596 



C. R. KEYES^ RELATIVE EFFICIENCIES OF EROSIONAL PROCESSES 



The evolution of an arid cycle from initial conditions of a plains sur- 

 face, analogous to the peneplain of the humid cycle, and without material 

 aid from running water, give land forms something as represented in 

 outline in the subjoined diagrams (figure 6). Contrasted with the geo- 

 graphic features of infancy under conditipns of a humid climate, the first 

 excavations are, instead of deep V-shaped channelways in the old pene- 

 plain, broad, plain-like valleys (figure 6a). The best example, perhaps. 



Figure 6. — Profile of Arid Physiographic Stages 



in the American arid country is in the Eaton Eange. There are other 

 plateau plains in the region where, on account of vast outpourings of 

 lava, deflation has lagged behind the general lowering of the country and 

 where the earliest inroads of erosion are now taking place. In this con- 

 nection the Chupadera Mesa, in central N'ew Mexico, may be noted 

 especially. 



Arid youth I take to be typically expressed in the northern part of the 

 Mexican tableland, including central New Mexico and southeastern Ari- 

 zona. The general plains surface is now a mile above sealevel, and the 



