Geologists’ Association Guide No. 68

The Geology of LondonIntroduction

Compiled by Diana Clements

THE CURRY FUND

HampsteadHeath

Frontispiece: Figure 1. Map of the London Area showing the locations of Itineraries described in relation to the Thames and the major road networks. The grey area is Central London. Red circles denote Itineraries.

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Notes: The details of routes given in this guide do not imply a right of way. The onus of obtaining permission to use footpaths and to examine exposures rests with the user of the guide, who should carefully observe the Code for Geological Field-work available from the Geologists’ Association. In particular those in charge of parties should ensure that no damage is caused to property.

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Typeset by: The Geologists’ AssociationPrinted by: CityPrint Ltd

Front cover: Snapshots of London Geology: From top left, the Itineraries clockwise round the Thames; cross stratification in the Reading Beds at Harefield, Pinner Chalk Mines, brickfield on Hampstead Heath c. 1880, London Clay exposure at Trent Park, sarsen at Chafford Hundred, sub-merged forest at Erith, Lower Shelly Clay at Charlton, Pulhamite grotto in Sundridge Park, Iguano-don, part of the Geological Illustrations in Crystal Palace Park, goat conservator at Riddlesdown Quarry. (Design Trevor Mill, Brand Engineering, after an initial idea from Sari Finch)

Introduction

INTRODUCTION TO THE GEOLOGY OF THE LONDON AREA

BGS 1:625 000 Bedrock Geology UK SouthBGS 1:250 000 sheets: Chilterns and Thames EstuaryBGS 1:50 000 sheets: 255, 256, 257, 269, 270, 271BGS: London Memoir (Ellison et al., 2004)BGS: Regional Guide (Sumbler, 1996)GLA, 2012: London’s Foundations web link (see below)

Contributors: Rory Mortimore (Overview and Chalk), Danielle Schreve (post-Anglian Pleistocene Gravels), additional material compiled by Diana Clements

Overview

The Late Mesozoic and Cenozoic sedimentary rocks of the region broadly known as the London Basin (Fig. 2) do not form a continuous succession. Several major hiatuses are present (Fig. 3) that span more time than is represented by preserved rocks. The sediments formed across an ancient tectonic block, the London or Anglo-Brabant Platform that represented a geological ‘high’ during much of the Early Cretaceous (Fig. 4). Along the southern, heavily faulted edge of the London high, episodic uplift provided source material for the Wealden sediments of the northern part of the Weald Basin (Allen, 1975; 1981). Rising sea levels in mid and Late Cretaceous led to flooding of the Weald Basin and the London Plat-form and widespread deposition of Gault and Chalk. Further tectonic movements along faults led to episodic fracturing of the Chalk while it was forming and growth of folds creating local highs on the sea bed within, and along the margins of, the London Platform, including the Greenwich and Purfleet anticlines and their underlying faults (Fig. 5). These intra-Chalk ‘Subhercynian’ movements culminated, towards the end of the Cretaceous and early Palaeogene, in differen-tial uplift and erosion of the Chalk (Laramide tectonic phase) prior to Palaeogene sedimentation (Fig. 3). By the Early Palaeogene, the Weald Basin had inverted to become a high and the London Uplands (Allen, 1981) had inverted to become a shallow, intra-platform basin. The Palaeogene seas then flooded this differentially eroded Chalk surface in the London Basin. Oscillations in sea level and further tectonic epi-sodes generated other hiatuses between Palaeogene formations, culminating in the Late Alpine (Miocene) uplift and erosion which must have removed much of the Late Palaeogene sediments from the region and led to further fault displace-ments of the Cretaceous and Palaeogene sediments within the London Basin (e.g. along the Greenwich, Streatham, Wimbledon en-echelon fault system). The re-sulting, differentially uplifted landscape was further modified by late Cenozoic and Quaternary weathering and climate oscillations.

1

2

Introduction

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3

Introduction

5

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20

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30

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40

45

50

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60

65

70

75

80

85

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Bagshot Fm

London Clay Fm

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Claygate Member

Harwich Fm

Camberley Sand& Windlesham Fms

Lambeth Gp

Thanet SandFormation

Reading FmWoolwich Fm

Upnor Fm

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Newhaven Ck Fm/ Margate Ck Mbr

Lewes Nodular Ck FmNew Pit Ck Fm

Holywell Nodular Ck FmZig Zag Ck Fm

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Phases of ‘Alpine’ uplift, foldingand erosion of Cenozoic sedimentsculminating in major Miocene tectonics

Quaternary ice ages and warm periodsformation of the Thames valley

Phases of Subhercynian and Laramideuplift, folding and erosion of the Chalk

Transgression of the Gault and Chalk seaonto the London Platform

London Platform basement rocks of variableage from Palaeozoic to Jurassic

Transgression of the Thanet seaonto the eroded Chalk of theLondon Platform

25 million yearhiatus

Coralline CragNorwich and Red Crags

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Figure 3. Late Cretaceous and Cenozoic succession and major events in the London Basin. (R.N. Mortimore)

4

Introduction

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5

Introduction

Further details on the structure of the London Basin will shortly become available from Mortimore (2011) and Mortimore et al. (2011) and from the Lon-don Basin Forum (De Freitas & Rowse, 2009). A more detailed overview of the geology of the London Boroughs can be found online in London’s Foundations (GLA, 2012). Quaternary The landscape of the London area was greatly modified during the Quaternary. Prior to the ice sheet that impinged on the north of London during the Anglian Glaciation, a series of fluvial deposits was laid down as former courses of the River Thames and its tributaries, many of which are thought to have crossed the London Basin from the Weald in the south to join the Thames on its course to the East Coast (Fig. 7) (Sumbler, 1996 fig. 31). See Figure 6 for the Chronology of the main Quaternary deposits of the London area. The greatest extent of the ice during the Anglian Glaciation was at least as far as the Colne Valley, the Finchley depression and the Hornchurch area where

LeatherheadSevenoaks

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Figure 5. Map showing the major NE-SW trending faults in the London Basin which are also associated with periclinal folds (e.g. the Greenwich and Purfleet anticlines) based upon Ellison et al. (2004) with the permission of the British Geological Survey; Mortimore et al. (2011). (R.N. Mortimore)

6

Introduction

remnants of glacial till can still be found (Fig 7; Sumbler, 1996 fig. 32; Ellison et al., 2004 fig. 31). During glaciations the whole area would have been subject to periglacial processes.

Post-Anglian Pleistocene GravelsA highly detailed record of climate change over much of the last c. 450,000 years is preserved in the lower reaches of the Thames Valley in the east of London. Here, a sequence of four river terraces is present, forming a ‘staircase’ of deposits

SYSTEMSERIES

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Figure 6. Chronology of the main Quaternary deposits of the Lon-don area. The Winter Hill Gravel (Itinerary 1) relates to MIS 13–12. CP14/013 British Geological Sur-vey © NERC. All rights reserved.

7

Introduction

ANG

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8

Introduction

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9

Introduction

where the highest is the oldest (Fig. 8). The Lower Thames sequence is, in its entirety, Anglian and post-Anglian in age, since the river was only diverted into this part of the valley by the Anglian glaciation. At Hornchurch, Essex, the Orsett Heath gravels directly overlie Anglian till or ‘Chalky Boulder Clay’. Each ter-race consists of a suite of cold-climate sand and gravel at the base (representing pre-interglacial, cold-climate conditions deposited on the warming limb of the glacial-interglacial cycle), overlain by fine-grained (and frequently richly fossilif-erous) deposits that are in turn covered by cold-climate gravels, laid down under deteriorating conditions before the river cut down to a new base level.

510000

510000

520000

520000

530000

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540000

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00

1600

00

1700

00

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00

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2000

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Superficial Deposits

Alluvium

Crag Group, including Stanmore Gravel

Clay-with-flints

River Terrace Deposits

Sand and Gravel, age and origin uncertain.

Derived from BGS digital geological mapping at 1:625,000 scale, British Geological Survey NERC. All rights reserved. CP14/013 British Geological Survey

Till

©

Figure 9. Simplified map of the superficial geology of the London Region showing the posi-tion of the Thames, the M25 and other major roads. Small squares represent 10 km.

10

Introduction

The four terraces are named the Orsett Heath terrace (= Boyn Hill ter-race of the Middle Thames), the Corbets Tey terrace (= Lynch Hill terrace) (see Itinerary 5, Location 1, Stops 1 and 2), the Mucking terrace (= Taplow terrace) (see Itinerary 5, Location 3, Stop X2) and the East Tilbury Marshes (= Kempton Park terrace). The interglacial deposits within them have been correlated with MIS 11, 9, 7 and 5e of the marine oxygen isotope record (Bridgland, 1994; Schreve, 2004). Although pollen is generally poorly preserved within the fluvial sediments, the interglacial deposits contain rich assemblages of fossil mammals, molluscs and Palaeolithic archaeology. The mammals, and to a lesser extent the molluscs, have proved particularly useful in distinguishing the interglacial de-posits by means of biostratigraphy. Each terrace has yielded a distinctive and unique suite of mammals, informally termed a ‘Mammal Assemblage-Zone’ (MAZ) by Schreve (2001) and allocated to a ‘type site’ where that fauna is best expressed (Fig. 8). The lowest terrace, represented by the East Tilbury Marshes Formation, now occurs below the modern river floodplain and no exposures are therefore visible. These deposits preserve evidence for the Last (Ipswichian) Interglacial, c. 125,000 years before present, the best-known site being Trafalgar Square in central London, where remains of hippopotamus, lion, narrow-nosed rhinoceros and other species were recovered from building foundation works in the 1950s. Itineraries

The itineraries in this guide are an introduction to this broad geology (see Figs 1 (frontispiece), 9 and 10). Older basement rocks of the Silurian, Devonian and Jurassic are not exposed at the surface and can only be seen in borehole cores. Faulting from the basement represented by the Purfleet and Greenwich structures can be seen at the surface in Itineraries 5 and 6. Episodic fracturing of the Chalk represented by different styles of fracture is seen in the Riddlesdown Quarry (Itin-erary 9). Chalk can also be seen in the remnants of former quarries in the Colne Valley (Itinerary 1). Two underground itineraries have been included as these are important sites, particularly for Chalk researchers. Chislehurst Caves (Itinerary 7) are open to the public but the Pinner Chalk Mine (Itinerary 2) requires the as-sistance of cavers. Details of Chalk stratigraphy are shown in Figure 11. The long hiatus/unconformity between the Chalk and the Palaeogene is represented by the basal flints of the Thanet Sand Formation Bull Head Bed, the flints being derived from the eroded Chalk (Itineraries 5 and 7). The overlying Thanet sands are best seen at Chafford Hundred (Itinerary 5). Further west, the Thanet Sand Formation thins and wedges out (Ellison et. al., 2004 fig. 9) but a similar horizon of Bull Head Bed-like flints can be seen at Harefield (Itinerary 1) where the top of the Chalk is intensively bioturbated with burrows descending from the overlying basal Lambeth Group (Upnor Formation).

11

Introduction

Figure 10. Simplified map of the solid geology of the London Region showing the position of the Thames, the M25 and other major roads. Small squares represent 10 km.

12

Introduction

Mt

Us

StratigraphicalrangeofChalkatsubcropbeneathLondon

MargateChalkatoutcropinNorthDowns

Portsdown Marl

Castle Hill Marls

Brighton MarlBuckle MarlsWhitaker’s Three InchFlintBedwell’s Columnar Flint(Michel Dean Flint)

Seven Sisters Flint BandBelle Tout MarlsShoreham Marl 2

Meeching Marls

Peacehaven MarlOld Nore Marl

Key boundary markers

New Southern Province Chalk Stratigraphy(Mortimore, 1983, 1986; Bristow ., 1997)et al

Traditional Chalk Stratigraphy

400

300

200

100

0

TraditionalUnits

Campanian

Mytiloides

labiatuss.l.Te

rebratulina

lata

Sternotaxis

planus

Micraster

cortestudinarium

Micraster

coranguinum

Offaster

pilula

Gonioteuthis

quadrata

Belemnitella

mucronata

Numerous ammonite

zones

Cenomanian

Turonian

Coniacian

Santonian

Approximatethickness (metres)

UpperChalk(redefined)

MiddleChalk

(redefined)

LowerChalk

GreyChalk

Subgroup

WhiteChalkSubgroup

LewesNodularChalk

SeafordChalk

NewhavenChalkCulver Chalk

Key marker beds

Barrois’Sponge BedBedwell’sColumnar Flint

Seven SistersFlint Band

Top Rock

Chalk Rock

Melbourn RockPlenus Marls

Grey Chalk

Chalk Marl

Glauconitic Marl Glauconitic MarlBase of the Chalk at baseGlauconitic Marl

Plenus Marls MemberMelbourn Rock

Tenuis Limestone

Sub-Plenus erosionsurface

Foyle Marl

Gun Gardens Main MarlMalling Street MarlsNew Pit MarlsGlynde MarlsSoutherham MarlCaburn MarlBridgewick MarlsLewes MarlsLewes Nodular chalksNavigation MarlsCliffe HardgroundHope Gap HardgroundBeeding HardgroundsLight Point Hardgrounds

Formations membersand bed divisions

Portsdown ChalkFormation

Spetisbury ChalkMember

Tarrant ChalkMemberBastion Steps

BedsMeeching BedsPeacehaven BedsOld Nore Beds

Splash Point Beds

Haven Brow Beds

Cuckmere Beds

Belle Tout BedsShoreham BedsBeachy Head BedsLight Point BedsBeeding BedsHope Gap BedsCliffe BedsNavigation BedsSouth Street BedsKingston BedsRingmer BedsCaburn BedsGlynde Beds

New Pit ChalkFormation

Holywell NodularChalk Formation

Zig ZagChalk Formation

West Melbury MarlyChalk Formation

ZoneStage

Figure 11. Chalk stratigraphy of the London area. (R.N. Mortimore)

13

Introduction

The Lambeth Group is a diverse sequence of sediments (Fig. 12; Sum-bler, 1996 fig. 27; Ellison et al., 2004 fig. 22). The basal Upnor Formation at Harefield is represented by a thin pebble bed, but in a thicker sequence at Pinner Chalk Mines (Itinerary 2), there is an in situ layer of silica-cemented pebble bed in the upper part known as Hertfordshire Puddingstone. It is probable that the cementation took place during a phase of global warming close to the onset of Reading Formation deposition. The overlying Woolwich and Reading Forma-tions are extremely variable and cause ongoing problems for the many engineer-ing projects in and around the capital. The SSSI at Gilbert’s Pit, Charlton (Itiner-ary 6) is the classic place for viewing the Woolwich Formation. The Harwich Formation comprises a number of contrasting lithologies (Fig. 12; King, 1981 text-fig. 5; Sumbler, 1996 fig. 28; Ellison et al., 2004 fig. 23), including clay with pronounced ash bands at Harwich, loams in the Tilehurst Member at Harefield (Itinerary 1), shell beds at Abbey Wood (Itinerary 6) and very distinctive round, black pebbles at Blackheath, best exposed at Gilbert’s Pit, overlying the Woolwich Formation (Itinerary 6). In several locations in southeast London these pebbles have been cemented with calcite, as at Elmstead (Itinerary 7), Dog Rocks, Plumstead (Itinerary 6) and on the top of Croham Hurst (Itinerary 9). Harefield, Abbey Wood, Gilbert’s Pit and Elmstead are all SSSIs. London is the type area for the London Clay but there are now few acces-sible exposures in the capital and temporary excavations provide the only oppor-

Thanet Sand Formation (marine)

Woolwich Formation (estuarine)Reading Formation (terrestrial)

Reading Formation (terrestrial)

Upnor Formation (marine)

Harwich Formation (marine)Tilehurst, Blackheath, Oldhaven & Swanscombe Members (King, 1981)

London Clay Formation (marine)5 Divisions A-E (King, 1981)Claygate Member at the top (Division E3)Walton Member at the base (Division A2)

Bagshot Formation (marine)

Formation

3

1, 3, 4,10

1, 6, 7, 9

1, 2, 6,(9)

5, (6),7, 9

EOCE

NEPA

LEO

CENE

Ypre

sian

Than

etia

n

Tham

es G

roup

Lam

beth

Gro

up

Period Stage Itinerariesseen

Figure 12. Chronology of the Tertiary deposits of the London area.

14

Introduction

tunities to examine the fresh ‘blue clay’. King (1981; 2006 fig. 16.12) recognised 5 coarsening-upward cycles A-E (see Itinerary 10, Fig. 4), which can be cor-related with better developed sequences in the Hampshire Basin. The youngest subdivision E3, becomes sandier towards the top due to shallowing of the London Clay sea and is known as the Claygate Member. Although the London Clay is largely obscured at outcrop, clues to the contact with the Claygate Member and the overlying contact with the Bagshot Formation can be inferred by looking at the geomorphology of Hampstead Heath (Itinerary 3) and Trent Park (Itiner-ary 4). Exposures of London Clay are described from the foreshore of the River Thames at Isleworth, Kew and Hammersmith where they are visible at very low tide, usually characterised by septarian nodules weathering out (Itinerary 10). The youngest Palaeogene sediments in the area comprise the shallow marine Bagshot Formation which caps the hills of Hampstead Heath (Itinerary 3), Harrow-on-the-Hill and the high ground around Epping Forest in the northeast. The Bagshot Formation also crops out around Esher and Weybridge to the south-east but the type area of Bagshot in Surrey is outside the area covered. A detailed chronology of the Tertiary deposits in the London area is shown in Figure 12. Quaternary sediments are included in several of the itineraries. Pre-An-glian fluvial deposits are featured in the Colne Valley (Itinerary 1) and the An-glian ice sheet is discussed in the geological walk around Trent Park (Itinerary 4) although exposures may more easily be observed in freshly dug graves of the Finchley cemeteries. The SSSI for the glacial till at Hornchurch is not included in the Guide as it is on a railway cutting and access is severely restricted, however, in the summer of 2010 this section was conserved and occasional access may be possible via Natural England (see below). The post-Anglian terrace gravels in east London are described in Itinerary 5 where periglacial features can also be seen. Two itineraries in the Guide discuss rocks that are not in situ. Itinerary 8, ‘Geological Illustrations in Crystal Palace Park’, features imported rocks of Car-boniferous age to illustrate the geological strata of the ‘Primary age’, and rocks of the ‘Secondary age’ to provide a backdrop to Waterhouse Hawkins’ magnificent giant reptile sculptures. In Itinerary 7 the artificial rock loved by Victorians, Pul-hamite can be seen at Sundridge Park Manor.

Websites (2014)

GLA. 2012. London’s Foundations: www.londongeopartnership.org.uk/publications.htmlNatural England (for access to the Hornchurch cutting SSSI): www.naturalengland.org.uk/about_us/contact_us/default.aspx

15

Introduction

References Allen, P. 1975. Wealden of the Weald. Proceedings of the Geologists’ Associa- tion, 86, 389–437.Allen, P. 1981. Pursuit of Wealden Models. Journal of the Geological Society, London, 138, 375–405.Bridgland, D.R. 1994. Quaternary of the Thames. Geological Conservation Review Series 7. Chapman and Hall, London.Bridgland, D.R., Allen, P. & Haggart, B.A. (eds) 1995. The Quaternary of the lower reaches of the Thames. Field Guide. Quaternary Research Association.De Freitas, M. & Royse, K. 2009. London Basin Forum. Geoscientist, 19 (No. 10 October), 20–21.Ellison, R.A., Woods, M.A., Allen, D.J., Forster, A., Pharoah, T.C. & King, C. 2004. Geology of London. Memoir for sheets 256 (N London), 257 (Romford), 270 (S London), 271 (Dartford). British Geological Survey, Keyworth.GLA. 2012. London’s Foundations: www.londongeopartnership.org.uk/publications.htmlKing, C. 1981. The stratigraphy of the London Clay and associated deposits. Tertiary Research Special Paper, 6, 1–158.King, C. 2006. Paleogene & Neogene: uplift and a cooling climate. In: Brench- ley, P.J. & Rawson, P.F. (eds) The Geology of England and Wales, 2nd edition. Geological Society of London. Mortimore, R.N. (2011) A Chalk revolution: what have we done to the Chalk of England? Proceedings of the Geologists Association, 122, 232–297.Mortimore, R.N., Newman, T., Royse, K., Scholes, H. & Lawrence, U. (2011). Chalk: its stratigraphy, structure and engineering geology in east Lon- don and the Thames Gateway. Quarterly Journal of Engineering Geology and Hydrogeology, 44, 419–444.Schreve, D.C. 2001. Differentiation of the British late Middle Pleistocene inter- glacials: the evidence from mammalian biostratigraphy. Quaternary Science Reviews, 20, 1693–1705.Schreve, D.C. 2004. The Quaternary Mammals of Southern and Eastern England. Field Guide. London. Quaternary Research Association.Sumbler, M.G. 1996. British regional geology: London and the Thames Valley. Fourth edition. London HMSO for the British Geological Survey.

MapsBritish Geological Survey, 1979. Ten Mile map South sheet. Solid Geology 1:625 000 scale. British Geological Survey, Keyworth, Nottingham.British Geological Survey, 2007. Bedrock Geology UK South. 1:625 000 scale. British Geological Survey, Keyworth, Nottingham.British Geological Survey, 1989. Thames Estuary. England and Wales Sheet 51N 00. Solid Geology. 1:250 000 scale. British Geological Survey, Keyworth, Nottingham.

16

Introduction

British Geological Survey, 1991. Chilterns. England and Wales Sheet 51N 02W. Solid Geology.1:250 000 scale. British Geological Survey, Keyworth, Notting- ham.British Geological Survey, 1996. Romford. England and Wales Sheet 257. Solid and drift.1:50 000 scale. British Geological Survey, Keyworth, Nottingham.British Geological Survey, 1998. Dartford. England and Wales Sheet 271. Solid and drift.1:50 000 scale. British Geological Survey, Keyworth, Nottingham.British Geological Survey, 1998. South London. England and Wales Sheet 270. Solid and drift.1:50 000 scale. British Geological Survey, Keyworth, Notting- ham.British Geological Survey, 1999. Windsor. England and Wales Sheet 269. Solid and drift.1:50 000 scale. British Geological Survey, Keyworth, Nottingham.British Geological Survey, 2005. Beaconsfield. England and Wales Sheet 255. Solid and drift.1:50 000 scale. British Geological Survey, Keyworth, Notting- ham.British Geological Survey, 2006. North London. England and Wales Sheet 256. Solid and drift.1:50 000 scale. British Geological Survey, Keyworth, Notting- ham.

Glossary Note: For stratigraphic units see text above.

Anglian glaciations: the most extensive of the Quaternary ice sheets that came as far south as north London (MIS 12). Anticline: fold where the rocks have been pushed up (convex upward). Younger rocks have then usually been eroded from the top leaving an inlier of older rocks exposed at the surface. Biostratigraphy: subdivision and correlation of sediments based on occurrence of their enclosed fossils. Bioturbated: the churning of sediment by burrowing animals, often destroying sedimentary structures. Calcite: a mineral form of calcium carbonate CaCO3 that is more stable than aragonite. Shells and tests of many marine organisms are formed of calcite. Fluvial: relating to a river or river channel Loam: mixture of sand, silt and clay in roughly equal proportions. Inlier: outcrop of older rocks completely surrounded by younger rocks.Mammal Assemblage-Zone (MAZ): one of a series of biostratigraphical inter-vals defined by the distinct fauna of mammals that occur within it. Marine (Oxygen) Isotope Stage (MIS): a system used for correlation which is dependent on the analysis of heavy and light oxygen isotopes. A higher propor-tion of heavy oxygen isotopes recovered from the shells of certain marine organ-isms is generally associated with cold temperatures because the lighter isotope is preferentially incorporated into ice and is thus less abundant in contemporary sea

17

Introduction

water during cold periods. Periglacial: strictly an area of frozen ground in front of a glacier, but often used to describe the residual effects of permafrost. Septarian nodule: a form of concretion (‘growing together’) that displays septa, or divisions caused by cracking. The spaces are often filled with calcite. Common in the London Clay, where they are also known as ‘cement stones’ owing to the suitability of their composition for that purpose. SSSI: Site of Special Scientific Interest. Designated sites have legal protection. Till: is a dumped mix of the products of ice erosion by glaciers, showing poor sorting and no stratification. The predominant rock type is clay but it also con-tains sand and chalk in the London area as well as a variety of more exotic clasts.

18

Introduction