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The London Clay is a marine deposit which is well known for the fossils it contains.

It is the most important member of the Lower Eocene Epoch (Ypresian, c.56-49 mya) strata in the south of England, and the only European source of diverse plant fossils from the Lower Eocene.

The fossils indicate a moderately warm climate, the flora being tropical or sub-tropical.

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 London Clay, and large septarian concretions are also common. These concretions have been used in the past for the manufacturing of cement, 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.

The London Clay is well developed in the London basin, where it reaches an average thickness of 130 meters (430 ft), though it is not frequently exposed. This is partly because it is to a great extent covered by more recent 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.

One location famous for London Clay fossils are the coastal exposures on the Isle of Sheppey, Kent. Two other notable coastal exposures from which fossils can be collected are Bognor Regis, West Sussex and Walton-on-the-Naze, Essex.

The London Clay is also exposed in the Isle of Wight, where it is 300 feet thick at Whitecliff Bay.

Animal fossils found in the London Clay include bivalves, gastropods, nautilus, worm tubes, brittle stars and starfish, crabs, lobsters, fish (including shark and ray teeth), reptiles (particularly turtles), and rarely birds. A few mammal remains have also been recorded.

Plant fossils, including seeds and fruits, may also be found in abundance. Plant fossils have been collected from the London Clay for almost 300 years. Some 350 named species of fossil plants have been found, making the London Clay flora one of the world's most varied for fossil seeds and fruits.

• Prophaeton, a prehistoric genus related to tropicbirds

• Geology of London
• Geology of the United Kingdom
• Geology of Hertfordshire
• Eocene
• List of fossil sites (with link directory)

• Collinson, M, 1983. Fossil plants of the London Clay, The Palaeontological Association.