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 A HISTORY OF KENT Oldhaven and Blackheath Beds. — These beds, now classed as forming the uppermost division of the Lower London Tertiaries, were originally regarded as the ' Basement Bed ' of the London Clay.^ In west Kent they are principally composed of peculiarly well-rolled flint pebbles mixed with fine sand ; but in the eastern part of their outcrop, except in an outlier at Shottenden Hill south of Selling, this predominant pebbly character is lost, and they consist of fine light-buff sand with dark grains, and sometimes with thin layers or patches of clay and a pebbly band or a bed of sandy brown iron-ore at the base. The rounded shape of all the pebbles is very characteristic and indicates long-continued attrition of the flints on the Eocene shingle banks. The fossils of the Oldhaven and Blackheath Beds are partly marine and partly estuarine, the marine species predominating in the eastern sandy portion of the formation. Westward the division thins out and disappears soon after crossing the Surrey border. LONDON CLAY The deposition of the sands, estuarine muds and shingle beds of the shallow-water Lower London Tertiaries was brought to a close by a subsidence of the land, which carried down the whole district once more beneath the sea and caused the earlier Eocene strata to be over- spread by a deep mass of marine clay — the London Clay — which constitutes the thickest and most widespread division of the Eocene sediments of the London basin. This great bed of tenacious brown and bluish-grey clay, attaining a thickness of from 400 to 480 feet where present from base to summit, preserves the same character over wide areas. It has usually an admixture of sand and flint pebbles in its lower- most stratum, and also contains here and there layers of nodular calcareous concretions, and segregations of pyrites. The calcareous nodules generally show shrinkage-cracks or septa lined with calcite or aragonite, and on this account are termed septaria ; these nodules have been collected in large numbers in the Isle of Sheppey for use in the preparation of cement. The widest tracts of London Clay lie beyond the boundaries of Kent to the north and west, but a glance at the geological map will show that it also covers much ground in the northern part of our county, lying always within a fringe of the Lower London Tertiaries. In the west it is broken up into numerous outlying patches and spurs, the remnants of a once continuous sheet which has been worn into shreds by denuding agencies. Farther east, though much obscured by the alluvium and other ' superficial ' deposits of the Thames and its tributaries, it underlies the Hundred of Hoo and the Isle of Grain ; and reappears from beneath the alluvium of the Medway in the Isle of Sheppey, where its uppermost beds are in places preserved, and where it is well exposed in cliff-sections long famous for their numerous and diversified fossils. On the mainland farther eastward it underlies the undulating well- 18
 * See Mem. Geol. Survey, 'The London Basin,' p. 239.