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London Clay
The London Clay is a marine geological formation of Ypresian (Lower Eocene Epoch, c. 56-49 Ma) age which crops out in the southeast of England. The London Clay is well known for the fossils it contains. The fossils from the Lower Eocene indicate a moderately warm climate, the flora being tropical or subtropical. Though sea levels changed during the deposition of the Clay, the habitat was generally a lush forest - perhaps like in Indonesia or East Africa today - bordering a warm, shallow ocean.

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. It is infertile for gardens and crops.

Distribution

The London Clay is well developed in the London Basin, where it thins westwards from around in Essex and north Kent to around in Wiltshire. 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 thick occurs at Whitecliff Bay on the Isle of Wight and around is spread along 6km of foreshore at Bognor Regis, West Sussex.

Formation

The clay was deposited in a sea up to 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.

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 thick at Claygate, Surrey. It is now believed to be diachronous, with the formation at Claygate for example being the same age as the end of the fourth cycle of deposition further east.

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 a large diversity of birds. A few mammal remains have also been recorded. Preservation varies; articulated skeletons are generally rare. Of fish, isolated teeth are very frequent. Bird bones are not infrequently encountered compared to other lagerstätten, but usually occur as single bones and are often broken.

Plant fossils, including seeds and fruits, may also be found in abundance. The flora demonstrates the much hotter climate of that time, with plants such as Nypa (Nipah palms) being frequently encountered. 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.

Vertebrates

Mammals

Birds

Reptiles

Crocodilians

Snakes

Turtles and tortoises

Bony fish

Cartilaginous fish

Invertebrates

Crustaceans

Molluscs

Cephalopods

Clams and other bivalves

Gastropods

Tusk shells

Echinoderms

Cnidarians

Other invertebrates

Ichnofossils

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.

Agriculture

"London clay is not hospitable to most plants... ploughing it up where it lies so near the surface as to be accessible to the plough is injurious to the surface soil and future crops. In Middlesex it is called 'ploughing up poison'"

See also

Footnotes

References

  • (2007): London Clay Species List Version of 2007-JUN-29. Retrieved 2008-JUN-16.
  • (1983). Fossil plants of the London Clay. The Palaeontological Association.
  • (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
  • (1992): The Hampshire Basin and adjoining areas (4th ed.). British Regional Geology series, British Geological Survey. ISBN 0-11-884203-X
  • (1996): London and the Thames Valley (4th ed.). British Regional Geology series, British Geological Survey. ISBN 0-11-884522-5

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Last updated on Sunday October 05, 2008 at 01:32:30 PDT (GMT -0700)
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