Huggett and Gale, 1995). The larger ones, known as septarian nodules, are generally 300 to 500 mm in diameter, but are locally more than 1 m. They are characterised by radial fractures generally filled with yellow-brown ferroan calcite but more rarely also with pyrite, baryte and vivianite. Many of the concretions have burrows preserved particularly on their outer surface. Thin, impersistent tabular beds less than 50 mm thick of siderite also occur (Hewitt, 1982) principally in unit B (see below and (Figure 26)).

Thickness of the London Clay ranges from 90 to 130 m; the full thickness is present only in outliers where the overlying Bagshot Formation occurs, for example at Hampstead Heath and around Brentwood (Figure 26); (Figure 27).

Subdivision of the London Clay

It has long been recognised that the upper part of the London Clay is more sandy than the lower part, which consists of relatively homogeneous clay (Whittaker, 1866), but attempts to subdivide it (see King, 1981 for a review) were hampered by lack of exposure and borehole cores (for example Wrigley, 1924, 1940).

King (1981) using a combination of biostratigraphy, lithological variation and the identification of marine flooding events defined five divisions (A to E) (Figure 26); (Figure 27). A further improvement in understanding the detailed succession was made with the drilling of stratigraphical boreholes as part of the BGS mapping programme in Essex (Bristow, 1985; Lake et al., 1986). More recently, cored boreholes have been drilled by BGS at Crystal Palace, Hampstead Heath and Stanmore Common. In addition Staines No. 5 Borehole (Ellison and Williamson, 1999), drilled for Thames Water Utilities about 6 km west of this district, has provided a useful reference section (Table 7); (Figure 26); (Figure 27). Samples from all these boreholes are held at BGS Keyworth.

From this extensive body of data and information from recent excavations, the main mass of the London Clay in the district is here divided into five informal units. The lowest four, informally denoted A to D (Figure 27) are not mapped, whereas the top part of the formation is mapped as the Claygate Member. The relationship of these informal units to the divisions of King (1981) is indicated in (Figure 27).

The boundaries between the units are gradational with subtle changes in grain size and glauconite content; sharp boundaries are exceptional. Gamma-ray logs of the cored boreholes (Figure 26) do not clearly differentiate all the lithological changes, probably because the higher content of mica and glauconite in the coarser grained units gives a similar response to the more clayey units. Biostratigraphical assemblages provide additional evidence which helps to define the boundaries (Figure 28).

There are no significant natural exposures of London Clay in the region. Foreshore and cliff sections are accessible along the north coast of the Isle of Sheppey in north Kent [TR 02 72], 50 km east of London, and are the best places to examine the strata. The only large section that is currently exposed is at Ockendon Clay Pit [TQ 611 834] (George and Vincent, 1978; King, 1981).

Lithostratigraphy

The London Clay Formation overlies the Harwich Formation throughout most of the district, but in parts of central and south London where the Harwich Formation is absent the London Clay rests disconformably on the Reading Formation or Woolwich Formation. The base is sharp, planar or undulating and penetrated by cylindrical thalassinoid burrows that descend at least 100 mm below the contact. The top of the London Clay was defined by King (1981) at the base of the ‘Bagshot sands’. The nature of the contact was then uncertain but is now thought to be erosional, and is sharply defined in places but transitional in others.

Unit A

This is a relatively sandy unit (7 to 14 m thick) at the base of the London Clay. The upper part of the unit is intermittently exposed in current workings at the Ockendon Clay Pit [TQ 611 834].

The basal bed consists of fine-grained sand with disseminated glauconite grains of fine to medium sand grade, sporadic, small, well-rounded flint pebbles and, in places, angular to rounded clay clasts derived from the Reading Formation; both clasts and pebbles are less than 10 mm across. Overlying the basal bed are alternations of rather poorly sorted silty clay, sandy silt and silty sand that is finely glauconitic and bioturbated. Disseminated pyrite, fine mica and fragments of lignite are common, and large fragments of wood occur, particularly in the lowest part. Calcareous concretions are rare, but are locally developed around logs. A characteristic of the unit, particularly in the lowest beds, is abundant clusters of horizontal, flattened white silt tubes, less than 1 mm across and up to 30 mm long. They resemble burrow fills, but are fragments of tubular agglutinating foraminiferids flattened by compaction.

Unit B

Silty clay is the dominant lithology and there are several sandy horizons. It is 7 to 18 m thick. The lower part of the unit consists of silty clay, finer grained, better sorted and containing less sand grains than the clay-dominated beds in Unit A; it includes also one or more layers of septarian calcareous concretions. This grades into bioturbated silty clay with fine sand laminae and lenses with a few interbeds of bioturbated sandy silt and silty very fine sand. The laminae are generally thicker and less disturbed by bioturbation towards the top. Many thin layers of semitabular siderite concretions less than 50 mm thick are present west of London (for example in the Staines No. 5 Borehole and in excavations at Heathrow Airport) but are absent farther east.

Unit C

This constitutes the clay-dominated middle part of the formation, about 40 to 52 m thick. A bed at the base, recorded only in the Staines No. 5 Borehole (Figure 26); (Figure 27) and probably restricted only to the westernmost part of the district, contains rounded black flint pebbles in a glauconitic sandy clay matrix. It probably passes eastwards into a thin bed of bioturbated sandy silt, locally with sporadic, relatively coarse sand grains. The bulk of the unit consists of homogenous bioturbated silty clay with layers of calcareous nodular concretions (septaria) up to 1 m in diameter and, in the east, common small phosphatic nodules, about 10 to 20 mm diameter, which are dark brown to black and rounded. In the west of the district there are beds of clayey silt with diffuse boundaries and containing fine-grained sand partings, a millimetre or so thick, and horizons with scattered glauconite pellets of fine to coarse sand grade. In north Kent and Essex, the unit is particularly strongly bioturbated, which has resulted in a uniform lithology throughout.

Unit D

This unit, 30 to 45 m thick, consists of interbedded bioturbated and glauconitic sandy clayey silt to sandy silt, in beds up to 5 m thick. Bed boundaries are mostly diffuse and transitional because of the bioturbation. Layers of septarian nodules occur at a number of levels and phosphatic concretions are present, mostly in the more clayey beds. Silt- and sand-dominated beds with a variable proportion of glauconite grains generally make up less than 10 per cent of the succession; these beds thicken westwards as the clay beds become thinner.

Claygate Member

This is shown on the 1:50 000 Series maps of the district. It was recognised initially in Surrey, around Esher [TQ 14 64] and Claygate [TQ 16 63] (Dewey, 1912), and described in the geological memoirs (Dewey and Bromehead, 1921; Dewey et al., 1924; Bromehead, 1925; Dines and Edmunds, 1925). The Claygate Member was defined by Bristow et al. (1980); it includes all the deposits above the base of the lowest fine-grained sandy bed that are thick enough to be distinguished from the underlying relatively homogeneous clays. This definition was adopted in the adjoining Southend (Lake et al., 1986), Chelmsford (Bristow, 1985) and Epping (Millward et al., 1987) districts and is also used here. Consequently, outliers of the Claygate Member in the north-east of the district in particular are now larger than shown on earlier maps. The criteria are however difficult to maintain in the mapping of outliers of Claygate Member in central London and westwards from there because of the increasing number of sand beds in the underlying Unit D of the London Clay (Figure 26).

In the central and eastern part of the district, the Claygate Member consists of alternating beds of clayey silt, very silty clay, sandy silt and glauconitic silty fine sand. Beds are generally 1 to 5 m thick, although the boundaries are generally diffuse as a result of bioturbation. Ripple laminated fine-grained sand, and flaser lamination with sandfilled burrows are common sedimentary structures in the top part of the succession. The lower part of the member is more generally bioturbated with relatively indistinct lamination and bedding; a bed of calcareous concretions is present near the base in many places. Compared to the underlying Unit D the sandy beds of the Claygate Member are slightly coarser grained and more conspicuously glauconitic. Typically, three principal beds of glauconitic sand are identified in the east (Bristow et al., 1980); some of them have been mapped around Brentwood [TQ 58 92].

In the west of the district, the Claygate Member is principally a finely interbedded and thinly laminated sequence of clay, silt and fine-grained sand with numerous interbeds of planar and lenticular bedded fine-grained, finely laminated sands up to 1 m thick.

Correlation of flooding surfaces in the London Clay Formation as a whole (King, 1981) suggests the Claygate member is diachronous and that deposits in the type area of Surrey are equivalent in age to the top part of Unit D in the east of the district.

Palaeontology

The collection of London Clay fossils was a popular pastime in the early 19th century (see for example Sowerby and Sowerby, 1812–1846), and many participants belonged to a ‘London Clay Club’. The London Clay Formation is probably even now best known to the majority of people through its fossils. Davis and Elliott (1957) briefly summarised the extensive palaeontological studies of the previous 150 years, and Curry (1965) gave additional information.

Organic-walled and phosphatic fossils (for example palynomorphs, crustacean carapaces and vertebrate skeletal debris) are preserved throughout the London Clay. Calcareous macrofossils and microfossils are also widely distributed, except in Unit A and in the higher sandier parts of the formation that are generally decalcified, probably due to the downward passage of acidic groundwater. Calcitic fossils (for example, foraminiferids, ostracods, pectinids and ostreid bivalves) are generally well preserved, although corroded at some levels. Less stable aragonite that makes up most of the mollusc fossils is commonly partially leached; the best preserved molluscs are usually those which had an early diagenetic infill of pyrite.

The distribution and relative abundance of the stratigraphically important fossils is shown on (Figure 28), and a brief overview of the highly varied fauna and flora is given in the following paragraphs.

Detailed analysis carried out by Dr R. Harland (BGS, internal reports) identified dinoflagellate zones D5 (Apectodinium hyperacanthum) to D8 (Charlesdowniea coleothrypta) in core samples from the Staines No. 5, Stanmore and Hampstead Heath boreholes. Microplankton (dinoflagellates and acritarchs) of the London Basin are described by Costa and Downie (1976) and Powell et al. (1996).

Plant macrofossils were reviewed by Collinson (1983). Plant debris, mainly small fragments of plant tissue and wood, occurs throughout the formation. The wood fragments vary in size from small fragments to large logs up to several metres long. At some levels these, although relatively widely dispersed, are the most prominent macrofossils, and commonly they form the nucleus of large calcareous concretions. Most logs are intensely bored by the calcareous tubes of teredinid bivalves (shipworms) (Huggett and Gale, 1995). Well preserved fruit and seeds occur sporadically, and may have internal pyrite-fills.

Calcareous nannofossil assemblages from Zone NP11 and NP12 (Martini, 1971) have been identified, mainly in the Unit C in the Staines No. 5 Borehole (Figure 26) in which reworked Cretaceous nannofossils were also recorded at a number of levels.

Diatoms are preserved, only sporadically, as pyrite moulds as their relatively unstable siliceous (opaline) skeletons were probably dissolved during early diagenesis. They dominate the microfaunal assemblages in Unit A, characterised by the association of Coscinodiscus sp. 1, C. sp. 2 and Triceratium spp. (Figure 28). Sporadic specimens of these taxa and others occur throughout the overlying strata and there is a distinctive assemblage in the top part of the Unit D in the central and western part of the region (King, 1981).

Radiolaria, in the form of pyrite replacing the original siliceous skeletons, survive only sparsely in Unit A, but are relatively common at some levels in the top part of Unit D.

Benthic foraminiferids are common throughout. Assemblages vary laterally and vertically; King (1981) recognised a succession of assemblages in the eastern part of the London Basin and thought they were controlled by transgressive events (1981). These events probably also controlled the distribution of planktonic foraminiferids. These show a low diversity, and they are generally less abundant and less evenly distributed than the benthic foraminiferids. A distinct increase in their abundance in the middle part of Unit C corresponds to the ‘planktonic datum’ of Wright (1972); in the top part of Unit D there is the first occurrence of several taxa, including Pseudohastigerina wilcoxensis.

Molluscs dominate the macrofauna, but in general specimens are widely dispersed and imperfectly preserved.

Unusually abundant and well preserved assemblages from the top part of Unit D at Highgate [TQ 282 873] were discovered in the early 19th century. Subsequently, Edwards and Wood (1849–1877) and Wrigley (1924; 1940) described numerous other taxa and lists from recent exposures in London are given, for example, in Rundle and Cooper (1970), Kirby (1974) and Tracey et al. (2002).

The most prominent molluscs are large nautiloids (chiefly Euciphoceras and Cimomia). Many of these are infilled with calcareous mudstone and their nacreous test layer is intact. They are disproportionately well represented in museum collections as they are often retrieved from excavations by construction workers. Bivalves, gastropods and scaphopods are dominant. In mudstone-dominated lithologies, molluscs are dominantly small deposit-feeding types (Nuculaceans and Thyasira). They are not evenly distributed, but occur most commonly in relatively thin units and are associated with the annelid Ditrupa. In silt and fine sand-dominated beds Striarca wrigleyi is locally common, associated with Pitar, Semimodiola and Modiolus. The gastropods (naticaceans, turrids, buccinids) are believed to have been predominantly carnivores and scavengers.

Planktonic molluscs (pteropods) occur throughout the London Clay and are abundant at some horizons (Curry, 1965). They are of considerable stratigraphical value, and four zones are differentiated (King, 1981).

Ostracods are not common but occur in relatively diverse assemblages in the lower part of the formation; they are more abundant but less diverse in the upper part, and absent in Unit A. A zonation scheme proposed by Keen (1977) has been revised by King (1981).

Crabs and lobster fossils form of the most striking part of the fauna. They occur throughout, but are most common in Unit C. They are preserved as isolated skeletal limb fragments and rarely as complete carapaces, which are usually filled by and partly enclosed in phosphatic concretions.

Isolated crinoid ossicles and parts of stems, mainly of Cainocrinus and Isselocrinus, are common in the middle of the London Clay and the restricted occurrence of I. basaltiformis in a 5 m-thick interval near the bottom of Unit C is a useful marker horizon. Sharks’ teeth are sparsely distributed, fragments of teleost scales and bones are common, but entire skulls and articulated skeletons are extremely rare. Representatives of other fossils groups include pyritised sponge spicules (rare), serpulids Ditrupa and Rotularia (fairly common throughout), bryozoa (rare but generally associated with detritus drifts that accumulated around large wood fragments), and the brachiopods Lingula (throughout) and Terebratulina wardenensis (sporadic in Unit C). Environment of deposition

The London Clay Formation was laid down in entirely marine conditions, either on open shelf or a more restricted lagoon or embayment. Claygate Member sediments are typical of tidal conditions, whereas the more sandy beds elsewhere in the succession are more likely to be storm-influenced but subtidal. Glauconite-rich sediments and lenticular sideritic concretions, such as those in Unit B, were formed during temporary breaks in sedimentation whereas septarian nodules are thought to have formed during periods of slow burial (Huggett, 1994).

Depositional sequences

King (1981) recognised that the London Clay was deposited during a succession of transgressive-regressive sequences. These are particularly well developed to the west of the London district and in the Hampshire Basin. The base of each is marked by an omission surface, overlain by thin poorly sorted glauconitic and locally pebbly transgressive units. These marker horizons were used to delineate King’s (1981) informal divisions of the formation, but they do not in all cases correspond to the boundaries of the main lithological units described in this account. The basal omission surface is recognised as the sequence boundary, and the overlying glauconitic sediment as the transgressive systems tract; the succeeding coarsening-upwards intervals that comprise most of the formation are the highstand systems tracts. These depositional sequences are difficult to recognise in the relatively deep-water sediments of the London district, but the transgressive sequence tracts, represented by thin glauconitic units are identified. Another method of correlating the depositional sequences is the use of a combination of lithological and biostratigraphical data, for example an influx of planktonic foraminifera and planktonic gastropods (pteropods) is interpreted as indicating a major rise in sea level.

Bagshot Formation

The formation is well known from historic accounts of sections and pits given in the memoirs covering the district, and a review by Wooldridge (1924). More recently details of the succession have been obtained from cored boreholes at Hampstead Heath, Crystal Palace, Stock and Westleigh Heights (Figure 27); (Table 7). The maximum thickness proved in the main outliers is: Hampstead Heath 18 m; Billericay 12 m; Brentwod 15 m; Stock 27.48 m; Westleigh Heights 17 m. The maximum estimated thickness in the Esher Common area is 10 m.

Bagshot Formation caps the highest ground in the district, mainly in the north-east and occurs as isolated outliers in central London and the south-west of the district (Figure 27); it gives rise to steep convex slopes up to 12°. It is characteristically free draining, and where undisturbed by development supports a typically heathland vegetation.

The base of the formation is well defined by a sharp lithological change, commonly marked by springs and a change of slope. Local erosion at the base has removed the top beds of the Claygate Member (Dewey and Bromehead, 1921, plate II). The basal bed in the outlier at Hampstead Heath is coarse grit with small wellrounded flint pebbles, but this is a local development. The formation comprises cross-laminated, yellow, ochreous brown and orange-brown, fine-grained quartz sand, which is silty in parts. The sand contains subordinate feldspar and white mica and grains of heavy minerals, mainly zircon and tourmaline. Laminae of pale grey clay, less than 10 mm thick, are common and there are sporadic units, generally up to 1 m thick, of thinly interbedded, flaser laminated, pale grey to greenish grey, silty clay, clayey silt and fine sand. Bioturbation is sporadic, but locally almost the entire succession is affected, as in the Hampstead Heath Borehole. The formation is oxidised throughout, and decalcified. Local iron pans, less than 50 mm thick, are developed in places.

The highest strata in the succession were proved in the Stock Borehole (Bristow, 1985) where 10.1 m of beds dominated by silt and clay occur above the typical Bagshot sand.

It is overlain by 4.22 m of mainly well-rounded flint pebble beds, known as the ‘Bagshot Pebble Bed’. Similar beds are recorded around Brentwood at Langtons [TQ 578 948] and possibly also at Holden’s Wood [TQ 5910 9135] but have not been mapped in this district. The origin of these pebble beds has been a source of debate (review in Bristow, 1985) but it is possible that they belong partly with the overlying Stanmore Gravel of Quaternary age (see p.52).

The formation was deposited in a shallow marine and estuarine environment, similar to that of the top part of the London Clay Formation, but supply of sediment was greater. Similar, but better exposed, sediments of the same age in the Hampshire Basin are thought to have had a tidal influence (Plint, 1984).

Chapter 6 Quaternary

Quaternary deposits, also known as drift or superficial deposits, were laid down in the London district during the last 1.65 million years or so. They provide evidence of an ancient river system, a precursor to the River Thames, a glaciation of Anglian age in the north of the district, and the development of the present River Thames valley. The climatic oscillations that led to the change from glacial to warm temperate conditions are now (tentatively) related to oxygen isotope stages (Table 9).

Pre-Anglian deposits

The Pre-Anglian deposits were formerly referred to as ‘high level pebble gravels’. They occur as numerous small, and in many cases isolated, patches of sand and gravel at a higher elevation than the main river terrace deposits, and are largely outside the limits of the Anglian glacial deposits. They have not been extensively exploited for aggregate and consequently are known in detail only from small pits and temporary exposures. Nevertheless, many attempts have been made to reconstruct the drainage history of the region based on the evidence of these deposits, most recently by Gibbard (1985, 1995) and Bridgland (1994).

Stanmore Gravel

These deposits crop out on the highest ground, mainly in the north of the district (Figure 29a). Most were formerly referred to ‘Pebble Gravel’ or ‘Plateau Gravel’, dependent largely on their altitude; a few outcrops were named ‘Warley Gravel’ (Dines and Edmunds, 1925). Exposures at Stock [TQ 685 988] in the extreme north-east of the district were mapped as the Bagshot Pebble Bed of Palaeogene age (Bristow, 1985). They are redefined in this account based on a description of the type area at Harrow Weald Common [TQ 147 929] (Bridgland, 1995; Gibbard, 1999).

The deposits almost invariably cap hill tops and give rise to clayey and silty soils containing abundant brown, red and black well-rounded flint pebbles and minor amounts of small, white vein quartz, subangular and nodular flint and rounded Triassic ‘Bunter’ quartzite pebbles. Many of the weathered flint clasts are partially desilicified, forming a white, bleached surface patina that in some cases extends deep into the clast. In the type area most of the flint clasts are between 16 and 150 mm across. Quartz clasts make up 30 per cent of the 4 to 8 mm fraction but only 0.3 per cent of the 16 mm or greater fraction, while Lower Greensand cherts are 10.5 per cent of the 4 to 8 mm fraction but only 0.4 per cent are greater than 16 mm (Moffat, 1986). The matrix consists of orange-brown, pale grey and bright red mottled clay and sandy clay with pockets of coarse sand in places. Cryoturbation structures are almost invariably present in the top 2 m of the deposit in the form of involutions and festoons containing pebbles with long axes orientated vertically and near-vertically. Beds of sand and gravel also occur locally; their thickness is highly variable up to about 5 m. No systematic work has been carried out to compare the pebble content of the widely spread deposits, but there is some regional variation as summarised in the (Table 10).

Red staining of clay matrix and flint pebbles is believed to be the result of pedogenic clay enrichment and rubification in a warm humid climate; an event tentatively correlated with development of the pre-Anglian sol lessivé (Moffat and Catt, 1982) that is widespread beneath till in Essex and Suffolk.

The origin of the Stanmore Gravel is uncertain. No absolute or comparative dates have been obtained from the deposits and their correlation remains speculative. An exhaustive review by Bridgland (1994) concluded that they were laid down in south bank tributaries of a precursor of the present Thames, presumed to be earlier courses of the Mole-Wey, Wandle, and Darent. However, the deposits appear to be too widespread to relate to individual river deposits, and a re-evaluation of the altitude of the deposits shows that they lie more or less on an extensive planar surface dipping gently to the north-east (Figure 29a). This surface coincides closely with the base of the Red Crag, constructed from isolated outcrops in the London Basin region combined with data from the more extensive distribution in Essex and Suffolk (Moffat et al., 1986; Mathers and Zalasiewicz, 1988). The outcrop of Well Hill Gravel (see p.57) also falls within the elevation range of this surface. The conclusion drawn from this re-evaluation is that the Stanmore Gravel and Well Hill Gravel may be marine in origin, supporting the view of Wooldridge (1960). It also concurs with the observations of Hey et al. (1971) who claimed that the surface texture of sand grains in the Stanmore Gravel indicate deposition in a fairly low-energy beach environment, although these textures could equally have been formed during deposition of the parent Palaeogene deposit.

Clay-with-flints

Clay-with-flints crops out in the south-east of the district forming a dissected spread largely on the dip slope of the North Downs. The surface of the outcrop is gently inclined, parallel to the regional northerly dip of the chalk. The Clay-with-flints almost invariably overlies chalk, only locally resting on Thanet Sand. A simplified profile through the deposit on the North Downs is shown on (Figure 30).

Clay-with-flints is heterogeneous and unbedded. Typically, it has irregular vertical and lateral changes of texture, colour and clast content that are too localised and complex to be shown on the geological maps. It gives rise to a reddish brown silty and sandy clay soil with angular and nodular flints. In some places it is dominantly sandy and in others there are abundant well-rounded flints. Variations between these types may occur within 100 m, but there is no apparent geographical pattern. The lithologies include reddish brown clay with large unworn flint cobbles, yellow fine- to medium-grained sand, reddish brown clayey silt and sandy clay with beds of well-rounded flint pebbles (Catt, 1986). At the base in many places is a dark brown to black, stiff and waxy clay, less than 100 mm thick, and containing relatively fresh nodular flints. These flints may be stained black by manganese precipitated from groundwater and have a green glauconitic cortex, similar to flints at the base of the Thanet Sand. In the main body of the Clay-with-flints the more argillaceous sediments tend to occur at the top (Figure 30). There are three main types of flint:

• very well-rounded, subspherical pebbles derived from Palaeogene strata
• complete or broken nodules derived directly from Chalk
• frost-shattered angular shards caused by alternate freeze-thaw of ground ice during periglacial periods

In general, the thickness ranges from 5 to 10 m, but is highly variable over short distances, probably due to dissolution of the chalk which has taken place mostly after the formation of the Clay-with-flints. The base is highly irregular with localised pipes and hollows between 1 and 50 m in diameter.

Clay-with-flints is a remanié deposit formed by weathering and solifluction of the original Palaeogene cover and earlier Quaternary deposits and dissolution of the underlying Chalk. Its formation probably commenced during late Pliocene or early Quaternary times during uplift and erosion of the Red Crag and underlying Palaeogene and Chalk. The Clay-with-flints has been subjected to pedogenesis and weathering during periods of warm climate, which has resulted in reddening and clay enrichment, and to several periods of periglacial climate that has produced intensive cryoturbation and solifluction.

The more argillaceous parts of the Clay-with-flints have many small fissures that provide significant permeability (Klinck et al., 1998). This fissuring was probably formed initially in permafrost conditions by small lenses of segregated ice. Some of the larger fissures, particularly those close to the base of the deposit and in karstic cavities, have slickensides that are indicative of slippage, possibly during subsidence which followed dissolution of the chalk.

Chelsfield Gravel

The Chelsfield Gravel is a newly defined unit. It occurs only in the south-east of the district in small outcrops at the type area at Chelsfield [TQ 476 642] and at locations between West Kingsdown [TQ 577 612] and Holly Hill [TQ 669 630] within the Clay-with-flints outcrop on the chalk dip slope at elevations between 125 and 170 m OD. The deposit consists of well-rounded flint pebbles in a clayey and silty fine-grained sand matrix, which gives rise to a pebbly soil. The gravel in the type area is beyond the main Clay-with-flints outcrop, and is interpreted as a head deposit, partly let down with the Thanet Sand into dissolution hollows in the Chalk. The deposit at Holly Hill, which forms a prominent steep-sided feature, is probably an outlier of Harwich Formation that has been only partly reworked. The other small outcrops were also derived from the Harwich Formation and have been incorporated in the Clay-with-flints.

Pre-diversionary Thames River Terrace Deposits

Deposits of the ancestral Thames river system were laid down in a valley that crossed the north-west of the district, more or less coincident with the present-day Colne. They comprise the Gerrards Cross Gravel and Westmill Gravel, and form gently sloping terrace-like features, degraded by dissection and solifluction. In general, the deposits overlie chalk, with a contact likely to be irregular due to dissolution. These deposits are part of the Kesgrave Sands and Gravels that extend across East Anglia from the Vale of St Albans, and form the largest body of sorted Quaternary coarse-grained sediments on the British land area (Rose et al., 2001). They are characterised by a relatively high proportion of quartz and quartzite pebbles derived from the English Midlands hinterland (Table 11).

The Dollis Hill Gravel and Woodford Gravel (Figure 29b) form hill-top caps that decline in elevation northwards, indicating deposition in south-bank tributaries of the ancestral Thames (see Gibbard, 1985). Information about these gravels and their composition is summarised in (Table 11) and (Table 12).

At Darenth Wood [TQ 580 727] and nearby to the south, outcrops of a dissected gravel deposit mapped as River Terrace Deposits undifferentiated occur at an elevation of 65 to 80 m above OD, intermediate between the Stanmore Gravel (see p.52) and the Black Park Gravel. The clasts are mainly rounded black ‘Tertiary’ flints, brown angular flints, and rare quartz and Lower Greensand. The origin of this deposit is uncertain, but it was probably laid down in a river flowing north from the Weald (Figure 29b) and turning east, possibly along the line of the modern Thames (Gibbard, 1994).

Other deposits

Outcrops of high level gravels not assigned to the Stanmore Gravel are the Well Hill Gravel [TQ 497 642] and Sand and Gravel of unknown age and origin at Crystal Palace, Norwood and Streatham Common (Figure 29a). Both contain noticeable amounts of Lower Greensand chert, up to 10 per cent in the Well Hill Gravel (Peake, 1982). The Well Hill Gravel lies at an altitude close to the inferred surface on which the Stanmore Gravel was deposited, and therefore the two deposits are interpreted as coeval. The gravels at Crystal Palace are well below the level of the Stanmore Gravel. They are interpreted as a fluvial deposit laid down by a river flowing in an easterly direction, perpendicular to the chalk dip slope. It seems unlikely that such a river would have eroded through the chalk escarpment and then flowed north along a proto-Wandle course as suggested by Macklin (1981) and reviewed by Gibbard (1994).

Anglian deposits

Details of localities and thickness are given in (Table 13).

Glaciofluvial deposits

Outwash from the Anglian ice sheet in the north of the district is mapped as glaciofluvial deposits. These deposits occur in relatively small outliers in the north-west of the district (Figure 31), close to the outcrops of the till and at a similar height. Locally the till overlies the glaciofluvial deposits, for example at Chigwell Row [TQ 468 931] and in the River Wid valley [TQ 610 975]. The extent beneath the till is not known, but is unlikely to be widespread as there is little evidence for large outwash channels either at outcrop or in boreholes.

The deposits consist of brown and yellow-brown variably clayey sand and gravel with clasts mainly of well-rounded flint (derived from the Stanmore Gravel) and subangular flint, with a few worn nodular flints and a smaller proportion of rounded quartz and quartzite pebbles and cobbles. The deposits at outcrop generally display cryoturbation structures.

Glaciolacustrine deposits

These crop out in the east of the district near Chapmans Farm, Upminster [TQ 565 890], close to the A127 (Figure 31). They were formerly exposed overlying till in the Upminster brickpits (Dines and Edmunds, 1925). The deposits consist of brown, grey, greenish grey and lilac, thinly interbedded sand, sandy clay, and silty clay. They are thought to have been laid down in a proglacial lake formed at or close to the Anglian ice front at the time of its maximum extent. An interbed of reworked (soliflucted) London Clay (Woodward, 1904) is interpreted as evidence of contemporary periglacial conditions.

Similar deposits of laminated silty clay and sandy silt deposits, up to 1.5 m, thick occur beneath till near Finchley e.g.[TQ 276 901]. They are interpreted as having been laid down in an ice-dammed lake (Gibbard, 1979).

Till

Till, formerly known as ‘chalky boulder clay’, crops out principally in the north-east of the district (Figure 31),with outliers around Finchley [TQ 26 91] and in the Colne valley in the north-west of the district. It was laid down at the southern margin of the Anglian ice sheet, and is part of a once-continuous sheet of till that extends from this district northwards across Essex and Suffolk.

The till is a hetereogeneous deposit consisting mainly of firm to stiff, pebbly, variably silty and sandy clay. At the surface it weathers to yellow-brown and ochreous colours; below about 2 m, where less oxidised, the till is pale grey becoming darker grey with depth. Clasts in the till are typically of chalk and flint with subordinate rounded Triassic ‘Bunter’ vein quartz and quartzite, minor Jurassic and Carboniferous limestones and sandstones, and rare granite and basalt (Moorlock and Smith, 1991). The chalk clasts are leached out of the weathered near-surface deposits in the top 2 m or so, but locally, for example around Battles Hall [TQ 496 959] near Stapleford Abbotts, the leached zone is less than 0.5 m. A relatively high proportion of well-rounded flint pebbles, derived from the Stanmore Gravel, are present in till outliers particularly south and east of Brentwood. The clay matrix is almost certainly derived from Jurassic as well as Palaeogene clays and, where not decalcified, typically contains a significant proportion of chalk flour.

The distribution of till and the altitude of the base of the deposit indicates the form of the ice sheet at the maximum extent of the Anglian glaciation (Figure 31). It is possible that two ice advances are represented in the district: an earlier one in the Finchley area, and a later one with ice tongues moving down what are now the Lea, and the Roding valleys and a narrow tongue extending to Hornchurch [TQ 550 879] (Baker and Jones, 1980; Bridgland, 1994).

Post-Anglian deposits

Post-diversionary river terrace deposits

Evidence suggests that the River Thames was diverted into its present valley late in Anglian times about 500 000 years ago (for overviews see Bridgland, 1994; Gibbard, 1985), thus marking the start of the deposition of extensive river terrace gravels in the district.

The deposits occur in a sequence of river terraces that are differentiated on the basis of altitude. The history and extent of research on the sequence, including the development of the nomenclature, is summarised in Bridgland (1994).

The terrace gravels occur at progressively lower elevations above the modern floodplain, a situation now considered to be a response to continuing neotectonic uplift (Maddy, 1997). The bulk of the gravels were deposited on a broad river braid plain during colder periods when periglacial activity made available the greatest volume of sediment. In the lower reaches of the Thames, aggradation of some terrace deposits was complicated by rising sea level, which increased the space available to accommodate sediment, and led to deposition of mainly fine-grained inner-estuary sediments.

Deposits from more than one cold climate episode may be represented in a single river terrace deposit. A basal cold climate gravel may be overlain by interglacial deposits that in turn are overlain by more cold climate gravels (Figure 32).

Sites with fossils and Palaeolithic artefacts are known from numerous localities in the river terrace deposits. These contain evidence for both cold and interglacial intervals, and allow the terrace deposits of the district to be correlated with oxygen isotope stages (Bowen, 1999; (Table 9)). The relationship of these sites to the river terrace deposits is shown in (Figure 34).

The earliest systematic subdivision of the river gravels, into Boyn Hill Gravel, Taplow Gravel and Floodplain Gravel, was made on the Geological Survey maps of the district published around the beginning of the 20th century. Hare (1947) made the next significant advances. He identified the (highest) Black Park terrace, formerly mapped by the Geological Survey as Glacial gravel, and the Lynch Hill terrace at an intermediate height between the Boyn Hill and Taplow Terraces. The gravels that underlie all these terrace features have subsequently been mapped as River Terrace Deposits.

Alterations to the terrace nomenclature, including the introduction of a numbering system (Table 14), were made as a consequence of a revision survey of the south London district, the results of which were incorporated on the 1:50 000 scale map published in 1981. An additional terrace deposit, numbered 3a, mapped between the Taplow and Lynch Hill Gravel, has been named Hackney Gravel on the most recent maps.

In a review of the Middle Thames terrace sequence, Gibbard (1985) correlated the terrace gravel deposits between Reading and central London. He extended the scheme erected by Hare, introducing the terms Kempton Park Gravel to replace the Floodplain Gravel, the Shepperton Gravel for even younger gravels that underlie the alluvium in the London district, and Staines Alluvial deposits for the fine-grained river floodplain sediments. His terminology, with the exception of the Staines Alluvial deposits, has been adopted in this account (Table 14) and on the geological maps of the district. The six principal river terrace deposits that have been mapped correspond in general to those of earlier workers, with minor additions and renaming as a consequence of analysis of new borehole data in combination with topographical evidence. In addition, one small outcrop of an additional terrace deposit, the Finsbury Gravel, at an altitude intermediate between the Boyn Hill and Lynch Hill Gravels, has been mapped.

Gibbard (1994) reviewed the sequence of terrace deposits in the Lower Thames valley, downstream from central London. In this area he introduced a series of local names whose equivalence with the Middle Thames sequence is shown on (Table 14).

Correlation of the terrace deposits in this district is illustrated on (Figure 34), which shows the base and surface of each deposit projected onto a notional river-centre line.

In many places the River Terrace deposits form a bench or terrace feature that is bounded by a concave break of slope on the margin farthest from the contemporary river channel, and a convex slope on the margin nearest the river. These terrace features are particularly well developed on the north side of the Thames, for example between Southall and Teddington and Stoke Newington and Bow. The terraces are also apparent in the centre of London, for example between Soho [TQ 296 811] and the River Thames at Whitehall [TQ 304 800]. In many places, however, where different terraces are adjacent they merge into a single planar or concavo-convex slope. The outcrop of the base of the deposits is defined clearly where it rests on London Clay. In some places head covers this contact (see for example (Figure 33)), and the boundary is determined on borehole data. The basal surface of the deposits may be irregular, either due to channelling or to dissolution where Chalk forms the bedrock.

Many of the older terrace outcrops are on hilltops, for example Wimbledon Common [TQ 235 735], Islington [TQ 314 838] and Dartford Heath [TQ 52 73], and in general have been dissected by erosion to a greater extent. The outcrop of the base of these deposits is commonly masked by downwash, and is more accurately determined on borehole evidence.

River Terrace Deposits consist of variable proportions of sand and gravel. They were deposited in a braided river system, an estimated 5 km wide. Gibbard (1995) in a review of the sedimentary features of the deposits identified gravel-dominated beds, generally less than 2 m thick, characterised by horizontal bedding with little internal structure and rare imbrication. These are cut by broad, shallow channels, which are infilled with tabular cross-bedded gravelly sand in fining-upwards units. Trough cross-bedded, sand-dominated beds, up to 3 m thick, are cut through the other units. Fine-grained sediment occurs locally as impersistent beds, less than 1 m thick; it consists of clayey and silty sand. In the east of the district, thicker sequences of fine- to coarse-grained sands occur and are interpreted as deposits laid down in estuarine conditions. The river terrace deposits have yielded vertebrate remains and Palaeolithic flint artefacts at numerous localities (see Gibbard, 1985,1995; Wymer, 1999).

The thickness of the deposits varies considerably as indicated by the range of maximum values given in (Table 15).

Individual River Terrace Deposits are lithologically indistinguishable although there are minor local variations in clast lithologies and proportions (Gibbard, 1985, pp.96, 146; Gibbard, 1994, pp.119, 216; Bridgland, 1994, p.181). For example, the Black Park Gravel contains most erratics and shows the greatest variability of clasts, presumably because they are derived from the glacial deposits. A higher proportion of Lower Greensand clasts are found in terrace deposits of the rivers Mole and Darent, and there is a consequent increase in these clasts in the Thames gravels close to the confluences. There is also evidence (Gibbard, 1985, 1994) of a minor decrease in the proportion of quartz and quartzite clasts and an increase in angular flints in the more recent gravels.

The Black Park Gravel (Figure 29c) is generally recognised as the oldest deposit laid down by the Thames in its current post-diversion valley. Gibbard (1985) traced the gravel from its type area west of this district to Wimbledon Common [TQ 235 735], and tentatively correlated it with the deposit capping Hangar Hill [TQ 182 819] (Figure 34).

The Boyn Hill Gravel (Figure 29d) was first defined near Maidenhead (Bromehead, 1912). It occurs in the main Thames valley and in the tributary valleys of the rivers Lea, Roding and Wey.

The Finsbury Gravel, first identified by this resurvey, crops out only at Finsbury [TQ 315 829]. It may be related to a phase of deposition of the Lynch Hill Gravel close to the confluence of the rivers Lea and Thames.

Much of the area now mapped as Lynch Hill Gravel (Figure 29e) was formerly shown on maps as Taplow Gravel. Hare (1947) first recognised it as intermediate between the Boyn Hill and Taplow gravels, and it was identified on the 1981 edition of the geological maps as Terrace 3b.

The Hackney Gravel (Figure 29f) formerly known as the Taplow or ‘Middle’ terrace (Bromehead, 1925), was identified as a separate deposit (Terrace 3a) on the 1981 edition of the geological maps. The altitude range of the Hackney Gravel is similar to that of the Lynch Hill Gravel, but as the base is generally lower it may represent the lower part of a single deposit encompassing both gravels, although this is not proved.

The Taplow Gravel (Figure 29g) is correlated from the type area [TQ SU 916 816] near Maidenhead eastwards to London. Extensive deposits of Taplow Gravel occur in the Thames valley and in the lower parts of the Brent, Wandle, Lea, Cray and Darent valleys. The outcrops are less extensive than formerly shown on geological maps because parts of them have been renamed.

The type section of the Kempton Park Gravel (Figure 29h) is at Kempton Park [TQ 118 703] (Gibbard and Hall, 1982). The deposits equate with the ‘upper floodplain gravel’ of Dewey and Bromehead (1921) and much of the Floodplain gravel of Bromehead (1925). They constitute the lowest terrace deposit above the floodplain of the Thames, but east of Woolwich [TQ 405 784] and Poplar [TQ 38 81] the deposits are concealed beneath alluvium.

Some anomalies of correlation of the River Terrace Deposits are illustrated in (Figure 34). Black Park and Boyn Hill gravels, in particular, have been the subject of a long standing, and as yet unresolved debate (summarised by Bridgland, 1994, and Wymer, 1999), concerning the relationship between the height and age of the deposits. The surface of the Boyn Hill gravel deposits at Dartford Heath [TQ 52 73] is at about 40 to 42 m above OD and the base is channelled down to about 25 to 28 m above OD. At Swanscombe, the surface level of these terrace deposits is about 8 m lower than at Dartford Heath, but the base is at a similar level to the floor of channels at Dartford Heath. The elevation of other outcrops of the Boyn Hill Gravel, at South Ockendon [TQ 595 834] and Orsett Heath [TQ 64 80], corresponds closely to that at Swanscombe. Bridgland (1994) put forward evidence that all the deposits should be regarded as Boyn Hill Gravel, the gravels of highest elevation being a ‘feather edge’ remnant of the deposit. However (Figure 34) indicates this is not the case, and that the height range of the Black Park and Boyn Hill gravels overlap. This may be because the deposits at Dartford Heath include gravels equivalent to both the Boyn Hill and Black Park gravels (Gibbard, 1995 p.18; Wymer, 1999, p.72), with the younger Boyn Hill Gravel at the surface, as shown on the geological maps.

The relationship of terrace gravels to till in the Thames valley is problematic for terrace correlation. The Black Park Gravel has been recognised as the first post-diversionary river gravel in the Thames valley, and was therefore deposited after the till. The base level of the river in which the Black Park Gravel was deposited would have cut down lower than the base of the till, which is at 25 m above OD in the Hornchurch railway cutting [TQ 547 874] (Figure 34). However, at Dartford Heath and Swanscombe, Hoxnian interglacial deposits that postdate the Anglian Till and Black Park Gravel are within the altitude range of the Black Park Gravel rather than below it as might be expected (Bridgland, 1994). This brings into doubt the relative ages of the till, Hoxnian deposits and the Black Park Gravel.

Deposits in deep depressions

Irregularities in the rockhead surface beneath Quaternary deposits are generally less than 5 m in amplitude, but locally beneath the Kempton Park Gravel, for example in central London between Battersea and Greenwich, there are enclosed depressions in the rockhead surface. These are known as ‘scour hollows’, which were documented by Berry (1979) and whose locations are shown on (Figure 35). Nearly all are eroded into London Clay, but some cut through into the underlying Lambeth Group and, exceptionally, the Chalk. In some cases the bedrock beneath the depression appears to have been uplifted and the underlying strata reduced in thickness. This is particularly apparent where the underlying strata is the London Clay Formation. The depth of ‘scour hollows’ is usually 5 to 15 m but exceptionally more than 33 m are recorded at Battersea and 60 m at Blackwall (where the base is not proved). In plan, the depressions may be irregular, roughly circular or boat-shaped, and vary from about 90 to 475 m wide. The sides are steep (locally with ‘cliff-like walls’; Berry, 1979), many with slopes of 20° or greater. Infill deposits consist mainly sand and gravel with some clayey beds. The clasts are predominantly flint, but where chalk forms the rockhead it also occurs within the fill, ranging from silt to large boulder grade. The deposits are generally stratified, but may be disturbed by soft sediment deformation as illustrated by core drilled through the deposits at Blackwall (Figure 36). Upward injections of London Clay that penetrate the deposits from the base have also been recorded. The deposits accumulated during the Devensian transition from cold to warm climate, and some contain palaeontological and palynological evidence of interglacial conditions (Berry, 1979).

Formerly, these depressions were thought to have been formed by the scouring action of seasonal, glacial meltwater in periglacial conditions (Berry, 1979). However, this mechanism does not entirely explain the formation of the deeper hollows, and some of the associated features such as the apparent bulging of underlying strata. Thus, it is now thought that the depressions were originally at the sites of open-system pingos, where massive bodies of ground ice grew by drawing water from unfrozen groundwater beneath the permafrost (Hutchinson, 1980); this could account for the bulging of underlying strata. During interglacial or interstadial times, the pingos melted, resulting in collapse of the overlying deposits into the hollow made by the melting ice. At the same time a sudden release of high hydrostatic pressure in the confined Chalk and lower ‘Tertiary’ sand aquifers would have caused a rapid ejection of water carrying with it Quaternary sediments and bedrock debris, and further deepening the existing hollow. The presence of large chalk blocks in the deposits at Blackwall, at least 15 m above the level of the Chalk bedrock (Figure 36) provides evidence of the powerful upward forces involved.

Brickearth

Deposits formerly mapped as ‘brickearth’ are divided into six units (Table 16) whose outcrops are shown on (Figure 37). They consist predominantly of silt and occur mainly on gentle slopes overlying river terrace gravels. In some places (for example at Hounslow West [TQ 11 77]) there is a veneer of this type of deposit extending across more than one terrace. In others, particularly at the back of terraces farthest from the present-day river, the deposits overlap onto bedrock. Where there is a relatively steep bedrock slope the deposits may be locally banked up against it, for example at Southall [TQ 12 81], Stoke Newington [TQ 33 86] and Crayford [TQ 514 767]. ‘Brickearth’ is generally less than 3 m thick (see (Table 17)). Formerly it may have been more extensive than shown on the geological maps, but much has been removed for brickmaking since Roman times, particularly in areas now built up. There are no significant lithological differences between the six units, and they are separated simply on the basis of their distribution rather than age.

The deposits consist of very fine-grained sand, silt, and clayey silt, which is brown to orange-brown in colour. They are unstratified with characteristic vertical columnar desiccation cracks. The top metre or so is decalcified but below this there are scattered irregular-shaped nodules of reprecipitated calcium carbonate (race), sometimes mistaken as chalk. Scattered angular flint fragments are common and there are sporadic pebbly seams. Gibbard (1985, 1994) concluded that the silt-grade grains are loessic (windblown) in origin, but gravel in the basal parts of the deposit was probably transported by solifluction; lamination, also present in the basal beds, indicates fluvial deposition.

Locally, laminated and cross-bedded sand, usually in association with interglacial sediments, beneath the typical ‘brickearth’ deposits is included in the thickness of the deposits classified as ‘brickearth’ (Table 17).

Interglacial deposits

Interglacial lacustrine deposits are mapped only around Peckham in south London [TQ 340 767]. Deposits of a similar origin are recorded from at least 25 sites in the London region (Figure 38); (Table 18), in association with River Terrace Deposits and the ‘brickearths’. They are fossiliferous, providing evidence of deposition in both cold and warm (interglacial) climates, and a key for correlation of Quaternary sequences in Britain.

The mode of formation and preservation of interglacial sediments and their relationship to river terrace deposits is not clear in all cases. The sediments occur mainly at or close to the back of River Terrace Deposits (see (Figure 34)) where they are overlain by head deposits derived from the contemporary river bluff. At several localities they form a channel fill in the river terrace surface (for example, at Aveley, (Figure 39)); in others they fill a channel cut into bedrock, beneath the river terrace gravels. Particularly in the east of the district, the relationship of interglacial deposits and associated terrace deposits is complicated because of estuarine influence on deposition.

Differentiation between interglacial deposits of different ages is based on several methods including absolute and relative dating. C¹⁴ absolute dating is the only one that has proved successful so far, but is of value only for sediments of less than 40 000 years old. Work on the amino-acid geochronology of shells from Quaternary deposits at interglacial sites in this district has yielded mainly reliable results (Bowen et al., 1989; Bowen et al., 1995). Other relative dating is based on the fauna and flora of the Quaternary sediments (for a review see Wymer, 1999). Pollen is used to a limited extent to separate interglacials. Mammal faunas differ from glacial to interglacial stages and some species occur in deposits of a particular age. For example the giant beaver became extinct after the Hoxnian (OIS 11), hippopotamus and horse bones have been found in the Ipswichian (OIS 5e) but not the Hoxnian (OIS 11) whereas the extinct water vole Arvicola cantiana is found in deposits interpreted as OIS 11, but not in earlier interglacial deposits. Numerous Palaeolithic flint implements have been collected in the district. These are also valuable for the determination of the relative dates of deposits (Wymer, 1999).

Alluvium

Alluvium forms a nearly flat surface in valley floors. It occurs principally in the River Thames valley, its main tributaries and also in minor valleys where there is a distinctive floodplain developed (Figure 40). In the Mar Dyke area, north of Thurrock [TQ 61 82] there are some surface irregularities in alluvium, perhaps related to drainage and consequent desiccation and shrinkage of organic-rich layers.

The base of the alluvium in the Thames valley falls regionally from about sea level in the west of the district to lower than 10 m below OD in the east. The deposits rest unconformably on river gravel with a gently undulating boundary, particularly in the east, probably caused by channelling similar to that identified at Westminster (Wilkinson et al., 2000) and Plumstead Marshes (Devoy, 1979).

The thickness of alluvium in the Thames valley varies from less than 1 m in places in the west to around 15 m at Tilbury (Table 19). It is less than 3 m thick, and absent locally, below the present-day river channel west of Docklands. In tributary valleys, it is generally 2 to 5 m thick. Alluvium consists largely of silty clay and clayey silt with locally developed beds of fine- to coarse-grained sand mainly less than 1 m thick but locally up to 4 m, for example in the vicinity of the Blackwall tunnel (Bates and Barham, 1995). There are also sporadic beds with scattered pebbles and granules. In the Thames valley from Erith eastwards, and to a lesser extent in the Cray and Darent valleys, chalk clasts of silt to granule grade derived from the underlying bedrock may occur in the basal part of the alluvium. East of Tilbury the succession contains a relatively high proportion of silt to fine sand.

Interbedded peat occurs in the east (Figure 40), with four main horizons recorded between Swanscombe and Tilbury (Devoy, 1979); the most widespread of these extends west to the Rotherhithe tunnel [TQ 355 805]. The total thickness of peat beds exceeds 2 m in large areas between the confluence of the rivers Thames and Lea and Tilbury (Bingley et al., 1999).

Tidal river or creek deposits

These are mapped only in areas between man-made sea defences and the mean low-water mark shown on the Ordnance Survey maps. They occur downstream of a point about 1.5 km west of the Thames Barrier [TQ 415 795] where there is a high suspended load and much sedimentation related to the position of the salt-water and freshwater mixing zone (Prentice, 1972). The preservation of these deposits is, in many areas, only temporary. The veneer of mud deposited during one tidal cycle is typically scoured off during another cycle, but in some places there may be a net accretion between the high and low water marks. However, this sediment eventually becomes unstable and slides towards the river channel where it is taken up in suspension.

Peat

Peat crops out only locally, as it is mostly interbedded with alluvium (see above). It occurs, for example, in a small valley fed by springs [TQ 685 695] emanating from the Harwich Formation, associated with abandoned river courses in the Kempton Park Gravel near Camberwell [TQ 323 792]; [TQ 362 774] and on the spring line at the base of the Claygate Member of the London Clay near Herongate [TQ 6408 9022]. It is forming at present only in areas of permanent wet land, for example in ponds fed by springs. Thin beds of peat have been identified as Devensian interstadial deposits of some rivers, for example interbedded in the Lea valley alluvium (see p.76 — Lea Valley Arctic Bed). More recent peat deposits occur in the fen areas of the Mar Dyke valley e.g. [TQ 62 85], which were poorly drained in former times.

Head

Head is defined as material that has moved downslope by solifluction (mass movement under periglacial conditions).

It formed principally beyond the ice limit during the glacial stages of the Pleistocene, when mass wasting was accelerated because of the arctic climate and lack of vegetation. When snow melted in the spring, debris of frost-weathered material formed a slurry, which gradually flowed downhill to form a poorly bedded deposit of variable character.

Periglacial solifluction has been the most potent agent of erosion in the district. As a consequence most of the hillslopes are essentially relict periglacial landforms that were largely formed during successive Pleistocene cold stages. The slopes have been modified only to a limited extent under temperate interglacial conditions (Ballantyne and Harris, 1994). The district has been beyond the glacial limit and thus subjected to longer periods of periglacial conditions than farther north. Consequently head deposits are widespread and almost certainly occur more extensively than are shown on the geological maps. However, as it has proved impractical to map solifluction deposits separately, the head that has been mapped includes these and surface wash and creep material.

In general, head occurs beneath concave slopes on the flanks and floors of valleys. Because of its local derivation, it is extremely variable in lithology, but composition closely reflects the source. For example, head derived from London Clay is clayey, and that derived from River Terrace Deposits is gravelly and sandy. Head derived from Chalk is generally calcareous, but may be locally decalcified depending on the acidity of ground and surface water.

The majority of the deposits are clay-dominated, derived from London Clay. Generally less than 2 m thick, they probably accumulated in shallow mudslides of softened and brecciated bedrock in the active layer. They consist of soft ochreous brown silty clay with blue-grey mottling in places and angular, frost-shattered fragments of flint occur sporadically throughout. At the base of these deposits and interbedded in places, there is a bed of pebbly clay, generally less than 0.2 m thick, with well-rounded flint pebbles derived from nearby outcrops of ‘high level’ gravel such as Stanmore Gravel. Beneath the head, there may be a low-angle shear surface or series of shears in the top part of the London Clay. Head derived dominantly from London Clay is mapped extensively south of Brentwood [TQ 60 90], around Wimbledon Common [TQ 23 72] and Richmond Park [TQ 20 73]. In these three areas there was considerable dissection in periglacial conditions. Head is likely to be widespread also in the areas of dissected topography in the north-west of the district, although it has not been mapped. (Table 20) gives the location of the thicker Head deposits known in the district.

Head deposits are extensive on the almost featureless and gentle concavo-convex slopes in the Mar Dyke valley between Upminster [TQ 56 87] and Bulphan [TQ 64 86]. These too consist of remobilised London Clay with a basal gravelly clay layer. Solifluction sheets collected in this area from the surrounding high ground to the north and east. The broad Mar Dyke depression, formed after deposition of the majority of the River Terrace Deposits, may have been a large nivation hollow in Devensian time.

Head in the dry valleys on Chalk bedrock consists mainly of sandy silt and angular to subangular flint derived from dissolution of the chalk and from the Clay-with-flints. It is likely to have been reworked periodically by ephemeral streams that flow after high rainfall and when the water table rises above the valley floor. Locally the head on Chalk bedrock consists almost entirely of chalk clasts that may be cemented in places (formerly known as Coombe Rock). It occurs around Greenhithe and Swanscombe where deposits up to 3 m thick appear to be in the form of a fan overlying a steep Chalk bedrock surface, and between Purfleet [TQ 56 78] and Grays [TQ 62 78] (Plate 4). The outcrops at the latter localities are small, mainly because much of the deposit has been removed by quarrying for chalk. Similar deposits of soliflucted chalk debris may also occur beneath the larger dry valleys in the south of the district.

Small patches of head occur on interfluves, for example near Longfield [TQ 59 68] and Meopham [TQ 655 665]. They are derived from Thanet Sand and Clay-with-flints and consist mainly of fine-grained sand with variable amounts of clay and a mixture of angular, subangular and well-rounded flint pebbles.

Head Gravel

These deposits are formed by solifluction and downwash of gravel-dominated deposits. In the area around Brentwood they occur generally on interfluves downslope from outcrops of Stanmore Gravel. Outcrops on the chalk in the south-east of the district are a mixture of fine-grained sand derived from the Thanet Sand, flinty clay derived from Clay-with-flints and well-rounded black flint pebbles derived from the Harwich Formation; many of the outcrops are in shallow dissolution depressions in the chalk.

Sequence of Quaternary events

The broad outline of Quaternary events in the district has been understood since the early work of the Geological Survey (see for example Whitaker, 1884; Prestwich, 1891). A modern account of the regional setting for these events is given by Sumbler (1996). Because of the fragmentary distribution of the Quaternary deposits, the correlation between many of them has been the subject of a long-running and continuing debate. There are also conflicting views about the age of some of the deposits in relation to the oxygen isotope stages (Table 9).

The oldest Quaternary deposits, the Stanmore Gravel, are interpreted as inshore or beach deposits, equivalent in age to the marine sands of the Red Crag that occur to the north-west of the district in Essex and Suffolk. These deposits were subjected to neotectonic uplift in early Quaternary times, which led to the removal by erosion of much of the Stanmore Gravel, and, in the south of the district, the exhumation of the Palaeogene deposits on the dip slope of the North Downs. Subsequently, Clay-with-flints accumulated on this exhumed surface.

The earliest major river system in the district drained northwards towards an ancestral Thames that flowed across East Anglia; this has been dated as Oxygen Isotope Stages (IOS) 16 to 14.

One tributary flowed along the present-day Colne valley and another across the western part of the district. Deposits of these rivers were the last to be laid down before the drainage system was radically modified as a result of ice advance during the Anglian glaciation, about 500 000 years ago. The Anglian ice blocked the rivers, which were diverted into what is now the Thames valley. The mechanism of this diversion is complex with four ice advances being recognised (Cheshire, 1981) in the Vale of St Albans to the north of this district. One of the earlier ice advances may have diverted the Thames into a spillway along the line of the present-day Lea valley, causing major erosion. This mechanism provides an explanation of the abnormal size of the Lea valley in relation to its current small catchment area. Later ice advances blocked the Lea spillway and diverted the Thames into its present valley. The last pre-diversionary Thames river deposit was the Westmill Gravel, preserved in the Colne valley in the north-west of the district. Small tongues of ice extended from the main ice sheet, to the north of the district, southwards along pre-existing valleys including the Colne in the north-west, the Roding at Hornchurch, and the ‘Finchley depression’ [TQ 26 91] (Figure 31). It is unlikely that these valleys were formed by ice scour because they were occupied by relatively small, marginal ice lobes with only minor erosive potential.

The relationship between the Anglian till and underlying gravels is uncertain. Two lines of evidence suggest that there was a considerable period of time between them. First, a palaeosol developed locally in the top part of the Gerrards Cross Gravel (Bridgland, 1994, p.121) may correspond to the widespread interglacial palaeosol developed on the correlative Kesgrave Sands and Gravels farther north and east. Second, the topography across which the valley ice tongues advanced was in several places significantly lower than the base level of the gravels. The latter evidence indicates a major period of erosion in the Thames valley before the ice advanced. It indicates also that there was already a depression in the present Thames valley prior to its occupation by the diverted River Thames.

It is generally agreed (for overviews see Gibbard, 1985; Bridgland, 1994) that the River Thames was diverted into its present valley late in Anglian times. This event led to the deposition of extensive river gravels that now form the River Terrace Deposits. Post-Anglian times were characterised by climatic oscillations, which were associated with marked changes of sea level that significantly influenced the depositional environment.

During cold periods rapid erosion and the downslope movement of vast quantities of material occurred in periglacial conditions. Much of the debris produced was transported in the contemporary river systems and laid down as river terrace deposits. Remnants of the periglacial deposits are mapped as Head and Head Gravel. In intervening warm periods, interglacial sediments accumulated in abandoned meanders that were cut into the River Terrace Deposits or in channels cut into bedrock, but because of later erosion these are preserved only locally. Interpretation of the relationship between River Terrace Deposits and Interglacial Deposits in the Thames valley suggests that there are cold fluvial deposits and warm interglacial deposits representative of all the oxygen isotope stages from IOS 11 and IOS 3 (Figure 41).

The oldest post-diversionary terrace deposit, the Black Park Gravel, is generally accepted as late Anglian (IOS 12) in age. The next oldest terrace deposits, the Boyn Hill Gravel, contains interglacial deposits at Swanscombe which are internationally important because of the discovery of the ‘Swanscombe Skull’ and associated flint artefacts. Although there are some uncertainties because of the lack of pollen data, the Swanscombe sediments are attributed to the first interglacial after the Anglian, now generally regarded as OIS 11 (Bowen, 1999). The subsequent Lynch Hill Gravel includes interglacial sediments at a number of sites, principally at Purfleet and Grays. These sediments include laminated beds of estuarine origin, recording an episode of high sea level. The fossil plants and animal remains are not diagnostic of any interglacial period but their position in the terrace sequence suggests deposition in the second of the four post-Anglian interglacials, generally ascribed to IOS 9 by Bridgland (1994), but regarded as earlier by Gibbard (1999).

Interglacial deposits have been found associated with the Taplow Gravel in several places, notably at Uphall Pit Iford, Aveley and Ockendon, again with evidence for estuarine conditions in part. These sites have been attributed to the last (Ipswichian) interglacial on the basis of pollen analysis, a view challenged by palaeontologists working on mammals and molluscs, who refer the deposits to the third of the post-Anglian interglacials, denoted as OIS 7, and termed the Ilfordian by some workers.

The last interglacial (Ipswichian sensu stricto) is now generally correlated with IOS 5. Sediments of this age have not been found downstream from London as they are now buried beneath the modern floodplain. Deposits referred to the IOS 5e occur at Trafalgar Square and Kew, within the Kempton Park Gravel.

The Devensian glacial stage, from about 122 000 to 10 000 years before present (BP), incorporates at least three short episodes of milder climate (interstadials). The first two of these, known as Chelford and Upton Warren interstadials, occurred before the main Devensian glaciation; the latter is represented by deposits in the top part of the Kempton Park Gravel at Isleworth, Twickenham and Kempton Park.

The main part of the Devensian Stage (IOS 4 to 2) corresponds to the period between about 70 000 and 13 000 years BP. Cold climate gravel deposits that form the later part of the Kempton Park Gravel (Bridgland, 1994), and the early part of the Shepperton Gravel Gibbard, 1985) were probably laid down during this stage. The latter occurs beneath the present Thames river course and is not exposed in the London district (see (Figure 34)). In the latest Devensian, during the Dimlington Stadial, 25 000 to 13 000 years BP, sea level was as much as 120 m lower than at present. The Thames river channel cut at that time now lies 5 m below OD in central London falling to 20 m below OD in the east of the district. As sea level rose again towards the end of the Devensian, this channel was infilled, initially, by sand and gravel deposited in a braided river. Lenses of organic silt and clay giving radiocarbon dates ranging from 28 000 to 21 000 years BP, are interbedded with and overlie these gravels beneath the floodplain of the River Lea at Ponders End [TQ 360 950] (Wymer, 1985; Gibbard, 1994). They are known as the ‘arctic bed’ and contain remains of mosses and dwarf birch and willow that indicate a severely cold climate. Deposition of the gravels appears to have ceased after about 15 500 years BP, probably as a result of declining river flow. The main river changed character and flowed in more restricted channels, with sand being deposited in point bars. Some of these sands crop out as islands or eyots within the alluvium, for example at Westminster [TQ 302 795] and Bermondsey [TQ 335 799]. The river maintained this low flow regime probably until about 7000 years BP at Tilbury and 3500 years BP in central London.

The final part of the Devensian stage began with the Windermere Interstadial, dated between about 13 000 and 11 000 years BP. Sediments of this age are recorded from the district only at Bramcote Green, South Bermondsey [TQ 350 785]. Here, deposits at the base of the alluvium have yielded birch pollen (Thomas and Rackham, 1996). Cold conditions returned briefly during the Loch Lomond Stadial from about 11 000 to 10 000 years BP. Organic deposits of this age occur in a shallow channel at the base of alluvium at Silvertown [TQ 401 805] (Wilkinson et al., 2000).

The climate finally ameliorated at the start of the Holocene about 10 000 years BP; this marks the start of the Flandrian stage of the British Quaternary. Sea level continued to rise through much of the Holocene resulting in the deposition of alluvial mud. Lenses and beds of peat within the sequence, mainly to the east of Docklands [TQ 39 81], are thought to indicate temporary minor falls in sea level (Devoy, 1979) or a slowing in the rate of sea level rise. Distribution of the peat was governed partly by a supply of fresh water from springs along the valley sides to emergent mudflats. A maximum of five peat beds have been recognised in the vicinity of Tilbury (Devoy, 1979) where the alluvial sequence is thickest (Figure 40).

The basal peat at Tibury is dated about 8300 years BP. Subsequently, as sea level rose rapidly (about 13 mm per year) the tidal head of the estuary probably reached Tilbury by around 7700 years BP. A regression at 7000 years BP is marked by a second bed of peat, which became submerged at 6600 years BP. Commencing at about 6200 years BP and lasting until about 2500 BP, the area of estuarine intertidal mudflats was reduced in size and there was a seaward expansion of substantial tracts marsh deposits along the river margins and in the tributary valleys. This coincided with the development of the thickest peat in the alluvial succession (see Long et al., 2000). Following this, sea level continued to rise slowly, and estuarine conditions once again migrated upstream so that in central London there was a gradual change from a freshwater system to an estuarine one. In the east of the district there was a return to brackish marine conditions in the estuary, and the channels cut into the peat were infilled with intertidal sediments. The uppermost peat in the alluvial succession marks a regression during the third and fourth centuries AD. This was followed by further sea level rise that is continuing at present.

Present-day sea level change and ground movement

Archaeological evidence indicates that the River Thames was not tidal in Roman times and occupation levels in London were at least 2 m below current high tide level. Today the River Thames is tidal as far as Teddington, and over the last two centuries there has been an increased tidal range caused by a decrease in tidal friction in the Thames estuary. This has been brought about by removal in 1830 of the Old London Bridge, which had always acted as a partial barrage. Also, the River Thames was extensively dredged in conjunction with the expansion of the London docks in the late 19th century. These two factors led to an increase in the tidal range from 4.6 to 6.3 m by 1877.

In more recent times, high quality tide gauge records have been used to produce mean sea level trends at coastal tide gauges, which form part of the National Tide Gauge Network (Woodworth et al., 1999). However, the only high quality tide gauge records available for the Thames estuary are those at Tilbury, east of London, and the coastal sites of Southend and Sheerness. Published estimates for the mean sea level trends showed a rise in relative mean sea level at all three sites. The trends and their standard errors were:

• +1.58 ± 0.91 mm/yr for Tilbury for the period from 1961 to 1983
• +1.22 ± 0.24 mm/yr for Southend for the period 1933 to 1983
• +2.14 ± 0.15 mm/yr for Sheerness for the period 1901 to 1996

Superimposed on these trends is a rise in global sea level. In 1990 and 1995, the Intergovernmental Panel on Climate Change (IPCC) reviewed the published evidence on the influence of global warming on sea levels (IPCC, 1995). They found that global sea level had risen by 100 to 200 mm over the past century. This is equivalent to a linear rise in the order of 1.0 to 2.0 mm/yr. In contrast, a rise in sea level of 8 mm per year was indicated by the height of storm tide surges and records from a Greater London tide gauge. An estimated 40 to 75 per cent of the difference (3 to 6 mm per year) could be due to the increase in tidal range, but the balance, up to 4 mm per year rise in water level, could be accounted for by changes in ground level in the Thames estuary and Greater London (Muir Wood, 1990).

To add to the uncertainty, there are few historical geodetic observations of changes in ground level in Greater London. In the Second National Geodetic Levelling of Great Britain carried out by the Ordnance Survey (OS) Greater London was surveyed between 1946 and 1951. The Third National Geodetic Levelling of Great Britain was carried out between 1952 and 1959. Not surprisingly, considering the precision of geodetic levelling at the time, Kelsey (1972) reported no significant change in the relative heights of OS benchmarks in Greater London between them. Later, the results from a north–south levelling traverse across London, carried out by the OS in the 1960s, indicated a consistent sinking of Central London of approximately 2 mm/year compared to ‘stable’ points to the north and south.

Chapter 7 Structure

The Palaeozoic strata of the district encompass three structural provinces. The largest is the London Platform, part of a relatively stable rigid crust known as the Midlands Microcraton. The concealed Caledonide fold belt of eastern England probably underlies the extreme north-east of the district (Pharaoh et al., 1987), and a zone of transition between the London Platform and the Variscan fold-thrust belt lies in the south.

North–south-trending seismic reflection profiles traverse the extreme southernmost part of the district. One line south of Croydon passes just west of the Warlingham Borehole and shows a gently southward-dipping reflector in the sub-Mesozoic sequence. Kearey and Rabae (1996) named this feature the Addington Thrust (Figure 2); (Figure 42), interpreting it as a structure analagous to the Variscan thrusts to the west (Chadwick et al., 1983; Donato, 1988). The structure was reactivated in Mesozoic times when it became a growth fault during the development of the Weald Basin. The residual gravity anomaly map (Figure 43) provides further insight into this structure. It shows a gravity low in the south of the district that was formerly interpreted as part of a concealed basin containing Carboniferous strata (Falcon and Tarrant, 1951) but information from the Warlingham Borehole (Worssam and Ivimey-Cooke, 1971) failed to confirm this hypothesis. The trend of the anomaly is oblique to the structure contours (Figure 2) and, therefore, it is unlikely to be caused by changes in the elevation of the basement surface; Kearey and Rabae (1996) concluded that it provides further evidence for the existence of the Addington Thrust. They interpreted the anomaly as a thrust-slice, wedge-shaped in cross-section, of relatively high density Upper Palaeozoic rocks.

Other inferred structures in the Palaeozoic strata are the Streatham Anticline and a fault beneath the Thames estuary, both bringing strata of Silurian age to subcrop beneath the sub-Mesozoic unconformity (Figure 2). Bedding dips of up to 45° are recorded in boreholes close to the structures, but the Palaeozoic strata elsewhere in the district have a low dip.

Between the gravity low in the south and a high in the north-east, an area with a steep gravity gradient trends north-east to south-west. Superimposed on this regional trend are three linear zones of locally steep gravity gradients, shown as dark shading on (Figure 43). They are interpreted as being due to pronounced variations in the thickness of Mesozoic strata, possibly brought about by a movement on syndepositional growth faults. Other structures in the Mesozoic syn-rift succession below the Cimmerian unconformity (at the base of the Lower Greensand Group), are interpreted largely on borehole evidence, but it is assumed that they are caused by reactivation of deeper seated structures. They include a small graben beneath central London inferred because of the preservation of Inferior Oolite proved in the Meux’s Brewery Borehole (Table 1) and a larger graben at Cliffe inferred from the distribution of the Oxford Clay in boreholes (Owen, 1971).

Structures in the post-rift Upper Cretaceous and Palaeogene strata of the district are better known than those in older strata because of surface exposure and information from a large number of boreholes. Data on the structure of the base of the Chalk (Figure 44) is sparse, but the structure contours of the Chalk– Palaeogene boundary and base of the London Clay (Figure 45) reveal considerable detail.

The geological structure of the district is relatively simple, being dominated by a broad north-east-trending syncline (the London Basin). The main limbs, coincident with the slopes of the North Downs in the south of the district and the Chiltern Hills in the north-west, dip generally less than 2°. Superimposed on the southern limb of the syncline are numerous east-north-east-trending periclines with limb dips up to a maximum of 7°. Close to faults, dip is steeper and vertical dips have been recorded (Plate 5). Some of these folds are asymetrical, with steeper north-facing limbs. In the axial part of the London Basin, subsidiary folds are broader in wavelength and lower in amplitude, with limb dips less than 2°.

The main faults at the surface are the en echelon Wimbledon, Streatham and Greenwich faults (Figure 45). Their location has been determined largely on the evidence of borehole data, but they have been exposed at several localities. For example, the Streatham Fault was proved in a sewer tunnel [TQ 317 740] at Brockwell Park where a maximum throw of 15 m was recorded, and two branches of the Greenwich Fault were formerly exposed in railway cuttings [TQ 3721 7605] and [TQ 3735 3638] (Whitaker, 1872). The main faults show a downthrow to the north in the order of 10 to 30 m. The Greenwich Fault passes north-eastwards into the steep northern limb of the Greenwich Anticline. It separates an area to the north with broad folds from one to the south where there are many periclines

Other faults shown on the geological maps are inferred from borehole records, for example at Richmond Park [TQ 19 72] and Camberwell [TQ 32 78]. Small faults in the Chalk, many with slickenslides, have been recorded at numerous outcrops and temporary exposures in the district. Most are normal faults with throws less than 2 m, for example, in a sewer tunnel at Nunhead [TQ 357 754], in the railway cutting east of Banstead [TQ 250 604], in a sewer tunnel at Carshalton [TQ 277 632] (Dewey and Bromehead, 1921), on the chalk escarpment near Crookhorn Wood [TQ 67 62], and at the Thames Barrier [TQ 415 795] (Carter and Hart, 1977).

In central London where there is a high density of boreholes, prominent kinks in structure contours on the base of the Palaeogene surface (Figure 45) are interpreted as north-west-trending faults with throws of up to 5 m. These have not been verified in exposures and appear to counter viewed towards the north-west, showing folds associated with the Greenwich Fault in the Upper Shelly Clay (including the Paludina Limestone) and Laminated beds of the Lambeth Group (Bromehead, 1922; Whitaker, 1899) (A1919). the conclusions of Skempton et al. (1969) who did not identify a regional pattern of joint orientation in the London Clay in surface exposures. More recently, planar deformation zones of tectonic origin have been described in the London Clay (Chandler et al., 1998), associated with minor folds.

Small faults are recorded also in the Lambeth Group at Bushey and in excavations for the Underground railway between Oxford Street and Regent’s Park (Bromehead, 1925). Small reverse faults were seen in a former London Clay pit near Lewisham [TQ 372 755] (Dewey and Bromehead, 1921), and in borehole core in the Lambeth Group between Stratford and Barking (unpublished BGS manuscript).

There is no published information about the regional distribution either of small faults or joints. A regional set of north-west-trending near-vertical extension fractures is apparent in the London Basin as a whole (Bevan and Hancock, 1986), although this orientation is modified, at least locally, as illustrated by the principal joints in the Chalk measured in exposures at Northfleet [TQ 632 737] (Figure 42).

Tectonic events

The timing of deformation events in the post-rift, Late Cretaceous to Palaeogene strata is not well known. Mortimore and Pomerol (1997) suggested that deep seated structures may have been active during deposition of the Chalk, leading to thicker successions of the Lewes Chalk and Seaford Chalk (late Turonian and Coniacian in age) in London and the Thames estuary area compared with the North Downs. Similarly, there is some evidence for minor inversion in Late Cretaceous–early Palaeogene times, which led to a greater amount of uplift and erosion of the Chalk in central London than in the North Downs (Mortimore and Pomerol, 1997).

Intra-Palaeogene tectonic activity, presumably driven by movement on the major deep-seated fractures, has been suggested from the London area but remains speculative (Ellison et al., 1996). Possible evidence for contemporary subsidence is the relatively thick Lambeth Group in west and central London compared with that found farther east (see for example (Figure 14)a) (see also Hester, 1965).

The main phase of folding in the district is assumed to have taken place in the Oligocene to early Miocene Helvetic phase of the Alpine orogeny. Interaction of the principal north–south compressive stress direction with the pre-existing deep-seated, east-north-east-trending structural fabric caused development of the en echelon Greenwich–Wimbledon Fault system and the en echelon, asymetrical folds in the Chalk and Palaeogene strata.

An assessment of tectonic activity in Pliocene and more recent times is hampered because there are few reference deposits in the London Basin. In regional terms, the height of deposits of probable Pliocene age is up to 180 m above OD in the Chiltern Hills and indicates a maximum uplift in the east of the London Basin equivalent to 0.9 mm per year over the past two million years (Worssam, 1963; West, 1972).

Further evidence for relatively recent neotectonic activity is found in the terrace gravels of the River Thames. The oldest terrace, at the greatest topographical elevation, is about 500 000 years old. The subsequent development of terrace gravels at progressively lower levels in the river valleys is now thought to be due to land uplift equivalent to 0.7 mm per year over the last 250 000 years (Maddy, 1997). There is also a possibility that neotectonic movement on the Greenwich Fault has led to the difficulty in correlating the River Terrace Deposits between central London and north Kent (see pp.61–62).

Whether or not uplift continues to the present day is currently in debate (Bingley et al., 1999) as there are conflicting lines of evidence concerning the interplay of sea level changes and land movement. For example, archaeological evidence indicates that the River Thames was not tidal in Roman times, and occupation levels in London were at least 2 m below current high tide level. This apparent relative rise in sea level was investigated further in the justification for the construction of the Thames Barrier. The increasing heights of storm tide surges and records from a Greater London tide gauge are thought to indicate an 8 mm per year rise in the level of the Thames estuary (Muir Wood, 1990). In contrast, estimates of global sea level rise are 1 to 2 mm per year (IPCC, 1995). Muir Wood (1990) estimated that 3 to 6 mm per year of the difference between these two figures could be due to the increase in tidal range, leaving up to 4 mm per year rise in water level possibly due to land sinking in Greater London.

Chapter 8 Applied geology

Geological factors have had a significant role in the expansion of London and its neighbouring towns, and will continue to be important in influencing the nature of future urban, rural and industrial development. By giving consideration to geological information at an early stage in the planning of work that impinges on the ground in any way it may be possible to mitigate some of the problems commonly encountered. The key ground-related issues in the London district are discussed briefly below; many of them are related to specific geological units (Table 21). Key issues in south-west Essex were identified and discussed in detail by Moorlock and Smith (1991), and those in the Thames Gateway (a corridor extending astride the River Thames from London Docklands to Gravesend and Tilbury) by Ellison et al. (1998).

Water resources

Chalk is the principal aquifer of the London region. It is confined by the London Clay in much of the district and is in hydraulic continuity with the overlying sands of the Thanet Sand and Upnor formations, which together are commonly referred to as the ‘Basal Sands’. The upper boundary of the aquifer is generally regarded as a clay layer with a thickness greater than about 3 m. In most places this clay lies within the Lambeth Group; it is coincident with the Lower Shelly Clay in much of central London, the Reading Formation in the west and north of the district, but probably the London Clay in the south-east of the district where the lower Palaeogene strata consist mainly of sand.

The Chalk aquifer is naturally recharged by rainfall at outcrop in the Chiltern Hills to the north and the North Downs to the south. The groundwater flows towards the centre of the London Basin. Prior to abstraction in the 19th century it discharged mainly at springs, many under artesian conditions, in Chalk valleys and along the Thames particularly between Erith and Gravesend (Figure 46).

Relatively minor aquifers in the district include the River Terrace Deposits, the confined Lower Greensand in the south-east and, locally, the Bagshot Formation.

The majority of the Chalk public supply sources in the Chalk aquifer are in the North Downs and the Darent and Lea valleys. Development is now taking place in the Greater London area to use the confined Chalk aquifer resource resulting from rising groundwater. Small quantities are abstracted from sources in the Lower Greensand from wells through the Chalk in the North Downs.

Development of groundwater resources in the London Basin

The growth of the City of London was constrained for many centuries by the availability of local water supplies, thus early expansion of the city was restricted to areas where river gravels are present. Until the 13th century water supplies for London were obtained from the Thames and its tributaries, and from springs and shallow wells in the river gravels. As the city expanded these resources became inadequate or polluted and further supplies were obtained via conduits, including the New River, from the Chiltern Hills. In the 18th century attempts were made to develop deeper groundwater beneath London. The first deep wells in the Chalk were constructed in the 1820s, although there was probably a reluctance to develop the confined aquifer because of difficulties in coping with the quicksands within the basal sands that were under artesian pressure (Barrow and Wills, 1913). By the 1890s, many of the early large diameter wells in the Chalk also had adits, some heading several hundreds of metres from the main shaft.

The rate of abstraction from the confined part of the aquifer peaked around 1940 and subsequently decreased, whereas that from the unconfined area continued to rise until the 1970s. Public water supply has dominated water use since the 1860s. The broad change in groundwater abstraction over the whole of the London Basin in the period 1820 to 1985 is shown in (Figure 47).

The yields of wells and boreholes in the Chalk (Table 22) depend mainly on the transmissivity of the aquifer at the site, the type of construction and the thickness of aquifer penetrated. There is a general decrease in aquifer permeability from the Chalk outcrop towards the centre of the London Basin.

In the confined Chalk, groundwater is considered to flow in corridors of high permeability Chalk separated by blocks of low yield. Where Chalk is deeply confined, yield is commonly low. Yields from individual large-diameter wells at favourable valley sites at outcrop may exceed 9000 m³/day. Yields from pumping stations in the outcrop area, which may have several wells and may include adits, often exceed 14 000 m³/day. By contrast, in the confined aquifer, yields are usually considerably less, commonly of hundreds of m³/day. Some very high yielding sites (in the order of 7000 to 10 000 m³/day) in the confined aquifer are probably related to the high permeability corridors.

A measure of the specific capacity of boreholes in the Chalk (Monkhouse, 1995) has been estimated from the yield drawdown relationship of boreholes, normalised to 305 mm diameter and standardised per metre of saturated borehole length. The data indicate an area of low yield in central London, with standardised specific capacities of less than 0.1 m/day, which is surrounded by an area with values between 0.1 and 1.0 m/day. Areas of high yield, with standardised specific capacities exceeding 1.0 m/day, occur in the vicinity of the River Lea, the eastern part of the Thames valley and in the north-west of the region (Figure 48).

Hydrogeological characteristics of the Chalk

The Chalk is hydraulically a highly complex aquifer. Its matrix has a high porosity, commonly of the order of 35 per cent (Bloomfield et al., 1996), but the pores are extremely small and thus the hydraulic conductivity of the Chalk is very low, with values averaging around 10^-3 m/day (Allen et al., 1997). The ability of the Chalk to act as an aquifer is therefore due almost entirely to its fractured nature.

Hydraulically significant fractures in the Chalk do not extend through its full thickness, but are concentrated in the upper few tens of metres. In the unconfined Chalk they are most prevalent in the zone of oscillation of the water table where many are enlarged by dissolution. Important water-bearing fractures have been shown to extend to depths of the order of 50 m below the water table. In the confined Chalk and ‘Basal Sands’ aquifer, the majority of inflows to boreholes, shown by geophysical flow logging, typically occur within 20 to 30 m of the upper surface of the Chalk.

The main controls on the transmissivity of the uncon-fined Chalk are depth and topography. The highest values, in excess of 1000 m²/d (Allen et al., 1997), are in valleys, for example the Darent and Cray valleys in the south-east of the district (Figure 49). Transmissivity is significantly lower in interfluve areas, where values in the order of a few tens of m²/d are likely. The main reason for the contrast is that in valleys the Chalk is likely to be relatively highly fractured and the fractures enhanced by dissolution (Younger, 1989).

Swallow holes and other karstic features in the Chalk strongly influence the transmissivity. For example, a swallow hole at Addington, Croydon [TQ 36 64] is connected to a complex shallow fracture system in which tracers moved a rapid 3 km/day (Richards and Brincker, 1908). The development of karstic features within the Chalk is related to the diversion and concentration of groundwater flow by relatively impermeable beds such as tabular flints, marl seams and hard, nodular chalk. These occur in particular parts of the Chalk sequence (see Chapter 3, Upper Chalk) and thus areas of relatively karstic Chalk can be predicted.

In the confined Chalk, transmissivity is generally highest in areas of greatest fracturing, thinnest overburden and largest groundwater flux. For example, it is low north-east of the River Lea, where the overburden is thick, but enhanced along the Colne and Lea valleys in the north and the Mole and Wandle in the south of the district. Groundwater associated with these valleys tends to be young, indicating they are zones of preferential flow. The zone of moderate transmissivity in central London (Figure 49) may be due to relatively enlarged Chalk fractures in areas of former springs. It may also relate to north-west-trending faults and fractures (see Chapter 7). In south London, zones of high transmissivity correspond approximately to the location of faults, notably the Greenwich Fault (see (Figure 44)), whereas permeablity across such faults may be low (Allen et al., 1997).

Water quality in the Chalk and ‘Basal Sands’ aquifer

The general distribution of groundwater types in Chalk and ‘Basal Sands’ aquifer in the London Basin is shown in (Figure 50). Chalk outcrops in the north and south of the district, and in the Lea and Wandle valleys, are characterised by hard, calcium bicarbonate water. In the axis of the London Basin soft, sodium-rich groundwater is present. Sodium chloride water occurs along the lower reaches of the River Thames because of saline intrusion (Water Resources Board, 1972).

Groundwater management and protection issues

In central London the original natural groundwater level was 7.5 m above OD. This was lowered to around 88 m below OD by the mid-1960s, due to pumping from boreholes, illustrated by the hydrograph of a well in Trafalgar Square (Figure 51); the top of the Chalk succession is dewatered over several square kilometres (Lucas and Robinson, 1995).

The fall in groundwater levels in the confined aquifer caused a reduction in spring flows and some river flows, and induced the intrusion of saline water from the Thames into the Chalk downstream of the Isle of Dogs. Reduction in groundwater abstraction, since around 1965, has resulted in recovery of the groundwater level and a reduction in the cone of depression; rates of water level rise approached 3 m/year in places in the 1990s (Figure 51). More recently, rates of rise in central London have decreased to around 0.5 m/year (Environment Agency, 2001), due to a combination of natural causes and a strategy to manage the water levels^{GARDIT (General Aquifer Research, Development and Investigation Team) is a group including the Environment Agency, water companies, and other organisations at risk from rising groundwater levels. Its strategy is to manage water levels by abstraction in order to maintain the integrity of underground structures and foundations in the London Clay, and also to ensure sustainable water resource management.}.

Artificial recharge

The lowered groundwater level resulting from pumping in the London Basin has provided the opportunity to use part of the dewatered Chalk and ‘Basal Sands’ aquifer in north London for artificial recharge. This technique involves using the unsaturated part of the aquifer as a reservoir into which water is emplaced during times of surplus, and extracted when required, mainly in times of drought. In the case of the Chalk and ‘Basal Sands’ aquifer, the high storage coefficient sands are able to store water while the high transmissivity of the underlying Chalk allows relatively easy flow of water into and out of the aquifer.

The principle has been applied in the Enfield, Haringey and Lea valley area (O’Shea, 1994). It works by pumping treated water, during the winter or other periods when there is excess, into wells that penetrate the aquifer. This water is stored in the Chalk and ‘Basal Sands’. When required, for example during times of drought, the stored water is pumped from the same wells either into reservoirs in the Lea valley or into a water transfer system which includes the New River, an engineered aqueduct through Enfield e.g. [TQ 340 980] and Hornsey e.g. [TQ 312 892]. The scheme has a drought yield of about 150 000 m³/d from 35 wells. A similar one is being investigated in south London where there is little or no unsaturated storage available in the Chalk or ‘Basal Sands’ aquifer. However, large abstractions in the Wandle valley [TQ 257 740] have increased the confined storage available for artificial recharge.

There is an apparent contradiction in artificially recharging an aquifer that elsewhere is causing problems as a result of rising water levels. However, there is no evidence that the artificial recharge in north London influences the rising groundwater trend in central London.

Theoretical studies, supported by monitoring since 1995, indicated that the temporary cones of impression caused by the injected water would be removed by abstraction before the water moves down the hydraulic gradient to affect the central London water levels.

Low flow alleviation

Until the late 1990s, the River Darent frequently dried in its middle reaches in years when recharge and groundwater levels were significantly lower than normal. This resulted in damage to river ecology as well as loss of amenity. Under average conditions, throughout the greater part of its length, the river should gain flow from groundwater. In fact river flow decreases north of Shoreham [TQ 52 62]. The main cause of this is leakage of river water through the river-bed and into the underlying . Chalk aquifer as a result of the lowering of the water table caused by abstraction from nearby groundwater sources. The impact on the river becomes more marked in times of naturally occurring ‘dry’ winters as experienced in 1992/93, when there was no flow at Horton Kirby [TQ 56 68].

In order to resolve the low flow problem, the Environment Agency and the water company concerned have agreed a phased programme to reduce abstraction from the Chalk in the middle part of the River Darent, with river augmentation boreholes being constructed and new Chalk groundwater resources developed to the north-east, outside the immediate catchment of the Darent.

Ground conditions

Engineering implications of changes in groundwater levels

The recovery of groundwater levels in London has several implications for engineering (CIRIA, 1989). Basements or tunnels above the water table and not sealed against the ingress of water are liable to flooding. Sealed structures submerged by rising water could become buoyant and liable to uplift pressures detrimental to stability. Structures originally below the water table may not be sufficiently watertight to contend with increased hydrostatic head, in which case remedial sealing or continuous pumping would be required.

Some tunnels are currently suffering from increased seepage, and chemical attack. One example is on the London Underground Northern Line, where highly acidic waters caused deterioration of the tunnel linings south of Old Street Station [TQ 328 826]. Investigations suggested that the source of the acid was oxidised pyrite in sands of the Harwich Formation and the Lambeth Group. These sands were originally saturated, but subsequently dewatered as the water table lowered. The pyrite was oxidised by air from the railway tunnels, in particular by the piston effect of passing trains and by changes in barometric pressure. Water seeping from the overlying London Clay into the newly created unsaturated zone resulted in the production of highly acidic, aggressive groundwater (Robins et al., 1997). This situation could be exacerbated as the water table rises (Rainey and Rosenbaum, 1989), but is being minimised by the GARDIT strategy.

During conditions of falling ground water level, the resultant underdrainage and consolidation of strata resulted in an estimated 300 mm of subsidence in central London (Water Resources Board, 1972). It also increased the bearing strength of the London Clay and clays in the Lambeth Group. As a result of rising groundwater, increase in pore water pressure and the swelling of clay, particularly the montmorillonite-rich London Clay, may result in a loss of shear strength, and hence bearing capacity. Chisholm (1984) for example showed that the bearing capacity of deep foundations and piles would experience a drop of between 25 and 50 per cent if the water level in the Thanet Sand Formation below central London rose to ground level. In addition there would be uplift pressures under basement slabs and increased earth pressure on the side walls causing differential movement and significant damage to buildings (CIRIA, 1989). The risk of these hazards having an impact is significantly reduced by implementation of the GARDIT strategy.

Deep drift-filled hollows

Deep drift-filled hollows (see (Figure 35)) encountered during excavation may require dewatering. It may also be necessary to control running sand during excavation and construction. The heterogeneous, non-cohesive infill and the potentially slumped and sheared sides of the hollow may cause instability problems in excavations within, or cutting across, the margins of a drift-filled hollow.

The unexpected penetration of a deep, steep-sided hollow filled with water-bearing gravel presents a serious hazard to shallow tunnelling operations in the London Clay due to potential flooding of the workings, the need to stabilise the drive and to seal the finished tunnel lining. The problems caused by such features during the construction of the Brixton extension of the Victoria Line required the use of timber support, grouting and compressed air to enable the works to proceed through the water-bearing gravel infill (Megraw, 1970).

Chalk dissolution and weathering

Weathering of the Chalk and the development of dissolution features are closely related to lithology, fracture density and water table fluctuations. The Chalk exhibits a wide variety of surface karst features. The most common of these are dissolution ‘pipes’. They are deep, narrow features extending down from the surface, possibly with no surface expression. Most of them are cylindrical, but they may be cone-shaped, pinnacled, or dish shaped (Farrant, 2001), and range from 1 to 20 m in diameter. Pipes may penetrate several tens of metres below rock-head and are usually infilled with Clay-with-flints, sand or clay derived from the overlying superficial or Palaeogene deposits (Figure 52). The larger dissolution features occur close to the outcrop of the base of the Thanet Sand, particularly where there is a large catchment area of the relatively impermeable Lambeth Group and London Clay. This leads to a relatively large volumes of runoff being directed down valleys cut into the permeable Thanet Sand and Chalk. The resultant development of pipes and swallow holes in the Chalk beneath the valley floor locally leads to collapse of Thanet Sand, and occasionally of higher strata. Numerous examples of sinkholes are known around Erith [TQ 51 78] and Crayford [TQ 51 75]. There are also many instances of them at or close to the boundary between the Chalk and Paleogene deposits, for example near Well Hill [TQ 495 644]. Large dissolution cavities filled with Thanet Sand and soliflucted superficial deposits are common in north Kent, particularly between Northfleet Green [TQ 627 711] and Cobham [TQ 670 685]. Dissolution of the Chalk occurs also in the floor of valleys, where joints are relatively common, and on the dissected dip slope of the North Downs in the south-east of the district. Where Chalk crops out on valley sides there is generally minor surface dissolution (Figure 52). Sinkholes can be reactivated artificially by concentrations of water resulting from leaking water pipes, drains or soak-aways, and in such cases collapse of the overlying ground into the cavity may take place.

Another form of karst is associated with flint and marl seams that are sufficiently continuous to act as local aquicludes. For example, the Belle Tout and Shoreham Marls and the Seven Sisters Flint (see (Figure 7)) in the Upper Chalk exposed in the Swanscombe quarries [TQ 580 735]; [TQ 59 73] display karstic features along their top surfaces formed above the water table.

At depth, below Palaeogene strata under London, joints in the Chalk are tight and coated with a black (ferrous iron oxide) powdery residue, but with little evidence of dissolution. In comparison, joints in the saturated Chalk directly below Thames gravels and alluvium are coated with red-brown, ferric iron oxide, but with little evidence of major dissolution even though there is water flow along discontinuities.

The Chalk weathering profile is the legacy of freeze-thaw action under periglacial conditions during the last ice age. Chalk at the surface is usually highly fractured, with a network of curviplanar discontinuities, typical of frost shattering, which extend to depths of 30 m. For example boreholes at West Thurrock [TQ 581 791] encountered 15 to 20 m of intensely fractured weak Chalk, and at sites between Erith and Crayford ‘rubbly chalk’ is locally recemented to form a breccia (Dewey et al., 1924).

The systematic description of Chalk (Spink and Norbury, 1990) is based on the Mundford scheme (Ward et al., 1968). On the basis of fracture spacing and dilation, six weathering grades, I–VI, are recognised. Grade VI, the most highly weathered, occurs at the surface and directly below superficial deposits. It is similar to a cohesive soil, consisting of coarse to fine gravel grade chalk fragments in a silt grade matrix. Grades I to V encompass a gradation from ‘structured chalk’, in which the original structure of joints, fractures and bedding can be identified, to ‘structureless chalk’ in which most of the primary structure is destroyed and lumps of soft chalk lie in a subordinate, fine putty, chalk matrix.

Chalk is sensitive to frost action. Soft chalk, such as the weathered Upper Chalk at outcrop in north Kent, starts to transform to a chalk putty after four to ten freeze thaw cycles. Harder chalk, such as the Lewes Chalk, is more resistant and may only suffer frost cracking (Bell et al., 1997). Lewis and Croney (1965) described contemporary frost heave caused by the formation of ice lenses up to 25 mm thick along chalk bedding planes. Higginbottom (1966) suggested this mechanism can lead to a total volume increase of 20 to 30 per cent. Freeze/thaw action may be significant also in the long-term stability of cut Chalk faces in engineered cuttings.

Engineering characteristics of the Chalk

Moisture content of chalk (Table 23a) is generally greater than 25 per cent and dry density values of the (Figure 52) Schematic block diagram (after Mortimore, 1997) to illustrate typical weathering and dissolution characteristics of Chalk in a dry valley.

Upper Chalk indicate it is soft to extremely soft (Higginbottom, 1966; Lewis and Croney, 1966). Unconfined compressive strength data of saturated samples of Upper Chalk from Kent fall in the weak rock range (1.25–5 MPa). Dry samples are stronger, in the moderately weak rocks range (5–12.5 MPa). The Upper Chalk of the London district is appreciably weaker by a factor of five or six than chalk from Norfolk and Yorkshire (Bell, 1977).

The load bearing capacity of both bored and driven piles in chalk is impaired due to skin friction being reduced by a coating of remoulded chalk paste left plastered on the hole during drilling. Similar chalk paste is formed in the process of pile driving. A comprehensive review of piling in chalk was carried out by Hobbs and Healey (1979) and updated by Lord (1990).

Artesian pressure in Chalk can lead to problems in construction. For example, during the construction of the Thames Barrier (Horner, 1984) it caused uplift of strata, and the excavation of concrete slabs cast to seal the formation at the bottom of a coffer dam. Extensive dewatering carried out from boreholes mitigated artesian conditions encountered during tunnelling in chalk for the Thames Cable Tunnel from Gravesend to Tilbury. Two major fissures were encountered during the drive that required temporary bulkheads to be built to stem the flow of water so that grouting could be achieved in static conditions (Haswell, 1969). In addition, recharge wells were installed to avoid subsidence damage to nearby property.

The efficiency of tunnel boring machines operating in Chalk is reduced by flint bands, and by the build-up of putty chalk. Haul roads in major excavations suffer from poor trafficability in wet weather where chalk becomes over compacted crushed and remoulded by the traffic. In particularly wet conditions it becomes a slurry.

Engineering characteristics of the Thanet Sand Formation

The Thanet Sand Formation is a non-cohesive granular deposit. It has the physical characteristic of a weak rock, but can be disaggregated by light finger pressure. The apparent cohesion is due to mechanical interlocking of subangular grains; this is characterisatic of a ‘locked sand’ (Barton and Palmer, 1989). Geotechnical Properties are summarised in (Table 23b) (see also Howland, 1991).

Weathered and/or cryoturbated Thanet Sand may have a low bearing capacity and suffer high settlement on loading. In comparison, undisturbed Thanet Sand has a high bearing capacity with no appreciable change with depth due to the dense nature of the deposit. The sinking of bored piles requires the use of bentonite slurry to avoid the hole collapsing below the water table. In the Canary Wharf Development [TQ 38 80], base grouting of bored piles was used (Troughton, 1992).

At surface outcrops, the Thanet Sand is easily dug using normal earth-moving equipment. Excavations above the water table stand at a high angle in the short term, but running sand conditions occur below the water table.

In tunnelling, the Thanet Sand is considered abrasive, due to the angularity of its grains, and causes high rates of wear on machinery. (Clarke and Mackenzie, 1994). Large unworn flints (‘bullhead flints’) in the basal bed can cause problems on the tunnel face. Sudden inflows of running sand can occur in excavations and tunnelling below the water table. For example pore water at a pressure of 3 bars caused water to flood a tunnel being driven for the London drinking water Ring Main, and filled it with 100 m³ of sand (Clarke and Mackenzie, 1994). Preventative measures against such hazards include balancing the air pressure in the workings with the water pressure of the groundwater, dewatering, ground freezing, grouting in advance of the face, the use of a full-face slurry-tunnelling machine or earth-pressure-balance tunnel-boring machine (Clarke and Mackenzie, 1994; Oliver, 1996).

Engineering characteristics of the Lambeth Group

The horizontaland vertical variabilityof the Lambeth Group is a source of continuing challenges for construction; tunnelling in these deposits, in particular, is a source of unforeseen expense (Table 23c). Generally, no tunnelling method is suited to a rapidly changing composition of the face, but problems can be minimised if techniques are determined in advance, using a ground model. The basis for such a model is recovery of good undisturbed ground investigation borehole samples, as described by Mair (1993) and Linney and Page (1996) for major infrastructure projects in London.

In engineering terms the clays in the Lambeth Group have broadly similar properties to London Clay (see p.97). The main exception is where they contain appreciable amounts of calcareous shell debris in the Woolwich Formation Upper and Lower Shelly Clay units. The sands are similar to the Thanet Sand but are generally coarser grained, more poorly sorted and not ‘locked’. Summary geotechnical properties are given in (Table 23c).

Lithologies in the Lambeth Group that can cause problems in excavation are flint pebble beds (in the Upnor Formation), calcareous nodules (Reading Formation) and other locally cemented beds (Woolwich and Upnor formations), weak to strong limestone beds (Woolwich Formation; Upper Shelly Clay), irregular-shaped sand bodies (Woolwich Formation; Laminated beds) and swelling clays (Reading Formation). Excavations are likely to encounter water-bearing sands and therefore support is required. Seepage from perched water tables on the side slopes of cuttings may cause slope instability, and slopes in sands need to be protected from erosion caused by surface run off.

The difficulty of tunnelling in the Lambeth Group was evident during construction of Sir Marc Isambard Brunel’s tunnel beneath the Thames from Wapping to Rotherhithe. It took 18 years to complete, from 1825 to 1843, and was flooded on five occasions (Skempton and Chrimes, 1994). The prospect of similar problems was largely responsible for the greater part of the London Underground railway system being constructed north of the River Thames, in the London Clay, rather than south of the river where the Lambeth Group is at or close to the surface.

Several designs of tunnel-boring machines were used to good effect in the Lambeth Group during the installation of a new sewerage system in London’s Docklands (Ferguson et al., 1991). In predominately clay strata, where there is minimal risk of water ingress, open-face tunnelling was used. In coarser material, a closed-face Earth Pressure Balance Machine (EPBM) or a slurry shield was effective. EPBM tunnelling in mixed lithologies, and progress with a slurry shield machine may be slow due to clogging of pipes and intakes by the clay component of a mixed face. Tunnelling through the Lambeth Group, which is composed of sand, silt and swelling clay with gravel lenses in the Bermondsey section of the Jubilee Line Extension, led to a problem with the removal of material through the discharge auger; this was solved with the addition of a foaming agent (Oliver, 1996b).

Control of groundwater inflow into Lambeth Group excavations has been achieved by ground freezing, removing water by well point dewatering, excluding water by grouting permeable horizons in advance of the face and by balancing water pressure by working in compressed air.

Engineering characteristics of the London Clay Formation

From an engineering point of view the London Clay is arguably the most important geological formation in the United Kingdom in terms of the number and value of structures founded on it (Table 23d).

The geotechnical properties of the London Clay Formation are fairly consistent. In the east of the district, the top 30 m of the formation, including the Claygate Member, is less sandy and more plastic than in the west (Burnett and Fookes, 1974). In comparison, vertical lithology changes in the sequence (see pp.45–47) are marginally more significant.

Weathered London Clay is brown in colour, and soft to stiff; unweathered London Clay, is grey in colour, and stiff to very stiff, becoming hard with depth (see (Figure 25)). Chandler (2000) examined the changes in geotechnical properties as a result of weathering. Permeability is low, in general, but silt and sand beds and fissures have higher values. Moisture content decreases and density increases with depth. Where the clay is desiccated at the surface it has a higher density and lower moisture content than the material immediately below it.

The bottom of excavations in London Clay may suffer heave due to stress relief and swelling on wetting. Railway and road cuttings may be similarly affected over a period of 30 years or more.

London Clay is well suited to tunnelling using a variety of techniques (Attewell and Farmer, 1974; Dick and Jaques, 1994), most of which have a face shield for support. The amount of subsidence caused by different tunnelling methods can be predicted (Heath and West, 1996), and several grouting techniques are used in its mitigation (Kimmance et al., 1995). Compensation grouting from fan-shaped spreads was used in the vicinity of the Jubilee Line Extension to protect buildings in Westminster, for example The Treasury and Big Ben (Winney, 1996).

Engineering characteristics of ‘brickearth’

‘Brickearth’ is typically firm to stiff and of low to medium plasticity. In places, it may be of low density, with an open structure that leads to instability. This can result in sudden collapse when a critical load is exceeded or the material is wetted under load.

The potential for instability is removed when the primary structure is destroyed by reworking and in this circumstance ‘brickearth’ can be used in earthworks.

There is little published information on the engineering properties and behaviour of the ‘brickearth’ of the London area, although similar, generally thicker deposits have been studied in south-east Essex (Northmore et al., 1996) and Kent (Derbyshire and Mellors, 1988).

Engineering characteristics of alluvium

The deposits mapped as alluvium and estuarine alluvium are normally consolidated, generally very soft to soft, with high compressibility exacerbated by peat beds (Marsland, 1986). A firm to stiff, desiccated surface zone may be present. The deposits are characterised by low bearing capacity and poor foundation conditions due to high and/or uneven settlement. Running sand conditions may be encountered in excavations below the water table, and excavations may need immediate support.

Chalk mining

Man-made cavities in the Chalk have been made during flint mining and chalk extraction, probably from pre-Roman times until the 19th century. The mines are typically narrow vertical shafts 10 to 20 m deep, known as dene holes that in some areas may be only 20 m apart. At the base of the shaft there may be a bell-shaped excavation or a number of short galleries. Edmonds et al. (1987) and Bell et al. (1992) described the methods of working. Mines are known in the Chalk at Pinner [TQ 114 906], Chiselhurst [TQ 4275 7015], at several locations near Plumstead e.g. [TQ 472 784]; [TQ 464 774], and Blackheath [TQ 383 767]. As there are few mine plans, there may be other, undocumented mines in these and other areas, for example at Blackheath Common [TQ 393 765] where there was discussion about the causes of sudden ground collapse (Laughton, 1881).

Mined cavities are usually stable, but the material with which they have been backfilled may suddenly subside and collapse due to the effects of natural drainage, leaking services or rainwater soakaways. Such instability may also occur due to slow deterioration of gallery roofs. In this case, the resultant collapse may lead to upward migration of the void and consequently sudden subsidence of the ground surface.

Slope stability

The natural maximum angle of repose of chalk is around 37° (Toms, 1966). The British Standard Code of practice for earthworks (BS 6031, 1981) recommends angles of repose for chalk embankments of between 33º and 37º.

Williams (1990) suggested that slopes less than 56º will not undergo significant degradation, but a slope of 45º or less is required for the establishment of a vegetation cover. Cut slopes up to 76º can be maintained if some spalling is acceptable.

Landslides on London Clay slopes are well known, and have been the subject of much research (for example Skempton, 1964; Hutchinson and Gostelow, 1976). The stable angle for natural slopes in London Clay was determined as 10º by Skempton and DeLory (1957), whereas Hutchinson (1967) concluded that an angle of 8º was the ultimate angle of stability. The Claygate Member, at the top of the London Clay, is more susceptible to slope instability than the bulk of the London Clay. It has high plasticity and high moisture content on account of water-bearing sand layers. Where the Claygate Member is overlain by water-bearing sand in the Bagshot Formation a spring line may develop, which raises pore water pressures and saturates the material below it. Slopes of 8º or greater on Claygate Member are potentially unstable.

Many London Clay slopes greater than 3° are covered with a veneer of head, which may not be shown on the geological maps. Culshaw and Crummy (1991) suggested that these too should be considered as potentially unstable. The head is composed of redeposited London Clay, including the Claygate Member; it is derived by downslope solifluction and soil creep and may contain relict shear surfaces. The shear strength is likely to be at, or close to, its residual value. Reactivation of the shear surfaces may occur if the slopes are undercut, loaded, saturated or the water table rises.

Low permeability clay layers within the Bagshot Formation may create perched water tables within it, which may impair slope stability. Unprotected cut slopes or embankment slopes in uncemented sands, such as the Bagshot, Harwich, Upnor and Thanet Sand formations, may suffer rapid erosion from surface run-off and require the slope to be drained, protected by vegetation or the water diverted.

Clay swell-shrink

Large areas of the district where London Clay is at or close to the surface, particularly north-west and south-west London, are affected by the problem of ground movement caused by alternate swelling and shrinking of the clay. This is caused by the smectite content of the clay, which expands when wet and contracts on drying out. Swell-shrink occurs, but to a lesser extent, on outcrops of till and Lambeth Group.

Annual moisture changes in soils developed on the London Clay can cause the ground surface of a grass-covered area to rise and fall over the year by 50 mm (Building Research Establishment, 1996). However, vegetation has a significant role in the desiccation of the soil and this figure can rise to 100 mm near to trees. Depending on the species, trees can reduce soil moisture to a depth of 5 m (McEntee, 1984). Unrecoverable heave of 12 mm was measured by Driscoll (1984) over a three year period and 80 mm was recorded on rehydration of the soil after the removal of trees (Building Research Establishment, 1996) a process that may continue for more than 20 years before moisture equilibrium with the surrounding clay is re-established (Cheney 1988). Thus, structures may be damaged by subsidence due to progressive desiccation of the soil over a series of drought years especially if the effect is enhanced by trees, or they may be damaged by heave when soils swell on re-hydration after the removal of trees.

Information sources

Further geological information held by the British Geological Survey relevant to the London district is listed below. It includes published maps, memoirs and reports. Enquiries concerning geological data for the district should be a addressed to the Manager, National Geological Records Centre, BGS, Keyworth. Geological advice for this area should be sought from the Regional Geologist, Southern and Eastern England, BGS, Keyworth. This list was compiled in 2004.

Other information sources include borehole records, fossils, rock samples, thin sections, hydrogeological data and photographs. A Geoscience Index System is available for consultation in BGS libraries and on the BGS web site (www.bgs.ac.uk). This is a developing system which searches indexes to the collections. It has a backdrop based on the 1:250 000 scale maps. Available indexes include:

• Index of boreholes
• Outlines of BGS maps at 1:50 000 and 1:10 000 scale and 1:10 560 scale County Series
• Chronostratigraphical boundaries and areas from British Geological Survey 1:125 000 maps
• Geochemical sample locations on land
• Aeromagnetic and gravity recording stations
• Land survey records

Maps

Geological maps

1:1 500 000

• Tectonic map of Britain, Ireland and adjacent areas, 1996

1:1 000 000

• Pre-Permian geology of the United Kingdom, 1985

1:625 000

• Solid geology, UK South Sheet, 2001; Quaternary geology, 1977

1:250 000

• 51N 02W Chilterns, Solid Geology, 1991
• 51N 00 Thames Estuary, Solid Geology, 1989
• 51N 00 Thames Estuary, Sea bed sediments and Quaternary Geology, 1990

1:50 000

• Sheet 238 Aylesbury, 1997
• Sheet 239 Hertford, 1978
• Sheet 240 Epping, 1981
• Sheet 241 Chelmsford, 1975
• Sheet 255 Beaconsfield, 1997
• Sheet 256 North London, 1994
• Sheet 257 Romford, 1996
• Sheet 258 and 259 Southend and Foulness, 1976
• Sheet 269 Windsor, 1981
• Sheet 270 South London, 1998
• Sheet 271 Dartford, 1998
• Sheet 272 Chatham, 1977
• Sheet 285 Guildford, 2001
• Sheet 286 Reigate, 1978
• Sheet 287 Sevenoaks, 1971
• Sheet 288 Maidstone, 1976

1:10 000

The 96 maps covering the four 1:50 000 series sheets of London (256, 257, 270 and 271) were resurveyed between 1970 and 1996 by A J M Barron, F G Berry, S J Booth, C R Bristow, R A Ellison, D H Jeffery, R D Lake, M McKeown, D Millward, R T Mogdridge, B S P Moorlock, A N Morigi, A Smith, P J Strange and I T Williamson.

Digital geological map data

In addition to the printed publications noted above, many BGS maps are available in digital form, which allows the geological information to be used in GIS applications. These data must be licensed for use. Details are available from the Intellectual Property Rights Manager at BGS Keyworth. The main datasets are:

• DiGMapGB-625 (1:625 000 scale)
• DiGMapGB-250 (1:250 000 scale)
• DiGMapGB-50 (1:50 000 scale)
• DiGMapGB-10 (1:10 000 scale)

The current availability of these can be checked on the BGS web site at:

http://www.bgs.ac.uk/products/digitalmaps/digmapgb.html

Geophysical maps

1: 1 500 000

• Colour shaded relief magnetic anomaly map of Britain, Ireland and adjacent areas, 1998
• Colour shaded relief gravity anomaly map of Britain, Ireland and adjacent areas, 1997

A geophysical information map (GIM) at the scale of 1:50 000 is available for the district. This shows information held in BGS digital databases, including Bouguer gravity and aeromagnetic anomalies and locations of data points, selected boreholes and detailed geophysical surveys

Geochemical maps

1:625 000

• Methane, carbon dioxide and oil susceptibility, Great Britain, South Sheet, 1995.
• Radon potential based on solid geology, Great Britain, South Sheet, 1995.
• Distribution of areas with above national average background concentrations of potentially harmful elements (As, Cd, Cu, Pb and Zn), Great Britain, South Sheet, 1995.

Hydrogeological maps

1: 625 000

• Hydrogeological map of England and Wales (1977)

Groundwater vulnerability maps

1:100 000

• Groundwater vulnerability of west London, Sheet 39, 1990 Groundwater vulnerability of the Thames Estuary, Sheet 40, 1990

Minerals maps

1: 1 000 000

• Industrial minerals resources map of Britain, 1996

Books

British Regional Geology

• London and the Thames valley. Fourth edition, 1996
• The Wealden district. Fourth edition, 1965
• The Hampshire Basin and areas adjoining. Fourth edition, 1982

Memoirs

• Geology of the country around Hertford (Sheet 239), 1924^{out of print; available as a facsimile copy at a tariff that covers the cost of copying}
• Geology of the country around Epping (Sheet 240), 1987
• Geology of the country around Chelmsford (Sheet 241), 1985
• Geology of the country around Beaconsfield (Sheet 255), 1924^{out of print; available as a facsimile copy at a tariff that covers the cost of copying}
• Geology of the country around North London (Sheet 256), 1925^{out of print; available as a facsimile copy at a tariff that covers the cost of copying}
• Geology of the country around Romford (Sheet 257), 1925^{out of print; available as a facsimile copy at a tariff that covers the cost of copying}
• Geology of the country around Southend and Foulness (Sheet 258/9), 1986.
• Geology of the country around Windsor and Chertsey (Sheet 269), 1925^{out of print; available as a facsimile copy at a tariff that covers the cost of copying}
• Geology of the country around South London (Sheet 270), 1921^{out of print; available as a facsimile copy at a tariff that covers the cost of copying}
• Geology of the country around Dartford (Sheet 271), 1924^{out of print; available as a facsimile copy at a tariff that covers the cost of copying}
• Geology of the country around Chatham (Sheet 272), 1954
• Geology of the country around Aldershot and Guildford (Sheet 285),1929^{out of print; available as a facsimile copy at a tariff that covers the cost of copying}
• Geology of the country around Reigate and Dorking (Sheet 286), 1933^{out of print; available as a facsimile copy at a tariff that covers the cost of copying}
• Geology of the country around Sevenoaks and Tonbridge (Sheet 287), 1969^{out of print; available as a facsimile copy at a tariff that covers the cost of copying}
• Geology of the country around Maidstone (Sheet 288), 1963

Sheet Explanations

• Geology of the Windsor and Bracknell district (Sheet 269), 1999 Geology of the Guildford district (Sheet 285), 2002

BGS Reports

BGS reports not included in the references are listed here. Most of them are available for consultation at the Library, BGS, Keyworth; those indicated as with a suffix ‘R’ or ‘C’ are restricted or confidential and permission may be needed before they can be consulted.

Lands and resources reports

• Geological report of a site at Guildhall Yard East.
• WA/93/50C. Lower London Tertiary strata of the Thames basin. Report for Union Rail. WA/94/10C.
• Overview of the Tertiary outliers between the Thames and Medway. Report for Union Rail. WA/94/15C.
• Geology of the Union Railways route from the Thames to the Medway. WA/94/78C.
• Core reference photography of Union Railways Limited Boreholes 1112B and 1131. WA/94/96C.
• Core reference photography of Union Railways Limited Boreholes 1131 (continued), 2112 and 2141. WA/94/96C.
• A review of the Tertiary strata proved in Union Railways cored boreholes, King’s Cross to Barking. WA/94/50C.
• Presentation of digital earth science information for planning in the Thames Gateway. WA/96/626.
• Geology of the East Tilbury district: 1:10 000 sheet TQ67NE. Part of 1:50 000 sheets 257 (Romford), 258/259 (Southend and Foulness), 271 (Dartford) and 272 (Chatham). WA/89/37.
• Geology of the West Thurrock district: 1:10000 sheet TQ57NE. Part of 1:50 000 sheets 257 (Romford) and 271 (Dartford). WA/89/35.
• Geology of the Upminster district: 1:10 000 sheet TQ58NE. Part of 1:50 000 Sheet 257 (Romford). WA/89/41.
• Geology of the Grays district: 1:10 000 sheet TQ67NW. Part of 1:50 000 sheets 257 (Romford) and 271 (Dartford). WA/89/36.
• Geology of the Billericay district: 1:10 000 sheet TQ69SE. Part of 1:50 000 sheets 257 (Romford) and 258 (Southend). WA/91/26.
• Geology of the South Ockenden district: 1:10 000 sheet TQ58SE. Part of 1:50 000 Sheet 257 (Romford). WA/89/38. Geology of the Orsett district: 1:10 000 sheet TQ68SW. Part of 1:50 000 Sheet 257 (Romford). WA/89/39.
• Geology of the West Horndon district: 1:10 000 sheet TQ68NW. Part of 1:50 000 Sheet 257 (Romford). WA/89/42.
• Geology of the Stanford-le-Hope district: 1:10 000 sheet TQ68SE. Part of 1:50 000 sheets 257 (Romford) and 258/259 (Southend and Foulness). WA/89/40.
• Geology of the Laindon district: 1:10 000 sheet TQ68NE. Part of 1:50 000 sheets 257 (Romford) and 258/259 (Southend and Foulness). WA/89/43.
• Geology of the Thorndon Park district: 1:10 000 sheet TQ69SW. Part of 1:50 000 Sheet 257 (Romford). WA/90/23.
• Geology of the Brentwood district, 1:10 000 sheet TQ59SE. Part of 1:50 000 Sheet 257 (Romford). WA/90/20.

Hydrogeological reports

• Results of investigations at the Hoe Lane site, Enfield. WD/90/53R.
• Results of investigations at the Chingford South site, Chingford. WD/90/54R.
• Results of investigations at the Coppermills site, Walthamstow. WD/90/55R.
• Results of investigations at Hadley Road, Enfield. WD/90/16C. Highway drainage to the Chalk aquifer: the movement of groundwater in the Chalk near Bricket Wood, Hertfordshire, and its possible pollution by drainage from the M25 motorway. WD/89/3.
• Correlation of electrical resistivity marker bands in the Chalk of the London basin. WD/82/1.
• Goundwater conditions in the vicinity of the Rothehithe Underground Railway Tunnel and their relation to the River Thames. WD/74/1.
• Groundwater conditions in the Woolwich reach of the Thames with reference to the effects of a tidal-surge and tidal barrier. WD/73/5.

Engineering geological reports

• A geophysical and geological investigation at Barking, Creekmouth, Essex. EG/72/7
• Investigation of possible subsidence in chalk near Meopham, Kent. EG/74/4
• Report on the Crystal Palace Borehole Geotechnical Testing Programme. EG/78/25
• Density, porosity and sonic velocity determinations on selected core samples of various rock types from the London basement. EG/97/5
• Groundwater conditions in the vicinity of the M1/M25 interchange, Hertfordshire. EGARP/KW/86/8/C
• A tracer test in the Chalk aquifer near Bricket Wood, Hertfordshire, in connection with the drainage of the M25 Motorway. WN/88/4/C
• Childerditch landslip, Essex. Field Report. WN/88/8
• S W Essex–M25 corridor. Engineering Geology. WN/90/2 The engineering geology of the London area. WN/97/27

Palaeontological reports

• Dinoflagellate cyst analysis of lowermost London Clay, Aveley Motorway cutting, Essex; Sheet 271. WH/PI/79/224R Calcareous microfossils: Charlton old gravel pit; Sheet 271. WH/PI/76/2R
• Bryozoa from the Richmond borehole; Sheet 270. WH/ PI/75/103R
• Palynology of selected samples from the Harpers Road (Peckham) borehole TQ37NW/2026; Sheet 270. WH/ PI/75/95R
• Calcareous nannofossil analysis of the Jubilee Borehole 404T. WH/91/314C
• Dinoflagellate cyst analysis of the Paleogene sediments from the Staines No. 5 Borehole. WH/92/273R
• Dinoflagellate cyst analysis of the Paleogene sediments from the Hampstead Heath Borehole. WH/93/28R
• Dinoflagellate cyst analysis of the Paleogene sediments from the BGS Stanmore Common Borehole. WH/93/256R
• A biostratigraphical review of the Chalk of the Dartford (Sheet 271) district and adjoining areas. WH/94/208R Chalk macrofaunas from the Dartford (Sheet 271), Kent. WH/95/7R
• Interpretation of geophysical well logs from the Chalk of the London Basin. WH/95/184C
• Chalk stratigraphy of the Cray valley, south-east London. WH/97/18C

Mineralogy reports

• Mineralogy and petrography of the Lambeth Group from the London and Hampshire Basins. WG/98/4R
• Stratigraphic variations in the heavy minerals of the Paleogene sandstones in the London Basin and the implications for sand provenance. WH/93/304

Mineral resource reports

• Mineral resource information in support of national, regional and local planning: Hertfordshire and Northwest London Boroughs. CR/03/075/N
• Mineral resource information in support of national, regional and local planning: Surrey (comprising Surrey and the London Boroughs of Croydon, Hounslow, Kingston upon Thames, Richmond upon Thames and Sutton). CR/03/073N
• Mineral resource information in support of national, regional and local planning: Essex (comprising Essex, Southend-onSea, Thurrock, London Boroughs of Barking and Dagenham, Havering, Redbridge and Waltham Forest). CR/02/127N
• Mineral resource information in support of national, regional and local planning: Kent (comprising Kent, Medway and London Boroughs of Bexley and Bromley). CR/02/125N
• Collation of the results of the 2001 Aggregate Minerals Survey. CR/03/53N

Documentary collections

Boreholes

Borehole data for the district are catalogued in the BGS archives (National Geological Records Centre) at BGS Keyworth on individual 1:10 000 scale sheets. For further information contact: The Manager, NGRC, BGS Keyworth.

BGS hydrogeology enquiry service; wells, springs and water borehole records. British Geological Survey, Hydrogeology Group, Maclean Building, Crowmarsh Gifford, Wallingford, Oxfordshire OXO 8BB. Telephone 01491 838800. Fax 01491 692345.

BGS Lexicon of named rock unit definitions

Definitions of the named rock units shown on BGS maps, including those of the London district are held in the Lexicon database. This is available on the BGS web site (http://www.bgs.ac.uk). Further information on the database can be obtained from the Lexicon Manager at BGS, Keyworth.

BGS photographs

Copies of the photographs used in this memoir are deposited for reference in the BGS libraries in Keyworth and Edinburgh. Colour or black and white prints and transparencies can be supplied at a fixed tariff. BGS also holds a large collection of photographs that date back to the early part of the last century. Part of the collection can be viewed online at the BGS web site.

References

Most of the references listed below are held in the Library of the British Geological Survey, Murchison House, Edinburgh and at Keyworth, Nottingham. Copies of the references can be purchased subject to current copyright legislation. BGS Library catalogue can be searched online at: http://geolib.bgs.ac.uk

Allen, D J, Bloomfield, J P, and Robinson, V K (editors). 1997.  The physical properties of major aquifers in England and Wales. British Geological Survey Technical Report, WD/97/34.

Allison, J, Godwin, H, and Warren, S H. 1952. Late-glacial deposits at Nazeing in the Lea Valley, North London. Philosophical Transactions of the Royal Society of London Series B, Vol. 236, 169–240.

Allsop, J M, and Smith, N J P. 1988. The deep geology of Essex. Proceedings of the Geologists’ Association, Vol. 99, 249–260.

Arkell, W J. 1933. The Jurassic System in Great Britain. (Oxford: Clarendon Press.)

Attewell, P B, and Farmer, I W. 1974. Ground deformations resulting from shield tunnelling in London Clay. Canadian Geotechnical Journal, Vol. 11, 380–395

Aubry, M P. 1986. Palaeogene calcareous nannoplankton biostratigraphy of north-western Europe. Palaeogeography, Palaeoclimatology, Palaeoecology, Vol. 55, 267–334.

Bailey, H W, Gale, A S, Mortimore, R N, Swiecicki, A, and Wood, C J. 1983. The Coniacian-Maastrichtian stages of the United Kingdom, with particular reference to southern England. Newsletters on Stratigraphy, Vol. 12, 29–42.

Baker, C A, and Jones, D K C. 1980. Glaciation of the London Basin and its influence on the drainage pattern: a review and appraisal. 131–175 in The shaping of southern England. Jones, D K C (editor.). Institute of British Geography Special Publication, 11. (London: Academic Press.)

Ballantyne, C K, and Harris, C. 1994. The periglaciation of Great Britain. (Cambridge: Cambridge University Press.)

Barrow, G, and Wills, L J. 1913. Records of London wells.  Memoir of the Geological Survey of England and Wales.

Barton, M E, and Palmer, S N. 1989. The relative density of geologically aged, British fine and fine-medium sands. Quarterly Journal of Engineering Geology, Vol. 22, 49–58.

Barton, M E, Palmer, S N, and Wong, Y L. 1986. A geotechnical investigation of two Hampshire Tertiary sand beds: are they locked sands? Quarterly Journal of Engineering Geology, Vol. 19, 399–412.

Bates, M R, and Barham, A J. 1995. Holocene alluvial stratigraphic architecture and archaeology in the Lower Thames area. 85–98 in The Quaternary of the lower reaches of the Thames. Bridgland, D R, Allen, P, and Haggart, B A (editors). (Durham: Quaternary Research Association Field Guide.)

Bell, F G. 1977. A note on the physical properties of the Chalk. Engineering Geology, Vol. 11, 217–224. (Amsterdam: Elsevier.)

Bell, F G, Culshaw, M G, Moorlock, B S P, and Cripps, J C.  1992. Subsidence and ground movements in Chalk.  Bulletin of the International Association of Engineering Geology. Vol. 45, 75–82.

Bell, F G, Culshaw, M G, and Cripps, J C. 1999. A review of selected engineering geological characteristics of English Chalk. Engineering geology, Vol. 54, 237–269.

Berry, F G. 1979. Late Quaternary scour hollows and related features in central London. Quarterly Journal of Engineering Geology, Vol. 12, 9–29.

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Bevan, T G, and Hancock, P L. 1986. A late Cenozoic mesofracture system in southern England and northern France. Journal of the Geological Society, Vol. 143, 355–362.

Bingley, R M, Ashkenasi, V, Ellison, R A, Morigi, A, Penna, N T, and Booth, S J. 1999. Monitoring changes in ground level, using high precision GPS. Environment Agency R&D Report, W210.

Bloomfield, J P, Brewerton L J, and Allen D J. 1996. Regional trends in matrix porosity and bulk density of the Chalk of England. Quarterly Journal of Engineering Geology, Vol. 28, S131–S142.

Bowen , D Q (editor). 1999. A revised correlation of Quaternary deposits in the British Isles. Geological Society of London Special Report, No. 23.

Bowen, D Q, Hughes, S A, Sykes, G A, and Miller, G M. 1989. Land-sea correlations in the Pleistocene based on isoleucine epimerization in non-marine molluscs. Nature, Vol. 340, 49–51.

Bowen, D Q, Sykes, G A, Maddy, D R, Bridgland, D R, and Lewis, G G. 1995. Aminostratigraphy and amino acid geochronology of English lowland valleys: the Lower Thames in context. 61–63 in The Quaternary of the Lower reaches of the Thames. Field Guide. Bridgland, D R, Allen, P, and Haggart, B A (editors). (Durham: Quaternary Research Association.)

Bridgland, D R. 1994. The Quaternary of the Thames. Geological Conservation Review Series, No. 7. (London: Joint Nature Conservation Committee/Chapman and Hall.)

Bristow, C R. 1985. Geology of the country around Chelmsford. Memoir of the British Geological Survey, Sheet 241 (England and Wales).

Bristow, C R, Ellison, R A, and Wood, C J. 1980. The Claygate Beds of Essex. Proceedings of the Geologists’ Association, Vol. 91, 261–277.

Bristow, C R, Mortimore, R N, and Wood, C J. 1997. Lithostratigraphy for mapping the Chalk of southern England. Proceedings of the Geologists’ Association, Vol. 108, 293–316.

Bromehead, C N. 1912. On diversions of the Bourne near Chertsey. 74–77 in Summary of Progress of the Geological Survey for 1911.

Bromehead, C N. 1922. Excursion to Brockley, Bromley Park and Beckenham. Proceedings of the Geologists’Association, Vol. 33, 77–79.

Bromehead, C E N. 1925. Geology of north London. Memoirs of the Geological Survey of Great Britian, Sheet 256 (England and Wales).

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Building Research Establishment. 1996. Desiccation in clay soils. Building Research Establishment Digest 412. (London: HMSO.)

Bullard, E C. 1940. Seismic investigations on the Palaeozoic floor of East England. Philosophical Transactions of the Royal Society of London, Vol. A239.

Burland, J B, and Hancock, R J R. 1977. Underground car park at the House of Commons, London. Geotechnical aspects. Structural Engineering, Vol. 55, 87–100.

Burland, J B, and Kalra, J C. 1986. Queen Elizabeth II Conference Centre: geotechnical aspects. Proceedings of the Institution of Civil Engineers, Vol. 80, 1479–1503.

Burnett, A D, and Fookes, P G. 1974. A regional engineering geological study of the London Clay in the London and Hampshire Basins. Quarterly Journal of Engineering Geology. Vol. 7, 257–293

Card, G B, and Carter, G R. 1995. Case history of a piled embankment in London’s Dockland. 79–84 in Engineering geology of construction. Eddleston, M, Walthall, S, Cripps, J C, and Culshaw, M G (editors). Geological Society Engineering Geology Special Publication, No. 10.

Carter, D J, and Hart, M B. 1977. Micropalaeontological investigations for the site of the Thames Barrier, London. Quarterly Journal of Engineering Geology, Vol. 10, 321–338.

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Chadwick, R A. 1986. Extension tectonics in the Wessex Basin, southern England. Journal of the Geological Society of London, Vol. 143, 465–488.

Chadwick, R A, Kenolty, N, and Whittaker, A. 1983. Crustal structure beneath southern England from deep seismic reflection profiles. Journal of the Geological Society of London, Vol. 40, 893–911.

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Chandler, R J, Willis, M R, Hamilton, P S, and Andreou, I. 1998. Tectonic shear zones in the London Clay. Geotechnique, Vol. 48, 257–270.

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Cheney, J E. 1988. 25 years’ heave of a building constructed on clay, after tree removal. Ground Engineering, 21 July, 13–27.

Cheshire, D A. 1981. A contribution towards a glacial stratigraphy of the Lower Lea valley and its implications for the Anglian Thames. Quaternary Studies, Vol. 1, 27–69.

Chisholm, F A. 1984. Rising groundwater levels under London and their Geotechnical significance. MSc Thesis, University of London.

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Clarke, R P J, and Mackenzie, C N P. 1994. Overcoming ground difficulties at Tooting Bec. Proceedings of the Institution of Civil Engineers, Civil Engineering, Vol. 102, 60–75.

Clayton, C J. 1986. The chemical environment of flint formation in Upper Cretaceous chalks. 43–54 in The scientific study of flint and chert. Proceedings of the Fourth International Flint Symposium, Brighton, 1983. Sieveking, G de G, and Hart, M B (editors). (Cambridge: Cambridge University Press.)

Cole, K W, and Burland, J B. 1972. Observations of retaining wall movements associated with a large excavation. Building Research Establishment, CP8/72.

Collinson, M E. 1983. Fossil plants of the London Clay. Palaeontological Association Field Guide to fossils 1. (London: The Palaeontological Association.)

Collinson, M E, and Hooker, J J. 1987. Vegetational and mammalian faunal changes in the early Tertiary of southern England. 259–304 in The origins of Angiosperms and their biological consequences. Friis, E M, Chaloner, W G, and Crane, P R (editors). (Cambridge: Cambridge University Press.)

Coope, G R, Gibbard, P L, Hall, A R, Preece, R C, Robinson, J E, and Sutcliffe, A J. 1997. Climatic and environmental reconstructions based on fossil assemblages from Middle Devensian (Weichselian) deposits of the River Thames at South Kensington, central London, UK. Quaternary Science Reviews, Vol. 16, 1163–1196.

Costa, L, and Downie, C. 1976. The distribution of the dinoflagellate Wetzeliella in the Palaeogene of north-western Europe. Palaeontology, Vol. 19, 591–614.

Cruickshank, J. 1991. Knows to the ground. Ground Engineering. June, 12–15.

Culshaw, M G, and Crummy, J A. 1991. SW Essex — M25 Corridor: engineering geology. British Geological Survey Technical Report, WN 90/2.

Curry, D. 1965. The Palaeogene beds of south-east England. Proceedings of the Geologists’ Association, Vol. 76, 151–173.

Daley, B, and Balson, P S. 1999. British Tertiary stratigraphy. Geological Conservation Review Series. No. 15. (Peterborough: Joint Nature Conservation Committee.)

Davis, A G. 1926. Notes on some chalk sections in N E Surrey. Proceedings of the Geologists’ Association, Vol. 37, 211–220.

Davis, A G. 1928. The geology of the City and South London Railway Clapham-Morden Extension. Proceedings of the Geologists’ Association, Vol. 39, 339–352.

Davis, A G. 1929. Excursion to Keston, Kent and Fairchildes, Surrey. Saturday, April 30th, 1927. Proceedings of the Geologists’ Association, Vol. 40, 103–104.

Davis, A G, and Elliott, G F. 1957. The palaeogeography of the London Clay sea. Proceedings of the Geologists’ Association, Vol. 68, 255–277.

Derbyshire, E, and Mellors, T W. 1988. Geological and geotechnical characteristics of some loess and loessic soils from China and Britain: a comparison. Engineering Geology, Vol. 25, 135–175.

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Dewey, H, Bromehead, C E N, Chatwin, C P, and Dines, H G. 1924. The geology of the country around Dartford . Memoirs of the Geological Survey, Sheet 271 (England and Wales).

Dibley, G E. 1909. Excursion to Gravesend. Proceedings of the Geologists’ Association, Vol. 15, 463–464.

Dibley, G E. 1918. Additional notes on the Chalk of the Medway valley, Gravesend, west Kent, northeast Surrey, and Grays (Essex). Proceedings of the Geologists’Association, Vol. 29, 68–105.

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Driscoll, R. 1984. The effects of clay soil volume changes on low rise buildings. In: Ground Movements and their effects on structures. (Surrey University Press.)

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Ellison, R A. 1979. Report of a field excursion to south Essex: 1st March 1975. Tertiary Research, Vol. 2, 51–55.

Ellison, R A. 1983. Facies distribution in the Woolwich and Reading Beds of the London Basin, England. Proceedings of the Geologists’ Association, Vol. 94, 311–319.

Ellison, R A. 1991. Lithostratigraphy of the Woolwich and Reading Beds along the proposed Jubilee Line Extension, south-east London. British Geological Survey Technical Report, WA/91/5C.

Ellison, R A, and Williamson, I T. 1999. Geology of the Windsor and Bracknell district — a brief explanation of the geological map. Sheet Explanation of the British Geological Survey. 1:50 000 Sheet 269 Windsor (England and Wales). (Keyworth, Nottingham: British Geological Survey.)

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Ellison, R A, Knox, R W O’B, Jolley, D W, and King, C. 1994.  A revision of the lithostratigraphical classification of the early Palaeogene strata of the London Basin and East Anglia. Proceedings of the Geologists’ Association, Vol. 105, 187–197.

Ellison, R A, Ali, J R, Hine, N M, and Jolley, D W. 1996. Recognition of Chron 25N in the upper Palaeocene Upnor Formation of the London Basin, UK. 185–193 in Correlation of the early Palaeogene in northwest Europe. Knox, R W O’B, Corfield, R M, and Dunay, R E (editors). Geological Society of London Special Publication, No. 101.

Ellison, R A, Arrick, A, Strange, P J, and Hennessey, C. 1998. Earth science information in support of major development initiatives. Summary Report for the Department of Transport, Environment and the Regions. British Geological Survey Technical Report, WA/97/84.

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Gibbard , P L. 1994. Pleistocene history of the Lower Thames Valley. (Cambridge: Cambridge University Press.)

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Gibbard , P L, and Hall, A R. 1982. Late Devensian river deposits in the Lower Colne valley, West London. Proceedings of the Geologists’ Association, Vol. 93, 291–299.

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Wrigley, A. 1940. The faunal succession in the London Clay, illustrated in some new exposures near London. Proceedings of the Geologists’ Association, Vol. 51, 230–245.

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Figures plates and tables

Figures

(Figure 1) The London district. a. Image of the district derived from a digital elevation model, illuminated from the north. B Principal roads and locations in the district.

(Figure 2) Distribution of concealed Palaeozoic rocks and depth contours on the Palaeozoic surface.

(Figure 3) Distribution of strata below the Late Cimmerian unconformity (base of Lower Greensand Group).

(Figure 4) Upper Greensand and Gault, distribution and thickness.

(Figure 5) Schematic cross-section showing correlation of the Chalk between the Chiltern Hills and the North Downs (see also (Figure 7)).

(Figure 6) Correlation of the Chalk using borehole resistivity logs (stratigraphy of Fetcham Mill Borehole is based on Murray, 1986 and Mortimore and Pomerol, 1987).

(Figure 7) Generalised vertical section through the Chalk.

(Figure 8) Correlation of key exposures in the Upper Chalk.

(Figure 9) Isopachyte map of the Thanet Sand Formation.

(Figure 10) Correlation of gamma-ray signatures in the Lambeth Group and Thanet Sand Formation.

(Figure 11) Particle size envelopes for the Lambeth Group and Thanet Sand Formation. a Laminated beds b Upnor Formation c Thanet Sand Formation

(Figure 12) Schematic diagram showing the relationship of informal lithological units in the Lambeth Group in central London.

(Figure 13) Isopachyte map of the Lambeth Group.

(Figure 14) Cross-sections in the Lambeth Group. Vertical bars drawn along the top of each section indicate the location of borehole data used to construct the section. a Channel Tunnel Rail Link route b Crossrail route c A102 (M) d Jubilee Line Extension route e South-east London

(Figure 15) Lithostratigraphical sections in the Lambeth Group. Location of sections is shown on (Figure 14).

(Figure 16) Upnor Formation distribution and thickness.

(Figure 17) Lower Shelly Clay distribution and thickness.

(Figure 18) Laminated beds distribution and thickness.

(Figure 19) Upper Shelly Clay distribution.

(Figure 20) Reading Formation distribution and lithology.

(Figure 21) Clay mineralogy of the Lambeth Group in Borehole 404T (central London).

(Figure 22) Schematic block diagram to illustrate the environment of deposition of the Lambeth Group.

(Figure 23) Ribbon diagram of the Harwich Formation showing the lithological variation and general thickness.

(Figure 24) Sketch of cross-stratification bedforms in the Harwich Formation in the A102 (M) road cutting at Eltham (based on information from Mr S Tracey).

(Figure 25) Weathering profile in the London Clay at Ockendon Clay Pit [TQ 611 834].

(Figure 26) Correlation of boreholes in the London Clay.

(Figure 27) Stratigraphical position of selected (current and previously described) exposures of the London Clay Formation within the London area. The thickness of the formation and its units are based on the BGS Crystal Palace Borehole, and may differ elsewhere in the district.

(Figure 28) Range of selected fossil taxa of the London Clay Formation in the London district.; 1 Apectodinium [Wetzeliella] astrum ; 2 Wetzeliella meckelfeldensis ; 3 Dracodinium similis ; 4 Eatonicysta ursulae ; 5 Dracodinium varielongitudum ; 6 Charlesdowniea [Wetzeliella] coleothrypta; 7 Coscinodiscus sp. 1 ; 8 Cenosphaera spp. ; 9 Bathysiphon ?sp. ; 10 Miliammina paleocenica ; 11 Spiroplectamina aff. thanetana ; 12 Percultazonaria [Marginulina] wetherellii ; 13 Cibicidoides eocaenus ; 14 Osangularia expansa ; 15 Alabaminoides [Epistominella] ‘vitrea’ ; 16 Fursenkoina sp. ; 17 Subbotina gr. triangularis ; 18 Subbotina patagonica ; 19 Globanomalina [Pseudohastigerina] wilcoxensis ; 20 Astarte subrugata ; 21 Striarca [Glycymeris] wrigleyi; 22 Cyclopecten ?[Pecten] duplicatus; 23 Eotibia sublucida; ; 24 Lentipecten corneum; 25 Astarte filligera; 26 Semimodiola elegans; 27 Spiratella taylori; 28 Spiratella aff. taylori; ; 29 Spiratella aff. Tutelina; 30 Camptoceratops priscum; 31 Isselicrinus subbasaltiformis; 32 Cytheridea unispinae; 33 Paracyprideis similis; 34 Cytheretta venablesi; 35 Cytheretta scrobiculoplicata; 36 Hazelina aranea; 37 Echinocythereis sp. ; 38 Echinocythereis reticulatissima; 39 Cytheridea newburyensis; 40 Cytheretta multicostata londinensis

(Figure 29) Evolution of the River Thames, distribution of fluvial deposits and inferred position of the penecontemporary flood plains.

(Figure 30) Schematic cross-section (not to scale) to illustrate the lateral variation in Clay-with-flints on the dip slope of the North Downs.

(Figure 31) Distribution of glacial deposits and associated geomorphological features.

(Figure 32) Illustration of the probable sequence of events during deposition of the Thames River Terrace deposits (after Wymer, 1999).

(Figure 33) Cross-section of Quarternary deposits along the M25 Motorway at Dartford.

(Figure 34) Longitudinal profile showing the vertical relationships of Thames River Terrace deposits and interglacial deposits between Hounslow and Tilbury. Black vertical bars represent the height range of selected interglacial deposits whose name and oxygen isotope stage are shown - see also (Table 18).

(Figure 35) Location of deep drift-filled hollows.

(Figure 36) Characteristics of a deep drift-filled depression at Blackwall. a Lithological section based on a cored borehole. b Detail of sediment infill (bi 55.50 m depth; bii 43.40 m depth) c Cross-section at the Blackwall Tunnel based on site investigation boreholes, showing zone of highly weathered chalk cross-cutting the Thames Sand and Lambeth Group.

(Figure 37) Distribution of ‘brickearth’ deposits.

(Figure 38) Location of key Quaternary interglacial sites.

(Figure 39) Cross-section to illustrate the relationship of interglacial deposits to River Terrace Deposits at Aveley.

(Figure 40) Distribution of alluvium and illustration of the alluvial sequence at selected locations along the River Thames. O¹⁴ dates obtained from peat beds are shown on the geological sections.

(Figure 41) Schematic cross-section through the Thames River Terrace Deposits showing their relationship to interglacial deposits and their oxygen isotope stages (IOS) (after Bridgland, 1994).

(Figure 42) Principal geological structures of the district.

(Figure 43) Colour-shaded Bouguer gravity relief image illuminated from the north. The map has been derived by wavelength filtering to remove the effects of deep-seated density variations and emphasise low-amplitude near-surface anomalies. Contours at 1 mGal interval (1 mGal = 1 3 10^-5m/s²).

(Figure 44) Structural contours illustrating the base of the Chalk. Contours in metres relative to OD.

(Figure 45) Structural contours illustrating the base of the Palaeogene: contours are at 25 m intervals. Perspective view of the Chalk– Palaeogene boundary, viewed from the south-west.

(Figure 46) Map showing areas with potentially artesian groundwater in about 1800 and groundwater levels in 1965 (based on information from the Environment Agency).

(Figure 47) Graph showing groundwater abstraction in the London Basin, 1820 to 1987. Based on information in CIRIA (1989).

(Figure 48) Variation in specific capacity of the Chalk where overlain by Palaeogene deposits in the London district. Based on information in CIRIA (1989).

(Figure 49) Schematic representation of chalk transmissivity in the London Basin.

(Figure 50) Groundwater types in the Chalk and ‘basal sands’ aquifer of the London Basin (after Smith et al., 1976).

(Figure 51) Hydrograph for a well in Trafalgar Square, showing the rise in water level following cessation of pumping in the mid 1970s

(Figure 52) Schematic block diagram (after Mortimore, 1997) to illustrate typical weathering and dissolution characteristics of Chalk in a dry valley.

Plates

(Front cover) Two of the massive steel-plated gates that form part of the Thames Barrier at Greenwich, completed in 1982 and designed to protect London from tidal surge floods. (Photograph supplied by www.libraryphotos.com.)

(Plate 1) View looking east of the Upnor Formation at Orsett Cock quarry [TQ 657 811] showing inclined sets of well rounded flint pebbles, interpreted as shoreface deposits, overlain by horizontally bedded tidal sediments (A12263).

(Plate 2) Photomicrographs of the Lambeth Group and Thanet Sand Formation. All samples are from Jubilee Line Extension Borehole 404T [TQ 3363 7960] (see (Figure 15)). a. Lambeth Group, Laminated beds: fine sandy clay and fine sand laminae with variable porosity: 30.05 m depth b. Lambeth Group, Lower Mottled Clay: clay has been patchily altered to a siliceous matrix (dark grey) pervasively stained by iron oxide (light grey); calcite rhombs (mid grey) form concretions: 34.20 m depth Lambeth Group, Upnor Formation: subangular to subrounded fine- to medium-grained quartz sand grains showing reduced porosity in clayey laminae (darker grey subhorizontal zones); the larger well-rounded grains are of glauconite: 38.01 m depth Thanet Sand Formation: well sorted fine-grained angular to subangular quartz sand grains, locally cemented by detrital clay; irregular dark grey areas are secondary voids due to feldspar dissolution: 52.80 m depth.

(Plate 3) Upnor Formation at Orsett Cock quarry [TQ 657 811] showing thinly bedded alternations of sand and clay, with well developed burrows of Ophiomorpha and Macaronichnus (A12267).

(Plate 4) Head (rests on chalk not in the photograph) in a disused chalk quarry at Purfleet [TQ 5574 7863]. The lower layer of paler coloured Head consists mainly of chalk clasts, and shows cryoturbation festoons emphasised by brown zones of dissolution. It is overlain by later Head, also cryoturbated, consisting mainly of brown silt with flint clasts (A10436).

(Plate 5) Railway cutting north of Brockley station [TQ 364 759].

(Inside front cover) Summary of the geological succession of the district inside front cover

Tables

(Table 1) Summary information for boreholes that penetrate strata below the Chalk.

(Table 2) Lower Palaeozoic strata proved in boreholes.

(Table 3) Summary lithology of Jurassic strata.

(Table 4)Summary of Wealden Group lithologies.

(Table 5) Subdivision of the Chalk Group. Traditional chalk subdivision after Jukes-Browne and Hill (1903; 1904).

(Table 6) Palaeogene lithostratigraphy and chronology (after Knox, 1996).

(Table 7) Key boreholes proving Palaeogene strata.

(Table 8) Lambeth Group nomenclature.

(Table 9) Chronology of principal Quaternary deposits and oxygen isotope stages (based on Sumbler, 1996).there is some debate about the allocation of oxygen isotope stages (see Ballantyne and Harris,1994;Hamblinet al., 2000)

(Table 10) Stanmore Gravel, summary information.

(Table 11) Clast composition (approximate percentages) of the pre-diversionary River Terrace deposits.

(Table 12) Pre-diversionary River Terrace deposits, summary information.

(Table 13) Glacial deposits, location details.

(Table 14) River Terrace Deposits, nomenclature and altitude.

(Table 15) Thickness of River Terrace Deposits.

(Table 16) ‘Brickearth’ associated with the River Terrace Deposits.

(Table 17) Details of ‘brickearth’ deposits.

(Table 18) Summary of deposits at interglacial sites.

(Table 19) Alluvium, location and thickness (based mainly on Gibbard, 1985, 1994).

(Table 20) Head, location and thickness.

(Table 21) Summary of potential ground constraints.

(Table 22) Typical water abstraction values and yield volumes from the Chalk

(Table 23a) Summary of geotechnical properties of the Chalk from selected published sources.

(Table 23b) Summary of geotechnical properties of the Thanet Sand Formation from published sources

(Table 23c) Summary of geotechical properties of the Lambeth Group from selected published sources.

(Table 23d) Summary of geotechnical properties of the London Clay Formation from selected published sources.

Tables

(Table 1) Summary information for boreholes that penetrate strata below the Chalk.

(Table 2) Lower Palaeozoic strata proved in boreholes.

(Table 3) Summary lithology of Jurassic strata.

(Table 4) Summary of Wealden Group lithologies.

(Table 7) Key boreholes proving Palaeogenestrata.

(Table 8) Lambeth Group nomenclature

(Table 10) Stanmore Gravel, summary information

(Table 11) Clast composition (approximate percentages) of the pre-diversionary River Terrace deposits.

(Table 12) Pre-diversionary River Terrace deposits, summary information.

(Table 13) Glacial deposits, location details.

(Table 14) River Terrace Deposits, nomenclature and altitude.

(Table 15) Thickness of River Terrace Deposits.

(Table 16) ‘Brickearth’ associated with the River Terrace Deposits.

(Table 17) Details of ‘brickearth’ deposits.

(Table 18) Summary of deposits at interglacial sites.

(Table 19) Alluvium, location and thickness (based mainly on Gibbard, 1985, 1994).

(Table 20) Head, location and thickness

(Table 21) Summary of potential ground constraints

(Table 22) Typical water abstraction values and yield volumes from the Chalk

(Table 23a) Summary of geotechnical properties of the Chalk from selected published sources.

Key

(Table 23b) Summary of geotechnical properties of the Thanet Sand Formation from selected published sources.

Key

(Table 23c) Summary of geotechical properties of the Lambeth Group from selected published sources.

Key

(Table 23d) Summary of geotechnical properties of the London Clay Formation from selected published sources

Key