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London Clay

1911 Encyclopedia Britannica

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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).