Geologists’ Association Guide No. 68 The Geology of London Introduction T H D E N CURRY FU Compiled by Diana Clements Hampstead Heath 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. The Geologists’ Association, founded in 1858, exists to foster the progress and diffusion of the science of Geology. It holds lecture meetings in London and, via Local Groups, throughout England and Wales. It conducts field meetings and publishes Proceedings, the GA Magazine, Field Guides and Circulars regularly. For further information apply to the Executive Secretary, Geologists’ Association, Burlington House, Piccadilly, London W1J 0DU: phone: 020 74349298, e-mail: [email protected], www.geologistsassociation.org.uk The Curry Fund of the Geologists’ Association provides support for geological publications and geological conservation projects, also contingency funding for other geologically based initiatives, either from groups or individuals. Rockwatch is the national geology club for children and is the junior arm of the Geologists’ Association. It publishes a magazine three times a year, organizes field trips around the country and runs ‘hands-on’ family days in museums and uni- versities. To find out more or to join Rockwatch: phone: 020 77345398, e-mail: [email protected], www.rockwatch.org.uk Registered Charity number 233199 Copyright: The Geologists’ Association encourages the use of its materials in promoting the geological sciences and gives non-exclusive permission to repro- duce free of charge for non-commercial purposes any figure or quotation from the text (not to exceed 250 words in length), subject to appropriate acknowledgement and compliance with any other stated terms and conditions of use related to the content of this guide. Full details relating to copyright can be obtained from the Geologists’ Association. This pdf is for personal use only and is not to be circu- lated or made available on a website. 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. Any information that would update and improve a revised edition of this guide would be welcomed by the Association Typeset by: The Geologists’ Association Printed 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 South BGS 1:250 000 sheets: Chilterns and Thames Estuary BGS 1:50 000 sheets: 255, 256, 257, 269, 270, 271 BGS: 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 Introduction 2 Norwich and other Crags Bury Bagshot Formation St Edmunds Thames Group (London Clay and Harwich Formations) Great Thanet Sand Formation and LambethGroup Melbourn Chesterford Sudbury ChalkGroup Arlesey Royston Key Chalkcored borehole(CBH) Totternhoe Key Chalk field exposureor quarry Layer-de-la-HayeLayer-de-la-HayeCBHCBH Localities Pitstone Kensworth Chinnor Brentwood 2 Hills n Syncline 00 Stokenchurch Basi London Fognam Chiltern s of Ogbourne Taplow Axi Lakeside Reading Rotherhithe Cliffe Marshes CBH Maisey Windsor Windsor Chislehurst Marlborough Croydon Swanscombe TanHill Newbury Fetcham Nort Faircross CBHBagshot Mill CBH h D Newbury Riddlesdown owns Faircross CBH Fetcham Mill Leatherhead CBH Warminster Guildford Farnham 4 0 40 km 6 00 500 00 Figure 2. Geological map of the London Basin region (based upon BGS 1: 625 000 Ten Mile Map, South Sheet, 1979 with the permission of the British Geological Survey.) (R.N. Mortimore) Introduction Quaternary ice ages and warm periods Era System Series Ma formation of the Thames valley Quat. Norwich and Red Crags Pliocene CorallineCrag 5 10 Miocene 15 NEOGENE 20 40 million year Phases of ‘Alpine’ uplift, folding hiatus and erosion of Cenozoic sediments 25 culminating in major Miocene tectonics 30 Oligocene 35 40 45 CENOZOIC Eocene Camberley Sand LAEOGENE PA &Windlesham Fms 50 Bagshot Fm Claygate Member 55 London Clay Fm Group Harwich Fm Reading Fm Thames Lambeth Lambeth Gp Woolwich Fm Group 60 UpnorFm Transgression of theThanet sea Thanet Sand onto the eroded Chalk of the Formation Paleocene London Platform 65 n 70 Phases of Subhercynian and Laramide Maastrichtia 25 million year uplift, folding and erosion of the Chalk hiatus 75 80 Newhaven Ck Fm Campanian / MargateCk Mbr CRETACEOUS 85 Santonian TE Coniacian SeafordCkFm Chalk LA Lewes Nodular Ck Fm 90 Turonian NewPit Ck Fm Holywell Nodular Ck Fm Group Cenoman. Zig Zag Ck Fm Transgression of theGault andChalk sea 95 West Melbury MarlyCkFm onto the London Platform E.Cret Albian Gault Formation Jur.&Palaeozoic Sub-Cretaceous rocks London Platform basement rocks of variable age from Palaeozoic to Jurassic Figure 3. Late Cretaceous and Cenozoic succession and major events in the London Basin. (R.N. Mortimore) 3 Introduction 4 Figure 4. Cross-section of the London Basin showing how the older outcrops can be seen on either side of the Thames Valley. The syncline is superimposed on the older anticline of Palaeozoic rocks. CP14/013 British Geological Survey © NERC. All rights reserved. Introduction M1 M25 A1 Lea Roding 1 Tottenham R. Harrow R. M1 Romford Plaistow Greenwich Formations in the core of anticlines Graben Anticline Thanet Sands Fm GreenwichFault Seaford ChalkFm 1 Pureet Anticline M4 Westminster Brixton Twickenham Greenwich Streatham Thames Fault Dartford M3 R. M20 Fault s Croydon ault Wimbledon 5 F M2 M26 M25 M23 Sevenoaks Leatherhead 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) 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