﻿Yol. 66. ,] THE WESTEKN END OF THE WEALD. 651 



Pliocene beds (17, p. 330), which according to him are largely- 

 composed of Lower Greensand material. Chert, too, as we have 

 seen, is scattered all along the Downs in positions to which it can 

 scarcely have been brought, except by the sea or by the earliest 

 streams that traversed the coastal plain ; therefore, whatever may be 

 the explanation of the Well Hill gravel, there must have been an 

 extensive exposure of the Lower Greensand in Diestian times, andi 

 a fortiori at the time of the Southern Drift. 



jNo satisfactory explanation is offered by Prestwich of the wide 

 distribution of chert along the Downs. While attributing some of 

 it to existing rivers, with broader valleys than at present (18, p. 171), 

 he recognizes that in other cases it is necessary to assume the 

 existence of a large number of small streams, each taking its share 

 in the transportation of drift (19, p. 136). It is, however, very 

 difficult to believe that a drainage-system which originated in 

 Eocene, or even Cretaceous times, and persisted through the 

 Oligocene and Miocene, should have possessed a number of con- 

 sequent elements in Pliocene times, and that practically the whole 

 reduction in their number has been effected since that comparatively 

 recent date. 



One of the objections raised by Prestwich against the hypothesis 

 of marine planation is founded on the relative thickness of the Chalk 

 in the London Basin and on the Downs. Ramsay's diagrams show 

 the planed-off Chalk extending for about 4 miles south of the' 

 present escarpment ; he was probably depending a good deal on the 

 general uniformity in level of the Downs and Lower Greensand 

 escarpment as evidence of a plain, but Prestwich, who thinks that 

 the diagram was only created to explain the distribution of flints, 

 says (19, p. 158): — 



' Taking the range of the Chalk from Crossness in the centre of the Thames 

 Valley, where its thickness is known, to the edge of the Chalk escnrpment 

 at Otford, a distance of 14 miles, we find it diminished from 650 to 450 feet, 

 a total reduction of 200 feet, or of 14| feet per mile. At this rate the Chalk 

 should have extended 31 miles beyond the escarpment, or, taking only the 

 Chalk- with-Flints, some miles (16 ?) less.' 



I am at a loss to understand the bearing of these data upon the 

 point at issue. It is universally admitted that some at least of this 

 thinning of the Chalk is due to pre-Eocene causes, and most of the 

 slope along which measurements are taken is actually covered by 

 Eocene beds. Kamsay was not considering a pre-Eocene plain • 

 aud, although no definite period is fixed, he makes it abundantly 

 clear that, if the Pliocene age of the Lenham and other beds could 

 be established, he would be willing to accept that date for his plain 

 of marine denudation (21, 5th ed. p. 345). 



Prestwich's attitude is the more remarkable, because he himself 

 has supplied evidence which points to a wholly different conclusion. 

 His paper on the Darent deals largely with the ' Chalk Plateau,' 

 which slopes away northwards from the escarpment, and he gives 

 two important sections (19, pi. vi, figs. 1 & 2) showing its relation 

 to the Eocene and Lower Cretaceous strata. If in these diagrams 



Q. J. G. S. No. 264. 2 y 



