Geology of London

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Geological map of the London Basin

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

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

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

limestones
because it is porous and earthy whilst others are compact and crystalline.

Tertiary period

An exposed section of the Lambeth Group, in Woolwich. Dark grey clays and light grey shelly clays overlay the yellow-brown weathering sands of the Upnor Formation.

The chalk basin has been infilled with a sequence of

Paleogene Period, then Neogene Period (1.6 to 66.4 million years old). Most significant is the stiff, grey-blue London Clay, a marine deposit which is well known for the fossils
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

clays were deposited 60-50 million years ago during the Eocene. Southern England 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.[3]

Quaternary Period

Above this is the

Staines and Tilbury Gravels.[4]

The sand and gravel terraces are made up of pebbles with flint, quartz and quartzite. In places, there are deposits of brickearth, which is a mixture of clay and sand that has supported London's long-standing brick-making industry. On top of these natural layers are the deposits of hundreds of years of human occupation. In the oldest parts the City of London and the City of Westminster this layer can be up to 6 metres deep.[5]

See also

References

  1. ^ Chilterns 1:250,000 map sheet, British Geological Survey (1991)
  2. ^ Sumbler, M.G., London and the Thames Valley, British Geological Survey (1996)
  3. ^ Story of London Archived 2006-11-28 at the Wayback Machine retrieved 14/1/07
  4. ^ BBC retrieved 14/1/07 Archived April 6, 2005, at the Wayback Machine
  5. ^ Shoreham by Sea Geology retrieved 14/1/07

External links

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