Vol. 4] Lawson. — Tehachapi Valley System. 439 



out to Mohave is wider in its bottom, carries little water except 

 during heavy rains, and in its upper part is encumbered with 

 stream gravels and sands. In its lower part, however, the grade 

 becomes too steep for this stream drift to linger on the canon 

 floor and the stream runs on bed rock. On the whole, the 

 Mohave outlet of the valley presents a more mature aspect than 

 the gorge of Tehachapi Valley. The divide between these two 

 drainages lies in the nearly flat, expansive, alluviated floor of 

 Tehachapi Valley near its middle part. The explanation of this 

 anomalous drainage must be deferred till certain other features 

 of the geology of the valley are presented. 



Although the fossil leaves of the sandstones and shales of 

 Cache Creek indicate that they were laid down in a fresh-water 

 basin, it is not supposed that this basin had any structural rela- 

 tion to the present Tehachapi Valley. The Cache Creek beds 

 are apparently much more extensive to the northeast than the 

 limits of the valley in that direction. Cache Creek Valley, more- 

 over, is a valley of erosion cut in part out of the sandstone 

 formation. As will be seen later there is ground for believing 

 that Cache Creek Valley, as a geomorphic feature, probably ante 

 dates the main Tehachapi Valley. The tuffs and agglomerates 

 of White Rock Creek are probably of about the same age as 

 the sandstones and were doubtless laid down in the same basin. 



In the northwest corner of Techachapi Valley, however, there 

 » is a group of formations which seem to have a somewhat more 

 intimate relation, as regards their basin of accumulation, with 

 the present Tehachapi Valley. These formations will be first 

 listed in the order of age and their characters and relationships 

 will then be briefly reviewed : 



1. An Ancient Alluvium, here designated the Atlas Forma- 



tion.- 



2. Andesitic lava flows and tuffs, here designated the Tank 



Volcanics. 



3. Fresh-water lake beds, here designated the Cable Forma- 



tion. 



4. Post-lacustrine alluvium, here designated the Tehachapi 



Formation. 



