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. I 17 3
