Hershev. 



Quaternary of Southern California . 



25 



present. By observing the inclination of flattened cobbles 

 I once thought the stream had flowed south through the pass, 

 but on other grounds I now think it did not. The cobbles and 

 boulders are of a great variety of granite and schistose rocks, 

 which are in place in the mountains about the pass. There 

 is little Tertiary volcanic material (none was with certainty 

 identified), such as must have been brought by a stream flowing 

 from the north. The gravel deposit is not developed in the pass 

 proper nor on the south side of the .mountains. My explanation 

 of it is as follows: 



At about the opening of the Quaternary era the Sierra Madre- 

 San Bernardino Range was differentiated from the country on 

 the north by the formation of a great fault along the northern 

 face of the mountains. This converted the Antelope Valley 

 area into a topographic depression, an enclosed basin. Mohave 

 River and its headwater tributaries carried sand and gravel out 

 of the high mountains about Cajon Pass and spread it over the 

 floor of Antelope Valley. Being in virtually an enclosed basin, 

 this alluvial gravel and sand began to accumulate at the time of 

 inception of the fault (which is presumed to have been at about 

 the close of the Pliocene period) and continued uninterruptedly 

 until about the close of the Red Bluff epoch, when the deposit 

 was uplifted and its dissection began. It may, therefore repre- 

 sent the whole of the Quaternary era until about the Illinoian 

 epoch of Eastern States geology. The deposit is, in fact, a 

 gigantic alluvial fan, 20 miles or more in length, gradually 

 thinning from a maximum thickness of at least 2,000 feet, and 

 merging into the ordinary detrital slopes, I suppose the early 

 Quaternary portion of the deposit to be confined to this one fan, 

 or, at any rate, to Antelope Valley. 



Near Newhall, in the valley of the Santa Clara River, there 

 has been, over an area of possibly fifty square miles, a most 

 remarkable tilting of the late Quaternary river terraces. They 

 are inclined toward a northwest- southeast axis on the line of 

 Newhall and Castaie stations. The slope is not infrequently 2° 

 to 5° and may even reach 10° in places. This is too steep for 

 detrital slopes in that region; besides, the same terraces in 

 Soledad Canon nearby remain horizontal. There are no 



