﻿690 ME. H. BURY ON THE DENUDATION OF [Nov. I9IO, 



Martin (11, p. 61) long ago called attention to the correspondence 

 of the northern and southern river-gaps, which he thought indicated 

 great transverse fissures; Topley (26, pp. 266-67) was also impressed 

 by the same phenomena, though he rejected Martin's inference, 

 and eliminated the ' wind-gaps ' under the mistaken impression 

 that they were unconnected with river-systems. More recently an 

 elaborate comparison has been attempted by Mr. Spurrell (24, p. 9), 

 and from his list I select the following pairs of valleys, without 

 expressing any opinion as to the remainder : Aldershot (Runfold) gap 

 and Cocking gap ; the Wey and the Arun ; the Mole and the Adur. 

 If we join these pairs by straight lines (see fig. 6, p. 670) we obtain 

 a degree of parallelism which is certainly very remarkable, and is 

 not, 1 think, generally appreciated. But, before accepting the 

 conclusion that these lines correspond with those of great transverse 

 movements or dislocations, extending right across the Wealden 

 area, we must consider certain other facts which previous writers 

 on the subject appear to have overlooked. Of these six valleys 

 four are still occupied by consequent rivers, while the former 

 course of the fifth is known ; and in each of these five cases the 

 main consequent river-trunk, before it reaches the Chalk, lies to 

 the east of the parallel line with which it is associated. It has 

 already been argued that the gradients of the plain were slight in 

 the region of the central watershed: the main fall was to north 

 and south, but there was also some inclination towards the west ; 

 and it may be that the westerly trend of the consequent rivers is a 

 resultant of the action of these two slopes. On the other hand, it 

 is worth noting that the trunks of the Blackwater, Wey, and Mole 

 are (or were) approximately parallel, not only one to the other, but 

 to the watershed between the two last-named rivers ; therefore, a 

 deeper-seated structural cause for their obliquity is by no means 

 improbable. 



Up to the present I have treated the plain as a direct product of 

 marine action alone ; and, so far as the origin of the rivers is con- 

 cerned, I see no reason no modify this view. If, however, the 

 stratigraphical evidence (mainly negative) is to be trusted, there 

 must have been two cycles of subaerial denudation — one very pro- 

 longed, extending through the Oligocene and Miocene Periods, and 

 the other beginning in the Middle Pliocene and lasting to the 

 present day. On the other hand, the cycle of marine denudation 

 which separated these was very short, being confined to the Diestian 

 stage of the Pliocene ; and it may be questioned whether, in so 

 brief a period, any great amount of planation was likely to be 

 accomplished. The speculation, therefore, may be permitted that 

 the removal of the Chalk was effected mainly during the first sub- 

 aerial cycle, which ended in a more or less complete peneplain ; and 

 that the marine cycle (as Dr. Barrois supposed) had little effect on 

 the rocks, being chiefly occupied in filling up the hollows of the 

 peneplain. The absence of rolled shingle from the beds supposed 

 to be Diestian would accord well with this view; and perhaps the 



