﻿676 ME. H. BT7EY ON THE DENUDATION OE [NOV. I9IO, 



in establishing a subsequent stream severing this part of the Chalk 

 from the Hythe Beds was, compared with the total length of the 

 supposed subaerial period, extremely short. This difficulty can be 

 avoided, (1) by assigning a marine origin to the gravels in question, 

 which, however, is opposed to Prestwich's conclusions as to the 

 Southern Drift ; (2) by assuming a much earlier origin (Miocene ?) 

 for this drift than is generally admitted ; (3) by postulating wide 

 oscillations of the River Wey on a nearly level plain of Chalk and 

 Tertiary strata — a peneplain, in fact. I am not prepared to say that 

 an} 7 one of these assumptions is wholly unreasonable, but am merely 

 pointing out what appear to me to be obstacles to the acceptance 

 of the subaerial theory, whether with or without a coastal plain. 

 The marine hypothesis, on the other hand, offers a simple solution of 

 the problem. The thin layer of planed-off Chalk was quickly re- 

 moved until a definite escarpment began to be formed ; whereupon 

 a subsequent system arose, which cut off all the less successful 

 consequent streams before they had had time to excavate any deep 

 notches in the Downs. 



On the view here adopted, the main differences in evolution 

 between the Blackwater and the Wey are traceable to differences in 

 the geological structure of the uplifted plain. The area belonging to 

 the former was at first almost wholly covered with a layer of Chalk, 

 which though thin, and perhaps easily removed, nevertheless caused 

 some delay in the formation of subsequent streams ; it was only 

 after an excavation of 200 feet (measured along the present summit- 

 line of the Downs) that the northern (anticlinal) subsequent was 

 finally completed, and it was still longer before the Tilford (syn- 

 clinal) River came into existence. 



In the case of the Wey, on the other hand, a vast surface of 

 Lower Greensand was already exposed ; and in this the develop- 

 ment of subsequent streams proceeded so rapidly that it is doubtful 

 whether any of the captured consequents had time to cut a valley 

 as deep as 150 feet before being beheaded. 



Again, the excessive upheaval of the anticline near Guildford 

 gave rise to a local dip towards both the east and the west, which 

 enabled subsequent streams to work outwards in both directions 

 from the^Peasemarsh inlier ; whereas in the case of the Blackwater 

 the beds fall away towards the west, so that the development of 

 subsequent tributaries was practically unilateral. 



(D) River Mole. 



Almost the whole main trunk of the Mole lies in the W'eald Clay, 

 and the most obvious peculiarity about it is the obliquity of its 

 course to the general line of the Chalk escarpment. The latter, it 

 is true, changes its direction at White Downs, some 3 miles west of 

 Dorking, and takes a more northerly trend ; but even to this line 

 the Mole is somewhat oblique, just as the Shalford Brook is to the 

 strike of the Chalk at Guildford. Across the Chalk itself the Mole 



