396 University of California Publications in Geology [Vol. 13 



The general structure east of Whitewater River is simple, but in 

 detail it is somewhat complicated by faults which have cut the Coa- 

 ehella fanglomerate into numerous blocks variously tilted (pi. 22A). 

 Along the crest of the ridge the strata dip 20° to 40° to the east, but 

 flatten out toward the east and clip beneath the Cabezon fanglomerate. 

 North of Red Dome horizontal beds of Cabezon fanglomerate overlie 

 lithologically identical beds which have dips up to 35°. This is prob- 

 ably due to their proximity to the Mission Creek fault, which might 

 bring about structural complexities, even though the general conditions 

 of deposition were unchanged and the time short. 



Quaternary faults. — A notable feature of the geologic structure of 

 the San Bernardino Mountains is a great system of Quaternary faults 

 whereby the mass has been uplifted and divided into several blocks. 

 Along the north side of the range is a prominent fault, the expression 

 of which, as a degraded scarp, is particularly bold between Black- 

 hawk Canon and Silver Creek (pi. 22B). From the head of Arctic 

 Canon the general relations can be seen clearly. To the south is the 

 rounded topography of Holcomb Valley. Immediately to the north 

 is the rugged, steep slope beyond which is the Mojave Desert. The 

 rounded topography of the second cycle, as exemplified by the country 

 around Bear and Holcomb valleys, represents late maturity ; the width 

 of the valleys and the gentle slopes which prevail indicate that the 

 streams which excavated them must have nearly reached base level. 

 Its present position, therefore, is discordant with its geomorphic devel- 

 opment and can only be explained by uplift. This fact, together with 

 the actual scarp, indicate the presence of a fault which is readily 

 traceable to the west beyond the quadrangle and to the east as far as 

 Blackhawk Canon. 



Just out from the base of the scarp and extending from Silver Creek 

 to Cushenbury Springs there is a more or less dissected bench consist- 

 ing partly of fanglomerate and partly of granite and limestone. At 

 the mouth of Furnace Canon, on the west side, this is represented by 

 a long flat-topped granite ridge. The interpretation of this is some- 

 what uncertain. It may mean that the uplift of the range was along 

 the face of this bench and that erosion cut back from the fault and 

 developed the surface of the bench. A recent movement would have 

 elevated it to its present position. There is also the possibility that 

 the surface of the bench represents an uplifted portion of the old 

 desert floor and that there has been step-faulting. 



