RELATIVE ASCENDENCY OF FOLDING AND EROSION 257 



dence of having been determined by long continued erosion. The 

 evidence may consist in close association with synclinal ridges of the 

 same age or by the presence within the anticlinal ridges themselves of 

 faults whose surface displacement has been effaced by erosion. The 

 cores of the anticlines are generally of more resistant rock than the other 

 strata, and have therefore persisted as ridges (see plate 24, figure 6). 



THE MONOCLINAL RIDGE OF EROSION 



Monoclinal ridges are usually minor and non-persistent features. They 

 generally occupy a single limb of a fold. When a deep valley has been 

 eroded along the axis of an anticlinal mountain and, together with the 

 valleys on either side of the main range, has been partially filled with 

 wash, it will then be transformed into a strip of desert dividing two mono- 

 clinal ridges, facing in opposite directions. Each limb of such a fold 

 may be made up of thick strata, which erosion, working most power- 

 fully along softer beds, may carve into a series of monoclinal ridges, all 

 facing the same direction (see plate 24, figure 5). 



THE ANTICLINAL RIDGE OF DEFORMATION 



A fourth and unusual class comprises anticlinal mountains without 

 cores of relatively greater resistance. These ranges generally consist 

 partly or wholly of upturned Tertiary rocks; therefore their upheaval 

 must have been comparatively recent. In some cases movement has 

 probably contiimed into the Pleistocene. Such mountains seem to be 

 the result of folding so rapid that erosion has been left behind. The 

 aridity of the region has, of course, been favorable to this result (see 

 plate 24, figure 4, and plate 25, figure 4). 



Eelative Ascendency of Faulting and Erosion 

 principal ascertained fa ults of great basin 



The faults in the Great basin belong to general systems. The two 

 chief sets run north-and-south and east-and-west, while important sets 

 are oblique, one trending northeast and another northwest. Following, 

 on page 258, is a rough table showing localities of actually ascertained 

 heavy faults. 



The east-west faults seem as numerous as the north-south ones and 

 their displacements equally great. 



The fault systems are intimately related to corresponding systems of 

 folds. Throughout most of Nevada the axes of folds trend north and 

 south, but there has been much minor compression along east-west lines, 



XXXVII— Bull, Geol. Soc. Am., Vol. 12, 1900 



