1922] 



Vaughan 



Geology of San Bernardino Mountains 



401 



above the floor of the San Gorgonio Pass. If this were entirely due 

 to erosion we should expect to find similar rocks at the same elevation 

 on the south side of the pass, but such is not the case. The align- 

 ment of the south side of the fanglomerate hills is even more sugges- 

 tive, the rectangular outline of the block west of Whitewater Canon 

 being particularly noticeable. The absence of such an area immedi- 

 ately east of Stubby Canon is probably due to the down-faulting of a 

 block. Further evidence of this fault south of Banning Height lies 

 in the fact that all streams wash the opposite side of the pass. The 

 bench has not been produced merely by streams cutting the pass down 

 and leaving the bench as a terrace. Also, on the opposite side of the 

 pass, south of Beaumont, the slope of the pass grades into the hills, 

 just as those on the bench grade into the surrounding hills. If the 

 searp were purely an erosional feature, we would expect to find one 

 on the south side of the pass similar to that on the north. 



The evidence thus adduced indicates, not only the presence of the 

 fault, but that there has been rather recent movement with downthrow 

 on the south side. This is of little importance, however, as compared 

 to earlier movements by which a great block was raised to form the 

 San Jacinto Mountains. South of Whitewater these present a bold 

 scarp rising about 9000 feet above the floor of the pass. Toward the 

 west the height of this scarp gradually diminishes and south of Beau- 

 mont it is insignificant. The San Jacinto Mountains slope to the 

 southwest, the sloping surface being of low relief and in marked con- 

 trast to the rugged declivity to the north. It therefore appears that 

 the San Gorgonio Pass was formed by a narrow graben between the 

 San Bernardino and San Jacinto mountains. Since the degradation 

 of the fault scarp on the south side of the pass the down-dropped 

 block has risen somewhat with respect to the San Jacinto block. 

 Therefore the present floor of the pass is to the south of the original. 



The south side of the range east of Mission Creek is a degraded 

 fault scarp, as is shown by its remarkable straightness and the fact 

 that when the line is continued to the west the fault itself may be 

 seen. Looking east from the crest of the ridge a quarter of a mile 

 northeast of Hog Ranch, a decided depression may be seen between 

 the fanglomerate and the schists. On the west side of the first small 

 ridge the fault is distinct and on it the schists have been overthrust 

 on the softer fanglomerate, the dip of the fault plane being 6!3° to 

 the north (fig. 11). On the west side of Mission Creek the fault trace 

 follows up a canon with fanglomerate on the south and schist on the 

 north and descends the west side of the ridge through a similar canon. 



