44-1 University of California Publications. [Geology 



composed are much larger in size and in the highest portions 

 spauls of rock 10 feet in diameter are occasionally met with while 

 masses from 3 to 5 feet in diameter are quite common. In gen- 

 eral, stratification is not apparent throughout the formation even 

 in excellent exposures except at its base, as above mentioned, 

 and it is difficult to estimate its thickness. Some idea of its vol- 

 ume may be obtained, however, from the fact that the highest 

 part of the formation is 350 feet above the level floor of the 

 valley at Tehachapi Station and assuming a gentle syneline for 

 the structure of the formation as a whole this figure may be 

 taken as a value for its thickness. 



This thick accunmlation of coarse alluvium is the record of 

 an important event in the diastrophic history of the southern 

 Sierra Nevada. It is considered by the writer to have been, 

 anterior to its deformation and degradation, a great cone such 

 as is found commonly where high grade streams emerge from 

 narrow mountain canons and spread out abruptly upon a lower 

 grade valley. Their situation is always along lines of geomor- 

 phic discordance, or where the geomorphic features of the moun- 

 tain are completely out of harmony with those of the adjacent 

 valley from the point of view of erosion al evolution. Such situa- 

 tions are produced by faulting or by other acute deformation of 

 the region, whereby the mountain mass has been uplifted rela- 

 tively to the valley. Occasionally a trunk stream may flow in a 

 canon which has been overdeepened relatively to its tributaries 

 and the conditions may in such cases be favorable for the devel- 

 opment of alluvial cones at the mouths of the hanging valleys. 

 These conditions, however, are most commonly found in glaci- 

 ated regions, and even then the geomorphic discordance between 

 the overdeepened canon and the tributary hanging valley is 

 slight compared with that which obtains between the recently 

 uplifted mountain ranges and the broad diastrophic valleys 

 which parallel them in Southern California and in Nevada. 



On the assumption which is here made of the strictly 

 fluviatile origin of the Tehachapi formation it is evident that an 

 acute deformation of the region occurred at the close of the Cable 

 lacustral period, and caused a rapid dejection of coarse moun- 

 tain detritus into the basin formerly occupied by the lake from 



