RELATIVE ASCENDENCY OF FAULTING AND EROSION ^59 



2. Where erosion has acted unequally along a fault on account of the 

 difference as to hardness between two rocks forced into juxtaposition or 

 between a crushed zone and an intact one the resultant cliff may be 

 termed an erosion fault-scarp. 



In the simple fault-scarp the cliff is always on the upthrown side of the 

 fault. In the erosion fault-scarp it may be on the upthrown or down- 

 thrown side, according to local conditions. In the former case it may 

 be called a normal erosion fault-scarp ; in the latter, a reversed erosion 

 fault-scarp. It is generally difficult, without collateral evidence, to dis- 

 tinguish a simple fault scarp from a normal erosion fault scarp. 



Since faults are closely related to folds as deformation features, their 

 stages of relative ascendency over erosion are also analogous. The sim- 

 ple fault-scarp is analogous to the anticlinal ridge of deformation, the 

 normal erosion fault-scarp to the anticlinal ridge of erosion, and the re- 

 versed erosion fault-scarp to the synclinal ridge of erosion. 



Where the solid rock on both sides of a fault is equally resistant, but 

 there is a zone of crushing, erosion produces a gully ; where the move- 

 ment has not materially weakened the rock the fault may have no effect 

 whatever on the topography. 



The accompanying sections of actual faults (see plate 23 and plate 24, 

 figures 1, 2, and 3) illustrate their various relations to topography. They 

 have been purposel}^ taken from the Great basin, and, so far as possible, 

 from other observers than the writer. 



RELATION OF FAULTS TO GREAT BASIN TOPOGRAPHY 



There is a general impression that the Great Basin mountain scarps 

 are topographic features peculiar to the region. Judging from his own 

 observations the writer believes this erroneous. These ranges have 

 about the same amount of steep or perpendicular faces as other moun- 

 tains—not so large as some and greater than others. Their most typ- 

 ical form of front has indeed a fairly moderate slope (see plate 21.) 



Nevertheless there are frequent bolder faces or scarps which might be 

 assumed to be due to faulting. Studying the field without prejudice, 

 the writer could not long entertain this general idea of their origin, since 

 he found no evidence positively corroborating it and much against it. 

 The points of unfavorable evidence are briefly as follows : 



(1) The faults actually observed in this region are comparativel}^ 

 few. Actually ascertained heavy faults along the main fronts of ranges, 

 are exceedingly rare. 



(2) The transverse faults which run across the general trend of the 

 Basin ranges are, so far as observed, equally or more numerous than the 



