EFFECT OF CLIMATE ON EROSION 83 



could get all the material it could carry, erosion would be at a maxi- 

 mum; but if the amount of material available for transportation is 

 slight, a large part of the force of the water could not be utilized in 

 erosion. While, therefore, it is not possible to say what distribu- 

 tion of rainfall favors most rapid erosion without knowing the 

 nature of the surface on which it is to fall, enough has been said to 

 show that the problem is not a simple one. Some of the most 

 striking phases of topography developed by erosion, such as those 

 of the Bad Lands (Figs. 68 and 69), are developed where the rain- 

 fall is distributed unequally in time, and too slight or too infrequent 

 to support abundant vegetation. 



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. If the arid region is high and 

 composed of heterogeneous strata, the topography w r hich erosion 

 develops is more angular (Fig. 70) 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. 70 and 73). 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 imme- 

 diate 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 CO2, etc., to descending waters), vegetation favors cer- 

 tain phases of weathering; but, on the other hand, it retards corra- 

 sion and transportation both by wind and water. This is well 

 shown along the banks of streams and on the faces of cliffs com- 

 posed of clay, sand, etc. Its aggregate effect is probably unfavor- 

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

 chemical processes. Winds have much to do with the rate of 

 evaporation and the distribution of rainfall, so that their indirect 

 effect on erosion is important. 



1 Button. Tertiary History of the Grand Canyon District, Mono. II, U. S. 

 Geol. Surv. 



