1922] Vaughan: Geology of San Bernardino Mountains 337 



The Desert 



The desert north of the San Bernardino Mountains has had a 

 physiographic history so different from that of the mountains them- 

 selves that it will receive special attention. This difference is not the 

 result simply of the difference between the effect of uplift and that 

 of depression on the gradients of the streams, but is also a result of 

 the difference in the climate and the processes at work. 



South of Old Woman Springs there is a considerable area of basalt 

 resting on a nearly flat granitic basement. This affords a clue for the 

 correlation of part of the desert surface with that of the mountains, 

 for, if the basalt is the same as that east of The Pipes, then the under- 

 lying granitic surface is probably the same in the two cases. Also, 

 the flat surface on the two areas of basalt may be correlative. Three 

 miles north of Old "Woman Springs, there is a similar area of basalt 

 resting on an old surface. Other small areas are found between this 

 place and Negro Butte, and of course these all help to convey the idea 

 that the pre-basalt surface was one of low relief and that the desert 

 and present mountains were at that time a continuous geomorphic 

 feature. 



Following the period of lava extrusion there was considerable 

 faulting throughout the region, and the comparison between the physi- 

 ographic development in the desert and in the mountains becomes very 

 difficult. Naturally the faulting increased the relief in the desert; 

 but the movements were not great, for we still find remnants of basalt 

 on Negro Butte and some of the nearby hills and also south of the hill 

 east of Negro Butte. The vertical offsets of these high and low areas 

 of basalt are not more than 500 feet. Fry Mountain is capped with 

 basalt which slopes 15° to the north and is continuous with a fault 

 area more than two miles in length. On the upper part of this hill, 

 extending southeastward from the peak and just below the crest of the 

 ridge, there are remnants of an old valley with stream beds of low 

 gradient. This part of the drainage system of the hill is in marked 

 contrast with the southwest margin, where the stream beds drop off 

 precipitously through steep, narrow canons. It is therefore certain 

 that during the uplift of Fry Mountain there was a period of rest, and 

 this was near the beginning of the movement. That the major move- 

 ment of the mass is of comparatively recent date is proved by the 

 geomorphic discordance. In consideration of this fact and of the 



