A HISTORY OF ESSEX The soils furnished by these Lower Eocene strata between the Chalk and London Clay are on the whole loamy, being mixed soils of sand and clay. The formations themselves exercise no marked influence on the scenery. LONDON CLAY The London Clay occupies a large part of the surface of Essex, more especially in the south, and where well developed it is upwards of 400 feet thick. It is naturally exposed in the cliffs at Southend, Shoeburyness and Clacton-on-Sea, where it is capped by valley gravels, and it is seen again in the cliffs at Frinton, Walton-on-the-Naze, and also at Harwich. It occurs at the surface over considerable areas along the valley of the Crouch, around Langdon Hill, over Hainault and Epping Forests and near Waltham Abbey. In north Essex the London Clay is exposed along the Stour valley below Middleton ; along the Colne valley from Castle Hedingham and Gosfield ; along the Pant and Blackwater valleys from Wethersfield ; and along the Pods Brook, Ter and Chelmer valleys at Braintree, Terling, Thaxted and Dunmow. In the Stort valley it occurs in a few places along the Essex borders. The formation where it appears at the surface is a stiff brown clay, which soaks up a good deal of water in wet weather, and shrinks and cracks in very dry weather. Where dug from a depth it is bluish-grey in colour, and in this unweathered portion of the deposit, or in the cement-stones, the fossils which characterize the London Clay are more usually found. These include mammals allied to the hyrax and tapir, some remarkable birds, turtles, a large serpent, sharks and other fishes, crabs and lobsters, as well as mollusca such as the Nautilus, Foluta, Cypraa, and a boring-shell or ' ship-worm ' called Teredina. Among plant-remains there are fruits and seeds of palms, euphorbias, mimosa? and acaciae. Towards the base of the London Clay there are occasional flint pebbles and also hard and impersistent bands of calcareous sandstone, yielding Aporrhais sowerbyi, Natica, Cytherea orbicularis, Panopcea and Pectunculus. 1 Blocks of this rock were obtained during an excavation at the gasworks at Beckton. As a rule however the London Clay rarely yields any fossils, and one may examine many a brick-pit or railway-cutting without finding a single specimen. The presence of selenite (or of moulds of its crystals) may have arisen from the decomposition of iron-pyrites and of calcareous organisms : hence in this and other formations the mineral may indicate the former occurrence of fossils. 2 The London Clay is characterized by nodules of argillaceous lime- stone known as cement-stones or septaria. These may be seen in the 1 See also Prestwich, Quart. Journ. Geol. Sac., vol. vi. p. 262. 8 See P. M. Duncan, ibid. vol. xxii. p. 12. 8
