﻿680 ME. H. BUKY ON THE DENUDATION OF [Nov. I9IO,. 



explanation. On the marine hypothesis it is most difficult to fix 

 the limits of the various strata upon the plain ; according to my 

 calculations, the Chalk prohably extended about 2 miles south of its 

 present position, although over part of this distance it was bevelled 

 off to a thin layer ; but the position and width of outcrop of the 

 Lower Greensand depend on so many uncertain factors that I can- 

 not attempt to define them. It is evident, however, that planation 

 would greatly reduce the horizontal distance through which these 

 strata have retreated, and would therefore lessen the opportunities 

 for the development in them of subsequent rivers ; the dip, too, 

 might conceivably be so great throughout their retreat as to keep 

 the outcrop permanently narrow. Although, therefore, many details 

 are obscure, yet I submit that the marine hypothesis offers a rather 

 better solution than the subaerial of this particular feature of the 

 Mole, as well as of the more general fact of the capture of Lower 

 Greensand areas by obsequent streams from the Weald Clay. To 

 sum up broadly, just as the Wey differed from the Blackwater owing 

 to the plain in its district having consisted more of Lower Green- 

 sand than of Chalk, so the Mole differs from the "Wey because so 

 large an area of Weald Clay was already exposed during its initial 

 stages. 



Attention may be called here to the fact that a straight line 

 drawn from White Downs to St. Leonard's Forest very nearly 

 marks the watershed between the Mole on the one hand and the 

 Wey and A run on the other. This is unusually straight for a 

 Wealden watershed, and is the more interesting because, as I shall 

 argue later on, it probably in the first place separated the Mole 

 from the W T ey only, the Arun's intervention being secondary. Such 

 a watershed suggests a structural cause, all the more because, in its 

 obliquity to the main escarpment line, it agrees in general direction 

 with the consequent trunks of the Wey and Mole; but, if such a 

 cause exists, I must leave to others its elucidation. Martin (12, 

 p. 195) asserts that there is an anticline in the Lower Greensand 

 along this line ; but I am unable to support his belief by personal 

 observation, and the fact that no such fold is alluded to by more 

 recent workers in this district (see especially 10) is strong negative 

 evidence against its existence. 



(E) River Arun. 



The chief consequent river, in this case, runs approximately at 

 right angles to the Chalk escarpment, and lies mainly in the Weald 

 Clay. Except at its extreme northern end, where a large branch 

 reaches it from the Horsham district, it is singularly deficient in 

 tributaries on its eastern side ; but three important streams join it 

 from the west, two lying wholly in the W 7 eald Clay, while the third 

 and largest (the Eother ! ) runs mainly in the Lower Greensand. 



i This must not be confused with the other river bearing the same name, 

 mentioned on p. 659. 



