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- 



* See Mem. Geol. Survey, 'The London Basin,' p. 239. 

 18 



