LAND WATERS STREAMS 153 



Erosion in 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 water-courses will be less numerous, and that fewer of 

 them will have permanent streams. There are certain other differ- 

 ences which are less obvious. If the arid region is high and com- 

 posed of heterogeneous strata, the topography which erosion 

 develops is more angular (Figs. 101 and 133) than that of the humid 

 region. This is because there is less rock decay, and less vegetation 

 to hold the products of decay. The more resistant beds of rock, 

 therefore, come into greater prominence, especially on slopes, where 

 they develop cliffs (Figs. 101 and 104). These general principles 

 find abundant illustration in the plateaus of the western part of 

 the United States, 1 where cliffs are by no means confined to the 

 immediate valleys of the streams. 



Indirect effects. Through vegetation, climate influences erosion 

 in ways which are easily defined qualitatively, but not quantita- 

 tively. Both by its growth (wedge-work of roots) and by its decay 

 (supplying C0 2 , 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 unfavorable to erosion by mechan- 

 ical means, and favorable to that by chemical processes. 



EFFECTS OF UNEQUAL HARDNESS 



Irregularities of hardness give rise to many peculiarities of topog- 

 raphy, and to many scenic features. To this category belong 

 many rapids, falls, narrows, terraces, and many striking hills and 

 ridges. 



Falls and rapids. Falls and rapids are most commonly developed 

 where streams pass from more resistant to less resistant rock. The 

 greater wear of the latter gives origin to rapids. At first the 



button. Tertiary History of the Grand Canyon District, Mono. II, 

 U. S. Geol. Surv. 



