﻿646 &IK. H. BURY ON THE DENUDATION OF [Nov. 1 Q. 10, 



writers, in part perhaps through its being confused with the previous 

 one : but the essential distinctness of the two is recognized when 

 we consider that, although the correspondence of the summits is 

 seen in both North and South Downs, it is only in the former 

 that even bevelling occurs (see 26, p. 265). Prof. W. M. Davis (3, 

 p. 136) insists strongly on the importance of this feature as a sign of 

 planation, and expresses the opinion that where it occurs, coupled 

 with a mature river-system, the existence of two cycles of subaerial 

 denudation can ' hardly be doubted.' If, on the one hand, the whole 

 of the Chalk has been removed by subaerial agency in one prolonged 

 cycle, the escarpment ought to be notched by the remnants of many 

 temporarily successful consequent streams rising in the central 

 axis and also, perhaps, by minor valleys which formerly scored the 

 dip-slope of the Chalk (without extending so far inland), precisely 

 as it is scored at the present day at a distance of only a few miles 

 from the escarpment. If, on the other hand, the removal of the 

 Chalk was mainly effected by marine planation, Prof. Davis (if I un- 

 derstand him rightly) thinks that the innumerable early consequent 

 streams, and their obsequent offspring, would have destroyed all 

 traces of the original bevelling before the present drainage-system, 

 well adjusted to the structures, and consisting largely of subsequent 

 rivers, had time to develop. This objection to marine planation 

 evidently rests on Prof. Davis's individual judgment as to the rate 

 at which a subsequent system is likely to be formed ; and, despite 

 the great weight attaching to his opinion, I venture to doubt 

 whether he has made full allowance for the extremely soft character 

 of the Wealden strata. 



(iii) Straightness of the escarpments. — This is another 

 feature pointed out by Prof. Davis (3, pp. 134-35) as likely to. 

 result from planation. According to him, in a mature system 



' the crest of the escarpment turns inland and reaches greater height about 

 midway between the consequent streams, but curves seaward and loses height 

 on approaching the transverse valleys that are kept open by the successful 

 consequent streams.' 



In extreme old age, however, as in the peneplain stage, these- 

 escarpments, reduced almost to the level of the rivers themselves, 

 straighten out again ; and this straightness is likely to be noticeable 

 for some time after the revival of activity in the second cycle. 



It is evident that this feature is in some degree bound up with 

 the last, for wherever the crest of the escarpment is deeply notched 

 we find obsequent valleys interfering with the regularity of the 

 outcrop : but I think that a very important factor in the Wealden 

 area is to be found in the longitudinal folds. One of the straightest 

 parts of the whole Chalk escarpment, between Dorking and Parnham, 

 runs closely parallel to an anticline, and a similar parallelism is 

 observable elsewhere, as, for example, in the South Downs, between 

 Butser Hill and Duncton Down, and again between the Aran and 

 the Adur ; though in the latter case the curve of maturity has 

 already begun to form. 



