Vol. 4] Lawson. — Tehachapi Valley System. 453 



plain was developed as a very notable feature of the region, being 

 cut across hard and soft rocks alike with great evenness. It is 

 very probable from our general notions of the conditions which 

 attend the evohition of stream-cut plains, that this flood plain 

 stood at a much lower level than its remnants do to-day (4000 

 feet A. S.). It marks an important datum plane in the evolu- 

 tion of the Sierra Nevada and is the probable correlative of 

 the high valleys of the Upper Kern.* 



Up to this time there was probably no such feature as the 

 modern Tehachapi Valley. But now the process of flood plain 

 expansion was abruptly terminated by another orogenic move- 

 ment, which besides effecting a general elevation of the region 

 far above the base level of the streams, also raised Tehachapi 

 Mountain as a fault block on the south of the present valley 

 and gave the latter its modern configuration as regards its 

 boundaries. The uplift of this block caused the dejection upon 

 the flood plain of the alluvium which forms the greater part of 

 the floor of the valley. Co-eval with this progressive obstruction 

 of the drainage on the flood plain, the head waters of its stream 

 were captured by head water erosion cutting back from the 

 Great Valley into the upraised mountains. The beheaded stream 

 was unable to cope with deluge of torrential debris from the new 

 mountain mass on the south and its course was reversed into 

 the same stream that had captured its head waters. Since this 

 reversal of the drainage the flood plain has been dissected. 



The geological history of the eastern outlet of Tehachapi Val- 

 ley is not so complex. The outlet is a rocky gorge so deeply allu- 

 viated as to give a rather broad bottom (200 yards). If we 

 imagine the alluvium removed below the depth of certain wells 

 that have been sunk in it but have not bottomed it, and suppose 

 the rocky slopes projected down we would have a quite narrow 

 and somewhat tortuous mountain gorge. This character estab- 

 lishes the fact that it was not the outlet for the stream that 

 corroded the broad terrace at the western end of the valley. The 

 gorge, ivith its present direction of drainage, could not very well 

 have been evolved after the present boundaries of Tehachapi Val- 



* Geomorphogeny of the Upper Kern Basin, Bull. Dept. Geol. Univ. Cal., 

 Vol. 3, No. 15. 



