Vol. 4] 



Lawson. — Tehachapi Valley System. 



451 



but slightly affected by stream erosion. It can scarcely be 

 regarded as other than a somewhat degraded fault scarp. The 

 date of its origin is, however, not quite clear. It appears to 

 have antedated the broad terrace which lies at its foot, and may 

 with much probability be referred to the period of orogenic 

 movement which deformed the Cable Lake beds and the 

 Tehachapi Alluvium, since the western edge of the trough in 

 which these formations lie is parallel to its trend and the trough 

 on this side is most acutely deformed so that, as before stated, 

 the Cable Lake beds and the plane of contact between them and 

 the overlying alluvium are in vertical altitude. It is probable, 

 however, as will appear in the sequel, that a second movement 

 of minor extent took place on this same fault at a later date. 



GEOLOGICAL HISTORY. 



Reviewing now the data that have been presented, the history 

 of the evolution of Tehachapi Valley in so far as the writer has 

 been able to decipher it, in the limited opportunities at his dis- 

 posal, may be summarily stated. It is not presumed that the 

 statement is complete. When the region is mapped and its 

 features are more closely studied it will doubtless be amplified, 

 and perhaps in some respects modified. 



After the profound degradation of the mountain mass which 

 resulted at the time of the mid-Mesozoic revolution in conse- 

 quence of the invasion of the region by the Sierran batholith, a 

 basin of deposition was developed in which accumulated the sand- 

 stones, conglomerates and shales of Cache Valley, and the tuffs 

 and agglomerates of White Rock Valley. The basin was prob- 

 ably lacustrine in character and is probably to be correlated with 

 early Tertiary lake basins of other portions of the Great Basin. 

 These lake beds and tuffs were folded, but not closely appressed, 

 and were then largely removed by erosion, the bed-rock surface 

 upon which they were deposited being in this way in large meas- 

 ure resurrected and subjected to renewed degradation. The next 

 event of which we have note is an orogenic disturbance of the 

 region and the formation of diastrophic valleys bordered by 

 abrupt mountain fronts. This event is an inference based on 

 the occurrence of a coarse alluvium. Alluvium of this character 



