174 PROF. J. PRESTWICH ON A SOUTHERN DRIFT IN THE THAMES 



Folkestcrie Beds between Sevenoaks and Maidstone, spread over the 

 Chalk-plateau to the south, renders a mistake impossible. 



The time of the first appearance of the Lower-Greensand debris is 

 significant. We have seen that the Wealden anticlinal was uplifted 

 after the deposition of the Leu ham Beds, while the first beds in which 

 ,we find chert audragstone of the Wealden area are those of the Red 

 Crag — at least I found none in the single small exposure of the 

 Pebble-bed at the base of the White Crag. Hence we may presume 

 that the Wealden range was raised and became exposed to atmo- 

 spheric waste at some time either towards the end of the White- 

 Crag epoch, or very early in the Eed-Crag epoch. But during the 

 time of the Bed Crag the waste was still small, and it was not 

 until the time of the Westleton Shingle and Southern Drift, that it 

 acquired larger dimensions — dimensions which had their climax in 

 the plateau- gravels of the Thames Yalley. These successive stages 

 are, I imagine, coincident with the increasing cold of the period, 

 and the increased disintegration and denuding action. If, on these 

 limited premises — and we have none others to judge by — we might 

 suggest time -equivalents for these several stages in the early 

 denudation of the Weald, I would take the Well-Hill Drift to 

 belong to the Red-Crag epoch, and the Plateau-Drifts to corre- 

 spond broadly in time with the Chillesford, Porest Bed, and Westle- 

 ton Shingle. A more definite concordance is hardly at present 

 practicable; for nowhere are the hill-Drifts found in juxtaposition, 

 and the extensive denudation of the subsequent Glacial period has 

 swept away any connecting-links that maj^ originally have existed, 

 and left only the fragmentary remainders we have described. 



While this glaciation was going on in the Thames Basin, it is 

 probable that the Wealden Highlands were the centre of another 

 snow- and ice-field, to which the denudation of that area is to be 

 more particularly ascribed. It was this range of high land which 

 obstructed the advance of the northern ice and the Boulder-clay, for 

 the Tertiary hills of the Thames Valley were then only the out- 

 posts of that more important Wealden range. This ice-field may 

 ultimately have been confluent in the Thames Yalley with the 

 great northern ice-sheet, though it may have yielded finally to it 

 as the destruction of the Wealden range proceeded, and the obstacles 

 to the further southward progress of the latter were removed. 



6. Relation of the ^oatUern Drift to the Westleton Shingle and 

 other Pre-Glacial Drifts. 



Let us now endeavour to follow the successive changes that took 

 place from the time of the early Crag deposits down to that of the 

 Westleton and Southern Drifts in South-eastern England and the 

 adjacent continental area, and note the relation of these drifts one 

 to another. 



There can be but little doubt that a marine deposit of early 

 Pliocene or Diestian age extended generally over a portion of the 

 south-east of England and adjacent parts of Erunce and Belgium, and 



