338 University of California Publications in Geology [Vol. 13 



preservation of the numerous areas of basalt, it seems probable that 

 at no time since the development of the pre-basalt surface has the 

 relief of this portion of the desert been much greater than at present. 

 This statement probably includes the hills west of Rabbit Springs 

 and around Means Wells, for the relief is of the same magnitude as in 

 the vicinity of Fry Mountain and Negro Butte. 



The topographic history of the desert cannot be separated into 

 parts comparable to the second, third, and fourth cycles so clearly 

 identified in the mountains. The changes which took place in the 

 desert during the development of the second cycle in the mountains 

 are still in progress, and a description of the processes now at work 

 and the forms developed by them must suffice. 



The erosional processes in the desert have certain distinguishing 

 features occasioned by aridity : the rocks break down by mechanical 

 disintegration rather than by chemical decay ; no definite drainage 

 system is maintained ; the distance of transportation of products of 

 disintegration is very limited. Another important consideration is that 

 the rainfall, though small in the aggregate, is torrential when it does 

 come. The topography consists of three elements : the rock masses, 

 the alluvial fans, and the playas. Lawson 5 has discussed the develop- 

 ment of the desert surface at length and certain rock forms which he 

 develops from theoretical considerations are well exemplified in this 

 region. 



The exposed rock masses break down by mechanical disintegration 

 and the products of this action are removed by rain wash and depos- 

 ited on the lower slopes. As this goes on the face of the exposed rock 

 mass retreats, leaving a bench protected by detritus. The retreating 

 rock face has been termed "The Subaerial Front," and the bench 

 "The Suballuvial Bench." It is an interesting fact that the subaerial 

 front maintains its steep slope as long as it exists at all. The coarser 

 material forming the alluvial embankment is on the higher slope and 

 may contain angular boulders of considerable size. Farther down the 

 material becomes finer and finally out on the playas it is fine silt. 



The granitic hills west of Negro Butte are a good example of desert 

 topography. They are extremely rugged, rather steep, and with the 

 exception of the north side they are approached by alluvial fans right 

 up to the bold rock masses. There are several isolated salients and 

 the general outline of the hills is very irregular. The alluvial fans 

 intersect those from other hills or end in playas. On the north side 



5 Lawson, A. C, The Epigene profiles of the Desert, Univ. Calif. Publ. Bull. 

 Dept. Geol., vol. 9, no. 3, pp. 23-43. 



