24 



University of California. 



[Vol. 3. 



cuts splendidly expose the material forming the floor of the 

 basin. It is everywhere in this region stratified gravel and 

 sand, the bedding being of the irregular sort common to alluvial 

 deposits. Fine cross- 1 tedding is developed in certain layers. 

 The material is not strictly local, as in ordinary detrital slopes, 

 and is more or less waterworn. 



Close to the mountains, as at the northern end of Cajon 

 Pass, the gravel deposit has been somewhat disturbed, and in 

 consequence deeply and extensively eroded. A western tributary 

 of Mohave River heads at Summit Station, and, flowing easterly, 

 has eroded a valley between the gravel deposit on the north and 

 a granite mountain on the south. Near the head this valley is 

 five hundred feet deep and a veritable canon. 



By headwater erosion, Cajon Creek has cut back through the 

 pass into the gravel on the north of the mountains, so that the 

 present summit is a ridge of Quaternary gravel and sand north 

 of the gap in the high granite range. The gulches at the head 

 of Cajon Creek are cut down into the gravel and sand deposit to 

 a depth exceeding 1,000 feet, and they splendidly expose its 

 interior to the very base. From Hesperia to near Summit, the 

 deposit remains in virtually a horizontal position, only sloping 

 to the north with the general and even slope of the basin floor; 

 but in the vicinity of Summit the gravel begins to show 

 tilting toward the northeast at a low angle, and from there to 

 the border of the granite mountains it is nearly everywhere 

 tilted to the north and northeast at angles of 10° to 15°, or 

 locally even greater. Looking toward Cajon Pass from Oro 

 Grande or Helen Station, the floor of the basin is seen to rise 

 evenly till near the pass, where a low and somewhat broken 

 ridge extending across the gap shows an abnormal elevation of 

 the gravel by orographic uplift near the mountains. 



The total thickness of the deposit as exposed in the head of 

 the valley of Cajon Creek must be about 2,000 feet. No 

 unconformities are apparent, but toward the base the deposit 

 becomes finer and more regularly stratified, much of it being 

 sand and sandy clay, with no pebbles or boulders. This 

 lower portion somewhat resembles the Upper Pliocene deposits. 

 Gravel occurs in abundance higher and some boulder beds are 



