Vol. 4] 



Lawson. — Tehachapi Valley System. 



449 



Creek emerges upon the valley. Here it is apparent that the 

 apex of the alluvial cone which spreads out from this point across 

 the entire breadth of Tehachapi Valley was established when the 

 rock bottom of the creek within the mountain was much higher 

 than it is at present. The canon bottom has been corraded down 

 to a depth of 250 feet below the apex of the cone and this deep- 

 ening of the rocky canon has necessitated the dissection of the 

 higher part of the alluvial cone. There is thus exposed in the 

 walls of the canon the contact betwen the buried face of the 

 mountain and the alluvium which reposes against it through a 

 vertical range of 250 feet. This contact, while not absolutely 

 vertical, is so nearly so that the rock surface against which the 

 alluvium rests can be interpreted only as a fault scarp. That is 

 to say, it is the lower portion of the general fault scarp of the 

 mountain front which has been preserved by burial from the 

 greater part of the degradation which has affected the upper 

 portion. 



In the recognition of the fault scarp nature of this moun- 

 tain front we have evidence of a third orogenic movement, and 

 its discreteness in time from the other two movements, which 

 have been recorded as affecting particularly the north side of 

 the present valley, is proved by the fact that the alluvial debris 

 which was produced by the torrential corrasion of the uplifted 

 block, was dejected upon the broad terrace which had been in 

 part cut out of the deformed Tehachapi Alluvium. 



Mature Geomorphy. — If now we continue our consideration 

 of the south boundary of Tehachapi Valley into its more eastern 

 part beyond the portion which has just been identified as a fault 

 scarp, we come upon a totally different type of geomorphic 

 expression. Here the valley is bounded on the south by a range 

 of hills of gently flowing profiles and contours. The hill tops 

 are flatly rounded and there are no incisive canons dissecting 

 them. The slope to Tehachapi Valley is not steep and abrupt 

 but descends as an undulating surface, on the lower parts of 

 w hich there are remnants of stream terraces strewn with well 

 washed cobbles and pebbles. The height of these hills is prob- 

 ably about 1000 feet above Tehachapi Valley or less than half 

 the height of the fault block mountain immediately to the west 



