652 



ME. J. V. ELSDEN ON THE SUPERFICIAL GEOLOGY OE 



A general review of the evidence in favour of this view has been 

 so recently given by Mr. Howorth* that it only remains here to 

 examine how far the theory of a sudden flood will explain the facts 

 described in this paper. 



In the first place, it becomes evident to anyone who carefully 

 examines these beds of drift that, instead of having been formed 

 after the present configuration of the land had been established, and 

 having been arrested here and there from some rapid current, all 

 the evidence points to an interruption in the continuity of the beds 

 by the subsequent lowering of the valleys by denudation. The 

 accompanying sections, drawn across the river-valleys, point to the 

 existence of an ancient drift-covered plateau lying between the base 

 of the Chalk escarpment and the margin of the central dome (see 

 figs. 10-12). Denudation has destroyed almost every trace of this old 

 plateau, except on the higher grounds, near the watersheds, and even 

 here the remnants of the drift are thin and fragmentary, and are cut 

 through by the smallest depressions. This is especially noticeable 

 around Hurstpierpoint, Ditchling, Eurgess Hill, and Berwick. In 

 fact wherever the smallest trace of the plateau-drift has been pre- 

 served it is invariably confined to the highest contours, and ends 

 abruptly on passing a certain level. From this ancient plateau the 

 flint-drift trails out in the direction of the streams, forming lower 

 terrace-gravels of more recent origin. 



Murchison's statement that the gravels become more waterworn 

 in the eastern portions of this area was not borne out by my observa- 

 tions, the gravels at Selmeston and Berwick being quite as angular 

 as any of the more western drifts. Moreover, the theory of a strong 

 current setting out from the west is scarcely in harmony with the 

 local character of the drifts of different areas. Thus atHardham, on 

 the right bank of the Arun (fig. 3), the gravels contain an abundance 

 of ironstone and chert from the Lower Greensand, and these sub- 

 stances occur in situ at no great distance from the river. But on 

 the left bank of the same river chert is much less plentiful, and 

 becomes quite rare in the gravels of the upper parts of the eastern 

 tributaries, which do not flow through districts in which chert is a 

 constituent of the rocks. Ironstone also, which is abundant in the 

 gravels of the Arun and Adur, becomes quite scarce in the valleys 

 of the Ouse and Cuckmere, where the Lower Greensand is lower 

 and thinner and only occasionally contains bands of ironstone. 



This same connexion between the materials composing the gravels 

 and the composition of the underlying strata may also be noticed in 

 the watershed gravels. Thus the drift at Petersfield is stated by 

 Murchison to consist only of flints : further eastwards it becomes 

 mixed with chert and ironstone : but at Longbury Hill, between the 

 Arun and Adur basins, scarcely any chert is to be found, although 

 an abundance of ironstone occurs both in the gravel and in the 

 underlying strata. At Ditchling and Burgess Hill even the iron- 

 stone is far less common, and at Berwick it becomes quite insignifi- 

 cant. The Arun gravels, again, contain no considerable quantity of 



* Geol. Mag., Nov. 1882. p. 509. 



