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Geology of London
The geology of London comprises various differing layers of sedimentary rock upon which London, England is built.
==Oldest rocks==
The oldest rocks proved through boreholes to exist below London are the old, hard rocks of the Palaeozoic. These consist of Silurian mudstones and sandstones,〔''Chilterns'' 1:250,000 map sheet, British Geological Survey (1991)〕 generally overlain by Devonian strata which are largely of Old 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 Palaeozoic rocks dip southwards and are more than 1,000 metres below the English Channel. Above this is a 60-metre thick layer of impermeable Gault clays. These clays are relatively young, only going back to the early Cretaceous 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 white chalk about 200 metres thick. In the Lower Chalk and in the lower region of the Middle Chalk there are abundant fossilized shell fragments, especially ''Inoceramus'' clams. In places these form the greater part of the rock but they decrease in amount upwards in the succession. Flints are abundant in the Upper Chalk.
These bands of chalk form the basis of the London Basin, a syncline the outcrop of which is v-shaped opening to the east and bounded by chalk outcrops forming hills: the North Downs to the south of London, and the Chilterns to the north. The chalk is a soft white limestone that is different in appearance to other limestones because it is porous and earthy whilst others are compact and crystalline.
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