198 THE RIVER VALLEYS OF 



And 3d, How the subserial agencies at the first formed 

 an escarpment in the chalk ? 



To consider the last first : subaerial agencies, un- 

 aided, seem incapable of cutting a ravine in homogene- 

 ous chalk or limestone. We do not know whether a 

 bare sheet of chalk exists, but many such of limestone 

 do, and in all cases meteoric abrasion denudes the 

 whole surface ; so that no ravine, and consequently 

 no escarpment can form, except the rock is faulted, 

 jointed, or traversed by some kind of shrinkage fis- 

 sures, which are acted on by the weather. But as in 

 such rocks master-joints and faults are nearly always 

 connected with an underground drainage, 1 they can 

 seldom be the forerunners of valleys that are denuded 

 solely by meteoric abrasion. Once a ravine or valley 

 is begun, whether it be due to the shrinkage of the 

 rocks, an anticlinal curve, marine denudation, or the 

 scooping out of ice, meteoric abrasion would be cap- 

 able of materially assisting in developing it; more 

 especially if the beds dip away from the valley, and 

 some are softer than the rest, as the basset of the 

 soft beds will be denuded more quickly, and cause a 

 hollow in which water will collect, and form a stream 

 at the base of a cliff; the water conveying off the 

 debris as fast as it falls. If, however, the beds dip 

 into the valley, the water will run away from the 

 cliffs, and all debris that falls will remain at their 



1 Whitaker, " Memoirs of the Geol. Survey," Sheet 7, England, p. 96. 



