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The Geology of London

With the Olympics upon us it is an appropriate time to reveal the geology of London for our visitors. A brief introduction to the geology is followed by five itineraries that have been selected to give an overview of what can still be seen, despite a population of nearly eight million.
This article appeared in Vol. 9, No. 3 - 2012

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Map of the London area showing locations of the itineraries selected. Source: M. Ridd, adapted with permission of the Geologists’ Association from GA Guide 68 2010 Geological strata in Greater London (not to scale). Source: Antony Slater
London sits within the London Basin formed as a distant downfold from the collision of Africa with Europe. The basin is bound by the 85 million-year old chalk hills of the North Downs in the south-east and the Chiltern Hills to the north-west. Inwards from these hills early Tertiary sediments of Thanet Sands and the variable Lambeth Group and Harwich Formation are exposed at the surface. Not surprisingly, it is the London Clay that is the bedrock under most of London. This is topped by the Bagshot Sands, deposited about 50 Ma.

A long interval of over 45 Ma followed, during which folding took place and most of the Bagshot Sands and Claygate Beds were removed, leaving isolated hillocks today. Some are capped by ancient river gravels relating to the former course of the Thames and its tributaries, many of which are thought to have crossed the area from the Weald to the south to join the Thames on its earlier course further north.

The spreading of the Anglian ice age from the north just 450,000 years ago changed everything: the ice reached as far south as Finchley and Hornchurch in North London and, as the ice melted, large pro-glacial lakes formed. When these burst they altered the route of the Thames, pushing it south to its present position through what was to become the centre of London. Vast amounts of gravel were caught in the melting waters’ bed load, which was spread over a wide area. Subsequent ice ages never again reached as far as London but each event provided fresh gravel deposits so that a series of terraces now remains either side of the Thames. Finally, in many areas, windblown loess was redeposited extensively over the gravels and has been a source of excellent brickearth.

All of the sedimentary rocks exposed have been extensively used for the building of London. During the 19th and early 20th centuries Greater London was peppered with quarries, and geologists were able to examine good exposures. Now it is only the Thames gravels in East London and up the Colne Valley that are still exploited. However, with the wealth of data collected during recent London-wide engineering projects, new discoveries, particularly about the structure, are still being made.

Where can the geology be seen? Here we give you a selection of interesting locations and geological walks in Greater London. You can find more in The Geology of London and London’s Foundations, which both give a general introduction on the geology with maps and details on these and other sites. Grid references are given in the text to help pinpoint the locations.

Springfield Park, Hackney - 1

Springfield Park (grid ref: TQ345873), on the west bank of the River Lea, is the closest location to the main Olympic Stadium. Its name comes from the numerous springs that arise along the junction between the Hackney Gravel and the underlying London Clay. Brickearth overlies the gravel at the top of the park, which was designated a Geological Nature Reserve in 1997. Interpretation boards in the park and leaflets available at the Visitor Centre explain the geology, although there are not usually any actual exposures. The River Lea has eroded a substantial valley since the big Anglian glaciation. The ice was very close to the park and, as it melted, would have formed deposits characteristic of the braided channels seen during glacial melts around the globe today. By the time the Olympic Park is reached, about 4 km to the south, the Lea has eroded both the London Clay and the Harwich Formation and excavations for the new Stratford station revealed excellent temporary exposures of the Lambeth Group. The gravels deposited within the valley have been extensively quarried in the past and the flooded holes now serve as boating lakes and reservoirs.

Green Chain Walk Geotrail - 2

Dog Rocks, Plumstead Common, formed of cemented Blackheath Beds pebbles. The blocks are in the shrubbery opposite the entrance to the playground. Source: Diana Clements The Green Chain Walk Geotrail runs from the Thames Barrier (grid ref: TQ415793) in south-east London to Abbey Woods Station (grid ref TQ473790), a distance of 11 km, which includes 12 geologically interesting stopping points. Dog Rocks in Plumstead, and two Sites of Special Scientific Interest (SSSI) at Gilbert’s Pit, Charlton and the fossil bed in Lesnes Abbey Woods are described below. The Geotrail passes right through the Olympic venue of the Royal Artillery Barracks on Woolwich Common and a diversion will be in place at this point during the Games.

Gilbert’s Pit was formerly one of several quarries in the area primarily exploiting the Thanet Sand, which was used as foundry sand at Woolwich Arsenal and for making bottles. The strata still exposed at the top of the quarry are the reason for the SSSI status. Here the sediments that make up the Lambeth Group and Harwich Formation are a valuable window into some of the most variable rocks that underlie London and the exposure is much visited by engineers. It is the type section of the estuarine Woolwich Formation. Beneath, a thin but important bed of mottled clay represents a period of high temperature during deposition of the terrestrial Reading Formation. This is underlain by marine Upnor Formation, mostly obscured by scree. At the top of the face the rounded black pebbles of the Blackheath Beds can be seen. These were laid down in marine conditions and are the local representative of the laterally variable Harwich Formation. Pebbles and occasional shells have rolled down the scree slope and can be picked up at the bottom. The Thanet Sand is now entirely covered by scree. Two faces can be examined from the bottom of the quarry. The main one is on
the eastern face, where there is also an explanation board, and the other on the south-east face, but both are currently surrounded by a fence. Access can be obtained via Greenwich Council (tel: 0208 856 0100).

The Dog Rocks (grid ref: TQ443779) on Plumstead Common allow close examination of the Blackheath Beds. Here the black pebbles have been locally cemented and the large lumps that can be seen were probably dumped by the quarrymen from within the quarry that now forms part of Plumstead Common and houses the Adventure Playground.

The fossil bed at Abbey Wood shows a third variation of the Blackheath Beds. Marine shells were noticed in sand in rabbit holes and with further investigation rare bones and teeth of mammals were found. The site is now opened up annually and the sand is wet-sieved on site, the residue being removed for further examination. All the mammal remains are housed in the Natural History Museum and are providing a valuable insight into their evolution. It is for the mammals that the site is an SSSI, but there are plenty of marine and estuarine shells as well as sharks’ teeth that come from the shell bed and some are abandoned when the dig is back-filled each year. It is not hard to find examples on the surface of the fossil enclosure.

For further details see:

www.greenchain.com/pdfs/Geologytrail.pdf

The Chalk Walk: Chafford Gorges Geotrail - 3

Dissolution hollow with Thanet sand piped down into Seaford Chalk, which can be seen from the ramp up the cliff face at Chafford Hundred. Bullhead Bed flints line the hollow. Source: Diana Clements This walk starts and finishes at the Visitor Centre, Chafford Hundred (grid ref: TQ599793), East London, where a leaflet describing the Geotrail is available. Upfolding of the chalk in north-east London brings it to the surface in the Purfleet area close to the Dartford Bridge where it was exploited in large quarries until 1976. Now one of the quarries houses the Lakeside Shopping Centre while those to the east have become home to the new town of Chafford Hundred. The planners have created an interesting area of ‘gorges’ mostly left in their natural state with the housing around the top. Vegetation and lakes fill the centre but the white cliffs with their prominent flint bands and the contact with the overlying Thanet Sand are still well-displayed. Periglacial features can be seen in places and there is a locked quarry displaying an unstable cliff of Thanet Sand. The area contains an SSSI for the overlying Thames gravels where a rich Middle Palaeolithic site has been found. Some fine examples of sarsen stones have been left by quarrymen at the top of one of the quarries.

For further details see:

http://www.essexwt.org.uk/visitor_centres__nature_reserves/chafford_gorges

Hampstead Heath between Hampstead and Highgate - 4

Geological interpretation boards on Hampstead Heath. A, Bagshot Sands on Sandy Heath; B, Claygate Beds being used for brick-making at the viaduct; C, Springlines on the Heath above the Boating Lake From the top of Hampstead Heath there are fine views across London to Shooters Hill and Sydenham Hill on the other side of the Thames, with the North Downs beyond. It is an excellent location to understand the overall structure of the London Basin. Views to the north are now largely restricted by trees but in winter the Chilterns can be glimpsed in the distance. At 134m this is the highest point in London and is where the youngest of the solid geology can be seen or, at least, inferred. Bagshot Sand tops the hill and, in the 1860s, it was dug by hand and carted away for the construction of the St. Pancras railway, leaving an interesting terrain of hollows and mounds on Sandy Heath. An interpretation board has been erected in a small enclosure around two old trees which shows the sand pits being worked (grid ref: TQ264869).

The Claygate beds beneath, at the top of the London Clay, were utilised for brick-making. The sandier texture was more suitable than the London Clay itself and former quarries have been noted right round the Heath. An interpretation board with a photograph of the brickfield operating in 1880 is placed beside the Viaduct (grid ref: TQ269865). The scene looks very tranquil, far from the reality when firing was taking place and sulphurous smoke invaded the neighbourhood.

The junctions between the sand, silt and clay are marked by springlines. These can be seen around the Heath, identified by moisture-loving plants and often by gullies running away downhill. A small new pond has been created at one of these springs above the boating pond. An interpretation board placed here demonstrates how the geology dictates the locations of the springs. Ultimately the streamlets give rise to rivers, notably three of the ‘lost rivers’ of London: the Fleet, the Westbourne and the Tyburn, whose courses can still be traced underneath London’s streets.

Riddlesdown Quarry (Croydon area)

Open University Geological Society students examining the chalk from the steps up the face of Riddlesdown Quarry. Source: Diana Clements Riddlesdown Quarry (grid ref: TQ336592) is the best location for seeing chalk exposed in the London area. Quarrying ceased in the 1960s and this very large quarry in the North Downs is now owned by the City of London Corporation, who are conserving it as public open space. Steps up the spoil heap adjacent to one of the faces allow close examination of the chalk. In all, nearly 50m of chalk are displayed in the quarry from the top of the New Pit Chalk Formation in the south-east corner, through the entire Lewes Nodular Chalk Formation up into the base of the Seaford Chalk Formation at the top of the quarry. For engineers this quarry provides essential viewing to realise how variable the chalk under London can be. Access to the quarry is only permitted when accompanied by a ranger (tel: 01372 279083).

About the Author:

Diana Clements has worked part-time in the Department of Palaeontology at the Natural History Museum for over twenty years, first specialising in the geology of London in 1996 when she began research on Islington, culminating in an exhibition in 1997: ‘Beneath our Feet: the Geology of Islington’, aimed primarily at residents and visitors with little geological knowledge. Since then, her researches have extended to Greater London, gathering itineraries for the Geologists’ Association Guide No. 68: The Geology of London, published in 2010. She is an active member of the London Geodiversity Partnership, whose aim is to identify, protect and promote London’s geological heritage. She teaches and gives talks on London geology and leads walks and field trips around the capital.

Further information:

London’s Foundations, 2012, GLA 1–59, pp. 119–249, GLA 43, 14, 8, 1, 42, 26 www.londongeopartnership.org.uk/Publications.html.

The Geology of London, 2010. Geologists’ Association Guide 68.

Geological maps of the UK. The British Geological Survey has a free iPhone app so that the geology can be called up in the field. Also available on the internet: mapapps.bgs.ac.uk/ geologyofbritain/home.html

Grid references can be put into www.streetmap.co.uk, which allows maps to be printed.

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