The Full Wiki

London Clay: Map

  
  

Wikipedia article:

Map showing all locations mentioned on Wikipedia article:



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 Englandmarker. 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 Indonesiamarker 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 Sittingbournemarker, and at Harwichmarker, 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 Wiltshiremarker. 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 Heathmarker, 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 Baymarker on the Isle of Wightmarker and around is spread along 6 km of foreshore at Bognor Regismarker, West Sussexmarker.

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 Claygatemarker, 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 Sheppeymarker in Kentmarker and Walton-on-the-Nazemarker, Essex in the London Basin, and Bognor Regismarker 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.

Dr. Birbal Sahni concluded from the fossils found in inter-trappean rocks that at that time estuarine conditions prevailed in Indiamarker, and the flora belonged to the genera of plants found in London clay. These plants must have migrated to Indiamarker by way of the Tethys Sea which stretched along the northern edge of the Gondwana land before the uplift of the Himalayasmarker. It has also been proved that Kashmirmarker and Rajasthanmarker once had a tropical forest, which later receded as a result of glaciation and the upthrust of the Himalayas. Prior to this upheaval, the Gangamarker drained northwards into the Sindhumarker. By this time man had already been evolved.

Vertebrates

Mammals



Birds



Reptiles

Crocodilians

Snakes

Turtles and tortoises

Bony fish



Cartilaginous fish



Invertebrates

Crustaceans

Exuvia of Hoploparia


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 Londonmarker 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'"

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



Embed code:






Got something to say? Make a comment.
Your name
Your email address
Message