PREVALENCE OF OLD AGE IN DESERT REGIONS 559 



The Great Basin appears to be a region long subjected to deflative 

 influences that is passing through the mature stage. 



Prevalence of old Age in Desert Eegions 

 postulated characteristics 



According to the normal standard adapted to arid conditions, topo- 

 graphic old age begins to set in when a general reduction of the high- 

 lands gives rise to a notable decrease in rainfall and consequently stream 

 action, and the process of drainage disintegration commences to pre- 

 dominate. Only at this time is wind-scour admitted to be at all effective. 



TOPOGRAPHIC FEATURES OF OLD AGE UNDER DEFLATION 



The drainage features of the latter part of the arid cycle may be en- 

 tirely neglected. Topographic expression alone can be considered. Com- 

 pared with that of maturity, there are scarcely any contrasts of relief. 

 Desert lowering will be much slower because weak rocks will have been 

 already largely removed. There is greater homogeneity of texture and 

 hardness in the older rocks than in the more recently formed sediments, 

 and consequently less opportunity for marked differential effects. 



The prevailing relief expression in arid lands must necessarily be that 

 of old age. The rapidity with which the several parts of an arid region 

 passes through the different relief stages depends partly on the strati- 

 graphic segregation of the hard as well as the soft rocks, partly on the 

 character of the deformation, partly on the nature of the geotectonic 

 pattern, and partly on the degree of aridity. Tlius it is that under the 

 same climatic conditions a certain provincial difference in the tectonics 

 of the Mexican tableland, the Great Basin, and the Colorado plateau of 

 xlrizona permits the first to represent arid youth, the second arid ma- 

 turity, and the third arid old age. 



Baselevel of Eolian Erosion 



The remarkable plains-forming tendency of deflation in dry regions is 

 one of its main characteristics. "^^ As already noted, the plain is the domi- 

 nant relief feature from the very beginning of the arid geographic cycle ; 

 in a humid climate it only becomes notably developed at the very end. 

 When Passarge'^ was conducting his investigations in the Soutli African 

 deserts he long had difficulty in understanding how it was possible under 

 conditions of arid climate for general planation of vast tracts to go on 

 without regard to sealevel, since the wind was thought to have no base- 

 level of erosion. So long as the waters of the sea are kept out, Penck^^ 

 argued that deflation could go on indefinitely below sealevel. 



'"Popular Science Monthly, vol. Ixxlv, 1900, p. 23. 



"Zeitsch. d. deut. geol. Gesellsch., vol. Ivi, Protokol, 1004, p. 191. 



