THE WORK OF RUNNING WATER. 131 



considerable resistance to the wind, and in the overturning, large 

 quantities of rock are sometimes loosened and carried down the slope 

 by gravity. This phase of destructive work is seen at its best on the 

 walls of gorges, where trees often flourish until their tops project 

 above the rim of the valley. 



Through vegetation, chmate influences erosion in ways which are 

 easily defined qualitatively, but not quantitatively. Both by its 



Fig. 109. — Characteristic cliffs of high arid regions. Right wall of Snake River canyon, 

 nearly opposite the mouth of Salmon River, Id. Two spring-formed coves, with 

 "Castle Rock" between. (Russell, U. S. Geol. Surv.) 



growth (wedge-work of roots) and by its decay (supplying COj, etc., 

 to descending waters) it favors certain phases of weathering; but, on 

 the other hand, it retards corrasion and transportation both by wind 

 and water. This is well shown along the banks of streams and on the 

 faces of cliffs, in clay, sand, etc. Its aggregate effect is probably un- 

 favorable to erosion by mechanical means, and favorable to that by 

 chemical processes. 



Erosion in high arid regions differs from that in regions of abundant 

 rainfall in several ways. It is obvious that the valleys will develop 

 more slowly in the former, that they will remain young longer, that 

 the period necessary for the dissection of the surface is greater, that 

 the watercourses will be less numerous, and that fewer of them will 

 have permanent streams. There are certain other differences which 

 are less obvious. If the arid region be high and composed of hetero- 

 geneous strata, the topography which erosion develops is more angular 

 (Fig. 83) than that of the humid region. This is because there is 



