598 C. R. KEYES RELATIVE EFFICIENCIES OF EROSIONAL PROCESSES I 



on their way to the sea, having their origin ontside of the area. Probably 

 their presence has tended more than any other one factor to prevent a 

 proper and early nnderstanding of desert erosion. 



5. In the arid region eolation is the chief gradational power to be 

 reckoned with, and its actual potency is probably far greater than water 

 erosion under the most favorable conditions. The characteristic aspects , 

 of the entire desert landscape are to be mainly ascribed to the pecnliai- | 

 ities of deflative sculpturing powers. ■ 



6. Deflative action is chiefly destructive in character; its constructive 

 effects, at least within the boundaries of the dry country, are ephemeral 

 and relatively unimportant. The clean swept rock floors of many inter- 

 mont plains clearly indicate that the latter are areas of constant denuda- 

 tion rather than of great aggradation, as has been generally supposed. 



7. The general relief tendency of deflation is preeminently plains- 

 forming. From first to last of the geographic cycle in an arid climate ' 

 the plain prominently persists. I 



8. In the arid region of western America a thickness of no less than I 

 5,000 feet of rock has been removed in attaining the present plains level. 

 This vast amount of excavation seems to be largely the work of the winds 

 alone. The process is going on today as actively as it has at any time in 

 the past. It is probable that the general lowering of the desert country 



is much more rapid than that of general stream corrasion toward base- 

 level in a humid land. 



9. The American arid country was at the beginning of the present 

 geographic cycle essentially a peneplain recently uplifted. This conclu- 

 sion is believed to be amply attested by the great remnantal plains which 

 are still found standing high above the existing plains surface. Above 

 the latter more than 4,000 feet rises, for instance, the unique Mesa de 

 Maya. The numerous plateau plains of the dry region appear to have a 

 like significance. | 



10. The evolution of the present relief expression of the desert country i 

 is not believed to be from a surface initially of rugged mountainous I 

 topography and through the sculpturing agencies of running water, but 



is thought to be from a plains surface through eolation principally. 



11. The origin of the Basin ranges and of the desert ranges generally 

 is, therefore, regarded as due in the main to extensive and vigorous differ- 

 ential deflation on a region that had been previously flexed and pro- 

 foundly faulted and then planed off, bringing narrow belts of resistant 

 rocks into juxtaposition with broad belts of weak rocks, the former now 

 forming the desert highlands and the latter the desert lowlands, or inter- 

 ment plains. 



