- Geology of London
The geology of London comprises various differing layers of sedimentary rock upon which
London ,England is built.Oldest rocks
The deepest layer below
London is that of the old, hard rocks of thePaleozoic . These consist ofSilurian mudstones and sandstones ["Chilterns" 1:250,000 map sheet, British Geological Survey (1991)] , generally overlain byDevonian strata which are largely ofOld Red Sandstone . The Devonian rocks are absent in parts of South London [Sumbler, M.G., "London and the Thames Valley", British Geological Survey (1996)] . The Paleozoic rocks dip southwards and are more than 1,000 metres below theEnglish Channel . Above this is a 60 metre thick layer of impermeableGault clay s. These clays are relatively young, only going back to the earlyCretaceous which began around 144 million years ago.On top of these clays is a non-contiguous layer of Upper
Greensand above which lies a rolling bed of whitechalk about 200 metres thick. In the Lower Chalk and in the lower region of the Middle Chalk there are abundantfossil ized shell fragments, especially "Inoceramus " clams. In places these form the greater part of the rock but they decrease in amount upwards in the succession.Flint s are abundant in the Upper Chalk.These bands of chalk form the basis of the
London Basin , a v-shapedsyncline bounded by chalk outcrops forming hills: theNorth Downs to the south of London, and theChilterns to the north. The chalk is a soft white limestone that is different in appearance to otherlimestones because it is porous and earthy whilst others are compact and crystalline.Tertiary period
The chalk basin has been infilled with a sequence of
clays andsand s of the more recentTertiary Period (1.6 to 66.4 million years old). Most significant is the stiff, grey-blueLondon Clay , a marine deposit which is well known for thefossil s it contains and can be over 150 metres thick beneath the city. This supports most of the deep foundations and tunnels that exist under London.Also in this area are the Reading/Woolwich sand beds and Thanet sands. Most of these
sand andclays were deposited 60-50 million years ago during theEocene . SouthernEngland at this time was covered by a warm tropical sea: this is shown by the fossil evidence. The sands contain animals that lived in both estuaries and freshwater. Some species burrowed into the underlying chalk. [ [http://www.storyoflondon.com/modules.php?op=modload&name=News&file=article&sid=348&mode=thread&order=0&thold=0 Story of London] retrieved 14/1/07]Above this is the
subsoil which consists of deposits ofgravel up to 10 metres deep. This was deposited during the lastice-age ½ million years ago when theRiver Thames was diverted to its present position. While establishing its new path, the river eroded its valley, creating a series of sand and gravel terraces. These terraces are named after the area they are best known in, for example:Dartford Heath Gravel,Swanscombe ,Orsett Heath, Corbets Tay,Mucking ,West Thurrock , Kempton Park,Shepperton ,Staines andTilbury Gravels. [ [http://www.bbc.co.uk/london/walks/london_story.shtml BBC] retrieved 14/1/07]The sand and gravel terraces are made up of pebbles with flint,
quartz andquartzite . In places, there are deposits of brick-earth, which is a mixture of clay and sand that has supported London's long-standing brick-making industry. Atop these natural layers are the deposits of hundreds of years of human occupation. In the oldest parts theCity of London and theCity of Westminster this layer can be up to 6 metres deep. [ [http://www.glaucus.org.uk/Geo2.htm Shoreham by Sea Geology] retrieved 14/1/07]References
ee also
*
Geology of the United Kingdom
*Geology of England
*Ancestral Thames External links
* [http://www.bgs.ac.uk/programmes/landres/glsb/segf.html British Geological Survey - SE England]
* [http://www.nature.com/nature/journal/v212/n5061/abs/212495a0.html Nature - Bagshot Beds]
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