366 



ME. C. REID ON THE ORIGIN OP DRY CHALK 



in these dry valleys ; for the Chalk is so porous that the heaviest rain 

 sinks in directly, and the most continued rainfall merely causes new 

 springs to burst out at some point rather higher up the valley than 



Fig. 1. — Contour-map of a portion o f the South Downs between the 

 Valley of the Adur and the Devil's Dylce. (Scale 1 inch to 1 mile.) 



1 2 34 



2 343 2 3423 43 23234 



Explanation of tints. 



0=1 , -100'. 1 = 100'-200\ 2=200'-300'. 3=300'-400'. 4 = 400 -500'. 

 5=500'-600'. 6= 600 -700'. 7 (black) =700'-800'. 



usual. The upper and steeper portion of the valley still remains 

 perfectly dry, and no running water can be found where the incline 

 of the bottom of the valley exceeds the slope of the plane of satura- 

 tion in the chalk. This is well shown in the Downs near Brighton 

 (see fig. 1). 



This difficulty in accountiug for the erosion of dry Chalk Valleys 

 by running water has been felt by other geologists, who have tried 

 to overcome it by an appeal to a former submergence and consequent 

 rise in the level of the plane of saturation ; or to a former higher 

 level of the plane of saturation before the valleys had been cut to 

 their present depth * ; or, thirdly, to an enormous increase in the 

 rainfall f. 



None of the explanations seem sufficient, though perhaps each of 

 these agencies has to some extent assisted in the denudation of the 

 lower portion of the valleys. 



There is no evidence of a submergence while these Coombes were 



* Prof. Prestwich, Presidential Address, 1872, pp. 58-63 ; and Dr. J. Evans,- J 

 Presidential Address, 1875, p. 37. 



t A. Tylor, " Action of Denuding Agencies," G-eol. Mag. dec. ii. vol. ii. 

 pp. 437-476. See also Prof. Prestwich, "On a peculiar bed of Angular Drift 

 on the Lower-Chalk high Plain between Upton and Chilton," Quart. J ourn. 

 Geol. Soc. vol. xxxviii. 



