London Clay
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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. It is the only European source of diverse plant fossils from the Lower Eocene. The fossils 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.

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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]

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]

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 be diachronous, with the formation at Claygate for example being the same age as the end of the fourth cycle of deposition further east.[3]

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[4].

Vertebrates[5]

Mammals

Birds

"Reptiles"

Crocodilians

Snakes

Turtles and tortoises

Bony fish

Cartilaginous fish

Invertebrates[5]

Crustaceans

Molluscs

Cephalopods

Clams and other bivalves

Gastropods

Tusk shells

Echinoderms

Cnidarians

Other invertebrates

Ichnofossils[5]

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.

See also

Footnotes

  1. ^ a b c Sumbler (1996)
  2. ^ Melville & Freshney (1992)
  3. ^ Ellison et al. (2004)
  4. ^ Collinson (1983)
  5. ^ a b c Clouter (2007)
  6. ^ Often called Eohippus
  7. ^ Includes "Primobucco" olsoni
  8. ^ Sometimes placed in Aprionodon
  9. ^ Sometimes placed in Physodon
  10. ^ Sometimes placed in Rhinoptera
  11. ^ Sometimes called Hypotodus robustus
  12. ^ Sometimes called Eugomphodus macrotus
  13. ^ Sometimes placed in Acanthius
  14. ^ Sometimes placed in Xendolamia
  15. ^ Sometimes called Necrozius bowerbanki
  16. ^ Sometimes placed in Eutrephoceras
  17. ^ Sometimes called Modiolus depressus
  18. ^ Sometimes placed in Striarca
  19. ^ Sometimes placed in Cyprina
  20. ^ Sometimes called A. rugatus
  21. ^ Sometimes called Pitaria tenuistriata
  22. ^ Sometimes placed in Ledina
  23. ^ Sometimes called Amussium corneum
  24. ^ Sometimes placed in Protocardium
  25. ^ Sometimes placed in Pteria
  26. ^ Sometimes placed in Ostrea
  27. ^ Sometimes considered a variety of Scala undosa
  28. ^ Sometimes called A. sowerbyii
  29. ^ Sometimes placed in Tibia
  30. ^ a b c d Sometimes in Hemipleurotoma
  31. ^ Sometimes placed in Conospirus
  32. ^ Sometimes S. bifaci or S. bifacsi
  33. ^ a b Sometimes included in T. teretrium
  34. ^ Sometimes placed in Galeodea
  35. ^ Sometimes called Newtoniella charlswoverification needed
  36. ^ Sometimes called Solarium pulchrum
  37. ^ Sometimes placed in Natica
  38. ^ Sometimes called Fusinus unicarinatus
  39. ^ Sometimes placed in Bartonia
  40. ^ Sometimes called Euthriofusus complanatus
  41. ^ Sometimes called Euthriofusus transversarius
  42. ^ Sometimes placed in Ficus
  43. ^ Sometimes called Murex argillaceus
  44. ^ Sometimes called T. tenuiplicaverification needed
  45. ^ Sometimes placed in Aurinia
  46. ^ Sometimes placed in Calyptraea
  47. ^ Sometimes placed in Adeorbis
  48. ^ Sometimes called Onutusus extensa
  49. ^ Sometimes placed in Asteropecten
  50. ^ Sometimes placed in Rhizochrinus

References

  • Clouter, Fred (2007): London Clay Species List. Version of 2007-JUN-29. Retrieved 2008-JUN-16.
  • Collinson, M. (1983). Fossil plants of the London Clay. The Palaeontological Association.
  • 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
  • Melville, R.V. & Freshney, E.C. (1992): The Hampshire Basin and adjoining areas (4th ed.). British Regional Geology series, British Geological Survey. ISBN 0-11-884203-X
  • Sumbler, M.G. (1996): London and the Thames Valley (4th ed.). British Regional Geology series, British Geological Survey. ISBN 0-11-884522-5
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