London Clay

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London Clay Formation
Stratigraphic range: Ypresian
Cliffs below Reculver Country Park - geograph.org.uk - 6927.jpg
London Clay at Reculver
Type Formation
Unit of Thames Group
Underlies Bagshot Formation (London Basin), Wittering Formation and Poole Formation (Hampshire Basin and English Channel)
Overlies Harwich Formation
Thickness up to 150 m
Location
Region southern England
Geological map of London Basin.jpg
Geological map of the London Basin; the London Clay is marked in dark brown.
Eroding London Clay cliffs at The Naze in Essex.

The London Clay Formation 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 is a stiff bluish clay which becomes brown when weathered. There are nodular lumps of pyrite and crystals of selenite (sometimes called "waterstones") frequently in the clay, and large septarian concretions are also common. These have been used in the past for making cement. They were once dug for this purpose at Sheppey, near Sittingbourne, and at Harwich, and also dredged off the Hampshire coast. The clay is still used commercially for making bricks, tiles, and coarse pottery in places such as Michelmersh in Hampshire.[1] It is infertile for gardens and crops.

Distribution and geology

The London Clay is well developed in the London Basin, where it thins westwards from around 150 metres (492 feet) in Essex and north Kent to around 4.6 metres (15 feet) in Wiltshire.[2] 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 (82 feet). 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 (299 ft) thick occurs at Whitecliff Bay on the Isle of Wight and around 101 metres (331 ft) is spread along 6 kilometres (4 miles) of foreshore at Bognor Regis, West Sussex.[3]

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.[2]

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.[2] 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.[4]

Fossil flora and fauna

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.

Plant fossils, especially seeds and fruits, are found in abundance and have been collected from the London Clay for almost 300 years.[5] Some 350 named species of plant have been found, making the London Clay flora one of the world's most diverse for fossil seeds and fruits.[6] The flora includes plant types found today in tropical forests of Asia and demonstrates the much warmer climate of the Eocene epoch, with plants such as Nypa (Nipah palms) and other palms being frequently encountered. The following plants list is incomplete and is based on the research by Marjorie Chandler.[5]

Plants

Pteridophytes

Gymnosperms

  • †Araucarites spp.
  • Pinus spp.
  • ?Taxaceae indet.
  • Cephalotaxus bowerbanki
  • †Cupressinites spp.
  • Podocarpus argillaelondinensis?

Angiosperms

  • †Caryotispermum cantiense
  • †Palmospermum spp.
  • Nypa burtini (syn. Nipa burtini)
  • Alnus richardsoni (syn. Petrophiloides richardsoni) - an alder
  • †Juglandicarya spp. (includes fruits comparable to Carya, Juglans and other members of the Juglandaceae)
  • †Crowella globosa
  • †Laurocalyx spp.
  • †Laurocarpum spp.
  • †Protoaltingia europaea
  • †Protoravensara sheppeyensis
  • ?Morus sp.
  • †Anonaspermum spp.
  • †Bowerbankella tiliacoroidea
  • †Davisicarpum gibbosum
  • †Eohypserpa parsonsi
  • †Microtinomiscium foveolatum
  • †Tinomiscoidea scaphiformis
  • Tinospora excavata
  • †Wardenia davisi
  • †Protobarclaya eocenica
  • Erythropalum spp.
  • †Plataninium decipiens[7]
  • †Urticicarpum scutellum

Animals

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.

The following fauna species list follows Clouter (2007).[8]

Vertebrates

Mammals
Birds
Reptiles

Crocodylians

Snakes

Turtles and tortoises

Turtle fossils from Sheppey
Bony fish
Cartilaginous fish

Crustaceans

Exuvia of Hoploparia

Molluscs

Cephalopods
Bivalves
Gastropods
Tusk shells

Echinoderms

Ophiura wetherelli from the London Clay of Bognor Regis

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, although this is also the reason why London has no true skyscraper buildings, at least to the same degree as many other cities throughout the world. Erecting tall buildings in London requires very deep, large and costly piled foundations.

London Clay is highly susceptible to volumetric changes depending upon its moisture content.[11] During exceptionally dry periods or where the moisture is extracted by tree root activity, the clay can become desiccated and shrink in volume, and conversely swell again when the moisture content is restored. This can lead to many problems near the ground surface, including structural movement and fracturing of buildings, fractured sewers and service pipes/ducts and uneven and damaged road surfaces and pavings. Such damage is recognised to be covered by the interpretation of subsidence in buildings insurance policies, and the periods of dry weather in 1976/77 and 1988/92, in particular, led to a host of insurance claims. As a result, many insurance companies have now increased the cost of premiums for buildings located in the most susceptible areas where damage occurred, where the clay is close to the surface.

Tunnels in London Clay

The London Clay is an ideal medium for driving tunnels, which is why the London Tube railway network expanded quickly north of the Thames, but south of the Thames the stratum at tube level is water-bearing sand and gravel (not good for tunnelling) with London Clay below, which is why there are few tube tunnels there. London Clay has a stand-up time long enough to enable support to be installed without urgency. It is also almost waterproof, resulting in virtually no seepage of groundwater into the tunnel. It is over-consolidated, which means that it is under pressure, and expands upon excavation, thus gradually loading the support, i.e. it is not necessary to stress the support against the ground.

Agriculture

Due to its impermeability especially when exposed by ploughing, London clay does not make good agricultural soil. In Middlesex it has historically been called "ploughing up poison."[12]

See also

Notes

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

References

  1. ↑ Lua error in package.lua at line 80: module 'strict' not found.
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  5. ↑ 5.0 5.1 Chandler, M.E.J. 1961. The lower Tertiary floras of southern England I. Palaeocene floras, London Clay flora. London: British Museum (Natural History).
  6. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  7. ↑ Poole, I., K.L. Davies and H.P. Wilkinson 2002. A review of the platanaceous woods from the Eocene paratropical rainforest of south-east England. Botanical Journal of the Linnean Society, 139: 181–191.
  8. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  9. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  10. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  11. ↑ Buildings on Clay The effects of geology, climate and vegetation on heave and settlement, Derek Clarke and Joel Smethurst
  12. ↑ View of the Agriculture of Middlesex: With Observations on the Means of Its Improvement, and Several Essays on Agriculture in General. By Board of Agriculture (Great Britain), John Middleton. Published by G. and W. Nicol, second edition, 1807. Page 20.
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