1915] 



Lawson : The Epigene Profiles of the Desert 



27 



classification of the salient features of the relief of the desert may 

 attain a notable degree of exactitude if mountains lithologically similar 

 be compared within the same climatic province. 



The degree of acclivity of the epigene slopes of the desert is, on 

 the other hand, no indication of the stage of advancement of the 

 degradational cycle. Under humid conditions hard rocks that are sus- 

 ceptible of chemical decay, and so form soil, tend to acquire more and 

 more gentle slopes as time goes on. A steep slope in such rocks is 

 indicative, in general, of geomorphic youth, whereas a gentle slope 

 is characteristic of old age. In the desert, however, hard rocks present 

 persistently steep slopes throughout the entire period of their degrada- 

 tion. The epigene rock slopes appear to be just as steep in old residual 

 mountains, almost buried in alluvium, as in youthful mountains with 

 but a small embankment of detritus at their base ; and it will be shown 

 that they are in reality somewhat steeper. 



If the mountain mass be heterogeneous the rock slope may not be 

 uniform, and it may so far depart from the law governing the slopes 

 of homogeneous material that it will assume temporarily the form of 

 a vertical cliff. This is particularly well exemplified where a hard, 

 resistant stratum lies at low angles upon soft beds, as in the case of 

 the Vermilion Cliffs. Here the factors of heterogeneity and structure 

 dominate the degradational process and the climatic control is relatively 

 slight. The result is an escarpment which is not peculiar to the desert, 

 but which may equally well be produced in humid climates, where the 

 same structure and lithologic heterogeneity prevail, as is exemplified 

 in the Niagara escarpment. 



But even where the condition of approximate homogeneity obtains 

 it is possible that other agencies than those noted may modify the 

 profile. The wind may be both a transporting and a corrading agent, 

 making for aggradation in one place and degradation in another. 

 While this agency is recognized as of possible importance in regions 

 where powerful winds prevail, particularly where mountains are largely 

 composed of soft, fine-grained, incoherent rocks, it will not here be 

 further considered. The wind, in regions of hard elastic rocks such 

 as are commonly found in the ranges of the Great Basin, is an 

 extremely inefficient agent in the evolution of the profiles of the relief. 

 It is concerned chiefly with local transport of the silts and sands of 

 the playas and with fitful whirling of sand over the fan slopes. The 

 occasional sandstorms doubtless carry notable quantities of fine sand 

 from one part of the desert to another and to regions beyond its 



