London Clay

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The London Clay is a marine deposit which is well known for the fossils it contains. It is the most important member of the Lower Eocene Epoch (Ypresian, c.56-49 mya) strata in the south of England, and the only European source of diverse plant fossils from the Lower Eocene. The fossils indicate a moderately warm climate, the flora being tropical or sub-tropical.

The London Clay consists of a stiff, bluish coloured clay which becomes brown when weathered. Nodular lumps of pyrite and crystals of selenite frequently occur within the clay, and large septarian concretions are also common. These have been used in the past for the manufacturing of cement. They were once dug for this purpose at Sheppey, near Sittingbourne, and at Harwich, and also dredged off the Hampshire coast. The clay itself has been used commercially for making bricks, tiles, and coarse pottery.

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[edit] Distribution

The London Clay is well developed in the London Basin, where it thins westwards from around 150 metres (490 ft) in Essex and north Kent to around 4.6 metres (15 ft) in Wiltshire.[1] though it is not frequently exposed as it is to a great extent covered by more recent neogene sediments and Pleistocene gravel deposits. One location of particular interest is Oxshott Heath, where the overlying sand and the London Clay layers are exposed as a sand escarpment, rising approximately 25 metres. This supported a thriving brick industry in the area until the 1960s. The London Clay is also well developed in the Hampshire Basin, where an exposure 91 metres (300 ft) thick occurs at Whitecliff Bay on the Isle of Wight and around 101 metres (330 ft) is spread along 6km of foreshore at Bognor Regis, West Sussex.[2]

[edit] Formation

The clay was deposited in a sea up to 200 metres (660 ft) deep at the eastern end. Up to five cycles of deposition (representing transgression followed by shallowing of the sea) have been found, most markedly at the shallower, western end. Each cycle begins with coarser material (sometimes including rounded flint pebbles), followed by clay which becomes increasingly sandy. The final cycle ends with the Claygate Beds.[1]

[edit] Claygate Beds

The youngest part of the London Clay, known as the Claygate Beds or Claygate Member forms a transition between the clay and the sandier Bagshot Beds above. This is shown separately on many geological maps, and often caps hills. It is up to 15 metres (49 ft) thick at Claygate, Surrey.[1] It is now believed to diachronous, with the formation at Claygate for example being the same age as the end of the fourth cycle of deposition further east.[3]

[edit] Fossil fauna and flora

Notable coastal exposures from which fossils can be collected are on the Isle of Sheppey in Kent and Walton-on-the-Naze, Essex in the London Basin, and Bognor Regis in the Hampshire Basin.

Animal fossils include bivalves, gastropods, nautilus, worm tubes, brittle stars and starfish, crabs, lobsters, fish (including shark and ray teeth), reptiles (particularly turtles), and rarely birds. A few mammal remains have also been recorded.

Plant fossils, including seeds and fruits, may also be found in abundance. Plant fossils have been collected from the London Clay for almost 300 years. Some 350 named species of plant have been found, making the London Clay flora one of the world's most varied for fossil seeds and fruits. [4]

[edit] Birds

[edit] Engineering

The presence of a thick layer of London Clay underneath London itself, providing a soft yet stable environment for tunnelling, was instrumental in the early development of the London Underground. The high degree of shrinkage and swelling of the clay with varying degrees of wetness can cause subsidence problems.

[edit] See also

[edit] References and further reading

  1. ^ a b c Sumbler M.G. (4th ed 1996), London and the Thames Valley, British Regional Geology series, , British Geological Survey, ISBN 0-11-884522-5
  2. ^ Melville R.V. & Freshney E.C. (4th ed. 1992), The Hampshire Basin and adjoining areas, British Regional Geology series, British Geological Survey, ISBN 0-11-884203-X
  3. ^ Ellison R.A. et al 2004, Geology of London: Special Memoir for 1:50,000 Geological sheets 256 (North London), 257 (Romford), 270 (South London) and 271 (Dartford) (England and Wales), British Geological Survey, Keyworth, ISBN 0-85272478-0
  4. ^ Collinson, M, 1983. Fossil plants of the London Clay, The Palaeontological Association.