ABSTRACTS OF PAPERS 65 



FALSE FAULT-SCARPS OF DESERT RANGES 

 BY CHARLES KEYES 



{ Abstract) 



Many of the conspicuous scarps bounding the mountain ridges of arid lands 

 ])rove not to be features of faulting, as commonly supposed, nor to have any 

 relationships with dislocations of any kind. When the major fault-lines of 

 some of these mountain blocks are finally located, they are found usually to be 

 not at the foot of the orographic slopes, but miles out on the adjacent plains. 

 Although Dr. J. E. Spurr's recent statement that no one has ever seen such 

 fault-planes among the Great Basin ranges may be too sweeping in character, 

 it appears to be nevertheless a fact that the majority of the recorded cases re- 

 quire rigid verification before their accuracy may pass unchallenged. 



In the case of certain desert ranges, as that of Fra Cristobel, an exception- 

 ally rugged block of tilted limestones in central New Mexico, the scarplike 

 face which rise.s out of the general plain of the Jornada del Muerto, is 500 

 feet high, but it is situated on the side of the mountain from which the strata 

 strongly dip. In other instances the mountain ridges present the so-called 

 fault-scarps on the side where the dips are into the mountain. Because of the 

 fact that the blocks are apparently upraised more on this side, and thus tilted, 

 the usual inference that there must be faulting to accovuit for the phenomenon 

 is not alwaj's correct. In still other cases, as the well known Sierra de los 

 Caballos, the mountain ridge is bounded on both sides by notable scarp-faces. 

 Finally, mountain blocks are not infrequently completely girdled. 



The unbroken mountain block, which is oftentimes three to five times the 

 width of the present mountain base, is found to be cut back on a level with 

 the general plains surface in the same way that on an exposed shore of the 

 ocean a great .sea-cliff is formed. 



This distinctly girdled belt appears to mark the zone of maximum lateral 

 deflation. Varying hardness of the rocks, their diverse attitudes, and their 

 relative arrangement or alternation seem to offer little resistance to the uni- 

 form extension of the surrounding plains. By means of the continual grind- 

 ing action of moving or wind-driven sands, constant flaking of soilless rock 

 surfaces, and deflation of comminuted rock-waste there appears to be, where 

 plain and mountain meet, a narrow zone, just above the sloping surface of the 

 former, where the destruction of the positive features of landscape goes on 

 much faster than anywhere else. 



At the base of the mountains the remarkable truncation of the transverse 

 ribs, which is so characteristic of so many of the desert ranges, is believed 

 to be not evidence of tlie existence of noteworthy faulting, as is so generally 

 assumed to be the case, but a result of the sharp contest between wind and 

 water to master the local features of land sculpturing. 



Presented bv title in the absence of tlic author, 



