GEOLOGY 



and near Reculvers. It ranges thence westward, with slight changes 

 of composition, along the northern slopes of the Chalk up to the Surrey 

 boundary, but dies out gradually in the eastern part of that county. 

 From the relatively insignificant thickness of this subdivision and from 

 the sUght resistance which it can offer to the erosive agencies, its out- 

 crop takes the form of a ragged irregular fringe to the Tertiaries, with 

 many detached patches or ' outliers ' surrounded by Chalk where the 

 wasting back of its mass has been unequal in rate at different spots. A 

 layer of unworn green-coated flints is constantly found at its base, these 

 having been derived from the Chalk either by the slow solution of the 

 original matrix by percolating waters after the deposition of the sands,^ 

 or by its removal under gentle current action before their accumulation. 

 The fossils of the Thanet Beds consist mainly of a few marine shells, 

 which are found chiefly in the eastern part of the county and are very 

 rare in the western part. These beds, with the overlying members of 

 the group, may be studied in numerous fine artificial sections in the 

 north-western part of the county,* as for example in the railway cuttings 

 near Chislehurst, where the recent widening of the South-Eastern line 

 has laid open the whole sequence.^ 



Woolwich and Reading Beds. — Next in the series we find a mass 

 of sediments — the Woolwich and Reading Beds — which represent a 

 period when the northern part of Kent lay at the mouth of a lagoon or 

 estuary, with open sea to the north-eastward. In east Kent the deposits 

 of this period consist of sharp light-coloured false-bedded sand contain- 

 ing a few marine fossils, usually with a greenish clayey layer and rolled 

 pebbles of flint at the base. Farther westward the beds are more variable, 

 light-coloured sands being interbedded with clay and loam and with 

 indurated bands of oyster shells and occasional layers of flint pebbles. 

 These sediments are often crowded with estuarine shells of the genera 

 Cyrena, Unio, Corbula, Ostrea^ Paludina, Melania, Cerithium^ etc., and some- 

 times contain fragmentary plant-remains. These estuarine beds have 

 been supposed to indicate the existence of a large river flowing from the 

 west, but they have also been explained as representing the deltas of 

 smaller streams flowing northward from the tract now known as the 

 Weald.* The outcrop of the Woolwich and Reading Beds and also that 

 of the overlying Oldhaven and Blackheath Beds border that of the 

 Thanet Sand, and are subject to the same general conditions. It is found 

 however that the overlying division in each case extends in certain places 

 southward beyond the limits of the underlying bed, and then rests directly 

 upon the Chalk. This ' overstep ' of the newer upon the older member 

 of the series is held to show that the bounds of the sea were again 

 expanding over a sinking land. 



* For discussion on this subject and references to literature see Mem. Geol. Survey, ' The London 

 Basin,' iv. 58. 



* For list of Kentish sections near London see ibid. ' Guide to Geology of London and the 

 Neighbourhood,' by W. Whitaker, pp. 38, 4.3, 47, 57. 



' See Proc. Geol. Assoc. (1900), xvi. 523, 533, and (1901), xvii. 69, 136. 



* See ' Guide to London,' op. cit. p. 40 ; also The Building of the British Isles, by A. J. J. 

 Browne, p. 315. 



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