580 C. R. KEYES — RELATIVE EFFICIENCIES OF EROSIONAL PROCESSES 



from the level of the plains on either side in the basal complex instead 

 of in soft elastics, their entire courses in New Mexico would probably 

 now rival the inner gorge of the Colorado. Their valleys, in place of 

 being 20 to 30 miles broad and widely open in character, would in all 

 likelihood be not more than a mile or two in width and would be upwards 

 of 2,000 feet deep. 



The controlling gradient of the valley sides of the through-flowing 

 streams is thus determined by the arroyo grade. The general leveling 

 and lowering of the country takes place mainly through means of wind 

 scour. The through-flowing rivers carry off the rock waste fully as rap- 

 idly as do the large streams of humid climates. In addition there are 

 large volumes of rock waste and soil transported and exported by the 

 winds. 



Deflation of arid Lands 



GENERAL CONSIDERATIONS 



Although some importance has been long attached to wind as a geologic 

 process, it is not one which the text-books on geology are accustomed to 

 place in the front rank of earth modifying agencies. Moreover, the de- 

 tailed etfects of eolian activities have been chiefly considered mainly only 

 in countries of moist climates, where water is the principal erosive force. 

 Under such circumstances not only are the effects of wind work greatly 

 obscured, but the real efficiency of wind as an erosive power is naturally 

 minimized. With theoretical conditions of this kind imposed, the general 

 role of the eolian influences as active and effective geologic agents has 

 come to be regarded as quite secondary. 



To be appreciated to its fullest extent the geologic work of the wind 

 must be critically examined in regions where water action is not the most 

 powerful of the erosive influences. The deserts furnish such favorable 

 climatic conditions. In the arid regions, where rainfall is notably defi- 

 cient, the extent and nature of constant wind scour, or eolic erosion, may 

 be studied under conditions comparable to the study of hydric erosion in 

 a normally wet climate. This basic feature has been in the past largely 

 overlooked. As a result the main erosive power in the desert has been 

 regarded as water. When water was found not to be adequate to produce 

 the orographic phenomena, resort has been made to tectonics. 



The several phases of wind work may be especially distinguished in the 

 same way as are those of water action. The general process may be 

 designated as eolation, a term first defined by McGee, corresponding to 

 the water equivalent of erosion. Its subordinate activities are insolation, 



