1922] Vaughan: Geology of San Bernardino Mountains 



325 



boulders and resembling crude monuments made by placing one stone 

 above the other. Such erosional forms are usually found in desert 

 regions which have become so reduced that the amount of wind-blown 

 sand is considerable. The results of present erosion here are quite 

 different ; many of the boulders are angular, and some of these are in 

 place where it is evident that they were derived by some sort of joint- 

 age. "While some have been rounded, this is not due to true exfoliation, 

 i.e., to spherically developed breaks, but to an irregular system of 

 cracks which can be seen near the surface of the rock (fig. 1). 



Fig. 1. Weathering of granite at San Gorgonio Mountain. Scale about 

 % nat. size. 



Below the basalt and between some of the basalt flows in the hills 

 east of The Pipes are beds of arkose. This material contains mica and 

 feldspar, as well as quartz, and appears identical with the material 

 now being derived from the granite in the arid hills to the south. It 

 is therefore probable that the surface on which the basalt rests was 

 developed under arid conditions, and that these conditions persisted 

 until some of the basalt had flowed over the region. 



The physiographic forms representing the second and third cycles 

 of erosion in the San Bernardino Mountains so resemble each other 

 and so grade into each other that their separation is often impossible, 

 and it seems, therefore, most convenient to discuss them together rather 

 than to treat each one separately. 



The second cycle is exemplified by the country around Bear and 

 Holcomb valleys. With its low hills and rounded outcrops of granite, 

 this area is in marked contrast with the more rugged country to be 

 seen- on all sides. Many of the hills are covered with debris on the 

 lower slopes, while on the upper slopes are often rugged outcrops ; as 

 at Gold Mountain, where the country rock is quartzite. On the south 

 side of Bear Valley the detrital slopes are similar. 



The streams of low gradient flowing through the meadows are the 

 headwaters of streams which plunge down steep canons in their lower 

 courses. The whole aspect of the country is that of a mature surface 



The Second and Third Cycles 



