| London Clay Formation | |
|---|---|
| Type | Formation |
| Age | Ypresian |
| Underlies | Bagshot Formation (London Basin), Wittering Formation and Poole Formation (Hampshire Basin and English Channel) |
| Overlies | Harwich Formation |
| Lithology | |
| Location | |
| Region | southern England |
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 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.
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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 6 km of foreshore at Bognor Regis, West Sussex.[2]
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]
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]
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]
Dr. Birbal Sahni concluded from the fossils found in inter-trappean rocks that at that time estuarine conditions prevailed in India, and the flora belonged to the genera of plants found in London clay. These plants must have migrated to India by way of the Tethys Sea which stretched along the northern edge of the Gondwana land before the uplift of the Himalayas. It has also been proved that Kashmir and Rajasthan once had a tropical forest, which later receded as a result of glaciation and the upthrust of the Himalayas. Prior to this upheaval, the Ganga drained northwards into the Sindhu. By this time man had already been evolved. [5]
Crocodilians
Snakes
Turtles and tortoises
Cephalopods
Clams and other bivalves
Gastropods
Tusk shells
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.
"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'"[53]
LONDON CLAY, in geology, the most important member of the Lower Eocene strata in the south of England. It is well developed in the London basin, though not frequently exposed, partly because it is to a great extent covered by more recent gravels and partly because it is not often worked on a large scale. It is a stiff, tenacious, bluish clay that becomes brown on weathering, occasionally it becomes distinctly sandy, sometimes glauconitic, especially towards the top; large calcareous septarian concretions are common, and have been used in the manufacture of cement, being dug for this purpose at Sheppey, near Southend, and at Harwich, and dredged off the Hampshire coast. Nodular lumps of pyrites and crystals of selenite are of frequent occurrence. The clay has been employed for making bricks, tiles and coarse pottery, but it is usually too tenacious for this purpose except in well-weathered or sandy portions. The base of the clay is very regularly indicated by a few inches of rounded flint pebbles with green and yellowish sand, parts of this layer being frequently cemented by carbonate of lime. The average thickness of the London Clay in the London basin is about 450 ft.; at Windsor it is 400 ft. thick; beneath London it is rather thicker, while in the south of Essex it is over 480 ft. In Wiltshire it only reaches a few feet in thickness, while in Berkshire it is some 50 or 60 ft. It is found in the Isle of Wight, where it is 300 ft. thick at Whitecliff Bayhere the beds are vertical and even slightly reversed-and in Alum Bay it is 220 ft. thick. In Hampshire it is sometimes known as the Bognor Beds, and certain layers of calcareous sandstone within the clays are called Barnes or Bognor Rock. In the eastern part of the London basin in east Kent the pebbly basement bed becomes a thick deposit (60 ft.), forming part of the Oldhaven and Blackheath Beds.
The London Clay is a marine deposit, and its fossils indicate a moderately warm climate, the flora having a tropical aspect. Among the fossils may be mentioned Panopoea intermedia, Ditrupa Plana, Teredina personata, Conus concinnus, Rostellaria ampla, Nautilus centralis, Belosepia, foraminifera and diatoms. Fish remains include Otodus obliquus, Sphyroenodus crassidens; birds are represented by Halcyornis Toliapicus, Lithornis and Odontopteryx, and reptiles by Chelone gigas, and other turtles, Palaeophis, a serpent and crocodiles. Hyracotherium leporinum, Palaeotherium and a few other mammals are recorded. Plant remains in a pyritized condition are found in great abundance and perfection on the shore of Sheppey; numerous species of palms, screw pines, water lilies, cypresses, yews, leguminous plants and many others occur; logs of coniferous wood bored through by annelids and Teredo are common, and fossil resin has been found at Highgate.
See Eocene; also W. Whitaker, "The Geology of London and part of the Thames Valley," Mem. Geol. Survey (1889), and Sheet Memoirs of the Geol. Survey, London, Nos. 314, 315, 268, 3 2 9, 33 2, and Memoirs on the Geology of the Isle of Wight (1889).
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