440 University of California Publications. [Geology 



Atlas Formation. — This formation is known from three ex- 

 posures, one at the nose of the ridge one mile west of Tehachapi 

 station on the north side of and immediately adjacent to the 

 railway; another in the cut one-eighth of a mile below Cable, 

 just north of the steel bridge; and the third in the bottoms of 

 certain canons tributary to Tehachapi Creek on the east side of 

 the railway and about midway between Cable and Tehachapi 

 stations. In all three of these exposures the formation has the 

 same characters and the same stratigraphic position. It is made 

 up of angular fragments of rocks of the Bed-rock Complex 

 including schists, quartz-diorite, granite, etc., pieces of quartz 

 and considerable granite sand or arkose. The fragments are 

 frequently from 6 to 12 inches in diameter but are prevailingly 

 less than 6 inches across. This aggregate presents the normal 

 characters of a typical alluvium, such as is deposited at the 

 mouth of a high grade, short mountain stream. This alluvial 

 debris has been well cemented ; but since its cementation it has 

 been thoroughly decomposed, so that the blocks and smaller 

 fragments of the crystalline rocks are now soft and incoherent 

 and crumble under the pressure of the fingers. Independently 

 of the stratigraphic position of this formation, it is easily recog- 

 nized and distinguished from other alluvial accumulations of the 

 region by these two characters : viz : its cementation and its de- 

 cayed condition. In the railway cut one-eighth of a mile below 

 Cable, near where one enters the rocky Tehachapi canon, there 

 are exposed 12 feet of this formation. Above it and resting strat- 

 igraphically upon it are 3 feet of yellowish white volcanic tuff; 

 and above this there is a thick sheet of andesitic lava. There 

 is no doubt, therefore, that the alluvium antedates the only vol- 

 canic rocks that are known in the region. 



The same relations are revealed in the canons east of the 

 railway between Cable and Tehachapi. Here the streams have 

 cut down through stratified tuffs into the Atlas Alluvium which 

 is thus revealed to a thickness of perhaps 30 feet. 



The thickest exposure of the formation is the first one men- 

 tioned at a point about a mile west of Tehachapi station. Here 

 there are about 50 feet of the formation shown in the railway 

 cut and in the adjacent steep slopes. A little beyond this point 



