NORMAL WATER ACTION IN DESERT REGIONS 561 



by water. The extent and character of water action are fully considered 

 later. From the very nature of the special climatic conditions imposed 

 by aridity, it follows that the erosional effects of the aqueous agencies 

 must be reduced far below what is commonly expected of them. It is 

 customary to regard desert landscapes as examples of normal water cor- 

 rasion identical in origin with those of moist climates except that it is 

 perhaps somewhat less rapid. Tn the present connection the importance 

 of water action in matters of landscape details is not questioned; but 

 the very secondary influence of stream corrasion in its broader operations 

 is premised, and as a general erosional agency the dominance of wind- 

 scour is recognized. Quantitative data on the relative efficiencies of the 

 two processes are at hand and they are discussed at length in another 

 place. Here the general results need be only briefly anticipated. 



Water action in desert regions assumes three distinctive aspects : That 

 produced by the through-flowing rivers, that of the intermittent torren- 

 tial arroyos of the mountains, and that of the rare and brief sheet- 

 floods. It is the second of these phases which mainly attracts the atten- 

 tion of the sojourner from less parched parts of the world. With a 

 natural proneness to extend his moist-climate conceptions, the impres- 

 sion is at first gained that nowhere else is there so eloquent attest of 

 energetic storm work as is presented on the desert ranges. Indeed the 

 dominant characteristic of the arid sierras is notable ruggedness. It is 

 apparently the same type of ruggedness which in moist-climate countries 

 is by general consent ascribed to vigorous stream work on a recently 

 upraised mountain tract. 



Preconceived notions concerning general erosional effects under moist- 

 climate conditions can not be in toto successfully transplanted to arid 

 lands. The sharp meeting of plain and mountain without the interven- 

 tion of foothills is certainly not a marked characteristic of water sculp- 

 turing in the mountains of moist climates. With much less amount of 

 water involved, how may it be plausibly converted into a conspicuous 

 feature in dry regions? The Castle Domes, Eagle Tails, Harquahalas, 

 and Plomas ranges of southwestern Arizona are notable examples. In 

 cliffs, picachos, minor and major crests they rise steeply out of the gen- 

 eral plains surface. Yet the annual rainfall of this district is less than 

 3 inches. Instead of being distinctive forms produced solely by water 

 action, there appear to be nowhere else so conspicuous illustrations of 

 general undercutting of hard masses of mountain rock by the wind armed 

 with sharp sands and aided by insolation. The locus of maximum lateral 

 deflation action, as has been recently shown,'^ is at the level of the general 



'» Science, n. s., vol. xxix, 1909, p. 752. 



