London Clay


London Clay
London Clay Formation
Stratigraphic range: Ypresian

London Clay at Reculver
Type Formation
Underlies Bagshot Formation (London Basin), Wittering Formation and Poole Formation (Hampshire Basin and English Channel)
Overlies Harwich Formation
Location
Region southern England

Geological map of the London Basin; the London Clay is marked in dark brown.

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 consists of a stiff, bluish coloured clay which becomes brown when weathered. Nodular lumps of pyrite and crystals of selenite (sometimes called "waterstones") 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.

Contents

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 (299 ft) thick occurs at Whitecliff Bay on the Isle of Wight and around 101 metres (331 ft) is spread along 6 km 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]

Tunnels in London Clay

The London Clay is an ideal medium for driving tunnels, which is why the London tube railway network expanded so quickly North of the Thames, but South of the Thames the London Clay is much deeper, and the stratum is water-bearing sand and gravel (not good for tunnelling) at tube level, 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 ground-water 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. Sometimes the London Clay is prettily coloured, and is called shepherd's plaid, but generally the colours disappear when the clay is fired.

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]

The following species list follows Clouter (2007).[5]

Vertebrates

Mammals

Birds

Reptiles

Crocodilians

Snakes

Turtles and tortoises

Bony fish

  • Acestrus elongatus, A. ornatus, Aglyptorhynchus sulcatus, A. venablasi, Xiphiorhynchus parvus and X priscusswordfish relatives
  • Acipenser toliapicus – a true sturgeon
  • Albula oweni – a bonefish
  • Ampheristus toliapicus – a scorpionfish
  • Ardiodus marriottiincertae sedis
  • Argillichthys toombsi – a lancetfish relative
  • Aulopopsis depressifrons, A. egertoni and Labrophagus esocinus – aulopuses
  • Beerichthys ingens and B. sp. – Two species of luvar or luvar-like fish[6]
  • Bramoides brieni and Goniocranion arambourgipomfrets
  • Brychaetus muelleri – an arowana
  • Bucklandium diluvii – a naked catfish
  • Cylindracanthus rectus and Hemirhabdorhynchus elliotti – Blochiidae
  • Cymbium proosti, Eocoelopoma colei, E. curvatum, E. gigas, E. hopwoodi, Eothynnus salmoneus, Scombramphodon crassidens, S. sheppeyensis, Scombrinus macropomus, S. nuchalis, Sphyraenodus priscus, Tamesichthys decipiens, Wetherellus brevior, W. cristatus, W. longior and Woodwardella patellifronsmackerel and tuna relatives
  • Diodon sp. – a porcupinefish
  • Egertonia isodonta and Phyllodus toliapicus – Phyllodontidae
  • Elops sp., Esocelops cavifrons, Megalops oblongus, M. priscus, Promegalops sheppeyensis and P. signeuxaeladyfish
  • Enniskillenus radiatusacanthomorph
  • Eutrichurides winkleri – a cutlassfish
  • Halecopsis insignis – Halecopsidae
  • Laparon alticeps and Whitephippus tamesisspadefish
  • Lehmanamia sheppeyensis – a bowfin
  • Myripristis toliapicus, Naupygus bucklandi and Paraberyx bowerbankisoldierfish
  • Percostoma angustum, Plesioserranus cf. wemmeliensi and Serranopsis londinensisgroupers
  • Podocephalus curryi, P. nitidus, Sciaenuropsis turneri and Sciaenurus bowerbankiporgies
  • Progempylus edwardsi – a snake mackerel
  • Pseudosphaerodon antiquus and P. naviculariswrasses?
  • Pycnodus bowerbanki and P. toliapicus – Pychnodontidae
  • Rhinocephalus planiceps and Trichurides sagittidenshakes
  • Rhynchorhinus branchialis and R. major – Eccelidae
  • Tetratichthys antiquitatis – a jack mackerel

Cartilaginous fish

Crustaceans

Exuvia of Hoploparia
  • Lobsters and shrimp
    • Archiocarabus bowerbanki
    • Callianassa sp.
    • Homarus morrisi [7]
    • Hoploparia gammaroides
    • Linuparus eocenicus & L. scyllariformis
    • Scyllarides tuberculatus
    • Scyllaridia koenigi
    • Thenops scyllariformis
  • Barnacles
    • Arcoscapellum quadratum
    • Scalpellum minutum and S. quadratum
  • Crabs
    • Campylostoma mutatiforme
    • Cyclocorystes pulchellus[Note 10]
    • Dromilites bucklandi & D. lamarki
    • Glyphthyreus wetherelli
    • Goniochela angulata
    • Mithracia libinioides
    • Oediosoma ambigua
    • Portunites incerta & P. stintoni
    • Xanthilites bowerbanki
    • Zanthopsis bispinosa, Z. dufori, Z. leachei, Z. nodosa and Z. unispinosa
  • Mantis shrimp

Molluscs

Cephalopods

  • Aturia ziczac, Cimomia imperialis, Deltoidonautilus sowerbyi, Euciphoceras regale, Eutrephoceras urbanum, Hercoglossa cassiniana and Simplicioceras centrale[Note 11]nautiluses
  • Belopterina levesquei, Belosepia blainvillei and B. sepioideacuttlefish

Bivalves

Gastropods

Tusk shells

  • Antalis anceps and A. nitens

Echinoderms

Ophiura wetherelli from the London Clay of Bognor Regis
  • Asteropecten crispatus, Coulonia colei, Hemiaster bowerbanki, Hippasteria tuberculata, Ophioglypha wetherelli and Teichaster stokesii[Note 48]starfish
  • Coelopleurus wetherelli and Schizaster sp. – sea urchins
  • Democrinus londinensis[Note 49]crinoid
  •  ?Ophiacantha sp., Ophioglypha wetherelli, Ophiomusium sp. and Ophiura wetherelli – brittlestars

Cnidarians

  • Paracyathus brevis and P. caryophylluscorals
  • Graphularia wetherellihydrozoan

Other invertebrates

  • Adenellopsis wetherelli, Aimulosia sp., Batopora clithridiata, Beisselina sp., Cribrilina sp., Didymosella sp., Dittosaria wetherelli, Entalophora sp., Idmonia sp., Lunulites sp., Nellia sp., Pachythecella incisa, Vibracellina sp. and Websteria crissioides – bryozoans
  • Hemiptera gen. et sp. indet. – true bug
  • Lingula tenuis, Terebratulina striatula and T. wardenensis – lampshells
  • Stelleta sp. – sponge

Ichnofossils

  • Ditrupa plana, Rotularia bognorensis and Serpula trilineata – polychaete worm tubes?
  • Scolithos

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. 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.

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'"[8]

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. ^ a b c Sumbler, M. G. (1996). London and the Thames Valley. British Regional Geology series (4th ed.). British Geological Survey. ISBN 0-11-884522-5. 
  2. ^ Melville, R. V. & E. C. Freshney (1992). The Hampshire Basin and adjoining areas. British Regional Geology series (4th ed.). 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. ISBN 0-85272478-0. 
  4. ^ Collinson, M. (1983). Fossil plants of the London Clay. The Palaeontological Association. 
  5. ^ Clouter, Fred (June 29, 2007). "London Clay Species List". http://www.sheppeyfossils.com/pages/london_clay_species_list.htm. Retrieved June 16, 2008. 
  6. ^ Alexandre F. Bannikov & James C. Tyler (1995). "Phylogenetic revision of the fish families Luvaridae and †Kushlukiidae (Acanthuroidei), with a new genus and two new species of Eocene luvarids" (PDF). Smithsonian Contributions to Paleobiology 81: 1–45. http://www.sil.si.edu/SmithsonianContributions/Paleobiology/pdf_lo/SCtP-0081.pdf. 
  7. ^ W. J Quayle (1987). "English Eocene Crustacea (lobsters and stomatopod)" (PDF). Palaeontology 30 (3): 581–612. http://palaeontology.palass-pubs.org/pdf/Vol%2030/Pages%20581-612.pdf. 
  8. ^ 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, accompanied with remarks of several respectable gentlemen and farmers. Published by G. and W. Nicol, second edition, 1807. Page 20.

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