404 University of California Publications in Oeology [Vol. 13 



largely of beds of coarse sandstone of granitic detritus from 5 to 6 

 feet thick, between which are finely laminated shales and some calca- 

 reous streaks. That these are in an abnormal position is shown by 

 the fanglomerate, containing huge blocks of granite over 8 feet across, 

 which overlies them unconformably, and is the normal accumulation 

 to expect in this position. The sandstone and shale have been tilted 

 to dips up to more than 30°, and this consideration alone is proof of 

 some sort of orogenic disturbance. There are two hypotheses as to 

 the nature of the faulting by which these sediments reached their 

 present position : first, the whole mountain range is an overthrust 

 block and the sandstone is exposed through a ' ' f enster ' ' ; second, the 

 sandstone is part of a graben. The evidence at hand favoring the 

 overthrust hypothesis is very meager. An overthrust block of suffi- 

 cient magnitude to bring the sandstone to the bottom of Santa Ana 

 Canon from either the north or south sides would require a strong 

 backing; but the San Bernardino Mountains rise rather abruptly 

 with no such support on either side. On the other hand, the possi- 

 bility of the sandstone having been faulted downward has much in 

 its favor. All the large faults in the region are practically vertical. 

 The mountain range itself has been raised between faults on the north 

 and south sides. The faults in the desert to the north are also vertical. 

 Since vertical faulting is so prominent a feature in the structure of 

 the range, it seems most probable that in some way the Santa Ana 

 sandstone-shale formation has been dropped relative to the schists and 

 gneisses on either side. This has been shown on the map by a simple 

 fault on each side of the canon, but in reality there may be a very 

 complicated system of faulting. 



There is some evidence of a fault along the north side of Bear 

 Valley. As compared to the south side and to the sides of Holcomb 

 Valley, it is rather straight. The most suggestive fact, however, is 

 that Holcomb Valley, which resembles Bear Valley in its topographic 

 development, lies 600 feet higher. Van Dusen Canon and the canon 

 just east of Poligue have cut back from Bear Valley with steep gradi- 

 ents and now tap Holcomb Valley. These facts certainly point to 

 faulting as explanation, although they do not actually prove the case. 



The nature of the faulting of the region may best be understood 

 from a consideration of the topography. This clearly points to a 

 progressive movement by several stages. The several fanglomerates 

 along the south front of the range with their unconformities confirm 

 this record. 



