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University of California Publications. [Geology 



supposition. Although rocky slopes, they have a mature aspect, 

 and the contour of the valley edge is indented with broad, wide- 

 open embayments separated by narrow ridges or points of rock, 

 the crests of which pitch down toward the valley and eventually 

 pass beneath the alluvium. Frequently, off from the base of the 

 hills, there are isolated knobs of rocks and rocky hillocks rising 

 from the alluvium like islands. In general, then, the valley is 

 bounded on this side by geomorphically mature slopes which pass 

 down, a little below the present floor of the valley, into an uneven 

 or lumpy terrace, which, however, is well exposed at the south- 

 western corner of the valley. There is thus no suggestion of 

 faulting on this side. But such a terrace could not have been 

 evolved with the present drainage scheme or the present configur- 

 ation of the valley. It clearly antedates the valley. In its south- 

 ern extension it abuts upon the fault block mountain to the 

 south of the valley and it thus appears to have been cut off in 

 the same way and at the same time as in the case of the stream- 

 cut terrace of Tehachapi Valley, with which it is correlated. 



The sudden drop from Brites Valley to Cummings Valley 

 would seem to indicate with little cpiestion a fault along the 

 eastern edge of the latter. In general it thus appears that on 

 three sides Cummings is bounded by fault scarps and that as a 

 result of the movements on these a geomorphically mature sur- 

 face, including a stream-cut terrace, now situated at an altitude 

 of between 4000 and 5000 feet above sea level, and immediately 

 above the edge of the Great Valley, has been tilted down toward 

 the southeast, so as to form a rock-rimmed trough, which has 

 since been filled by alluvium, arising from the degradation of 

 the fault block on the south. 



This conception of the character of Cummings Valley involves 

 the recognition of the fact that, before it became filled to its 

 present level by alluvium, it must have been occupied by a lake. 

 Should the central part of the valley, therefore, even be pierced 

 by wells deep enough to reach its rocky floor it may confidently 

 be predicted that such wells will pass through lake sediments. 

 It is questionable whether this lake had an outlet or not. The 

 present catchment area tributary to Cummings Valley is only 

 about 45 square miles, or 3-| times the area of the valley floor. 



