Vol. 6] R e id: The Geomorphogeny of the Sierra Nevada. 113 



least three distinct periods of faulting and uplift are recorded 

 there. The next most important feature, connected with proofs 

 of the faulting of different ages, is the large number of lesser 

 crust blocks found in every part of the fault zone. Also, there 

 are two other examples of well-developed longitudinal valleys, 

 though their size is very small compared to Little Valley. Lastly 

 a few minor details of topography call for explanation, such as 

 the peculiar floor of the north end of Little Valley. 



The analysis of this great fault zone is of vital importance to 

 the history of the evolution of the Sierra Nevada, for by it we 

 can obtain a cross-section, as it were, of the diastrophic move- 

 ments producing the range, and this knowledge is complementary 

 to that record gained from a study of the west slopes of the 

 Sierra. Furthermore, according to Lindgren, there has been no 

 important post-andesitic faulting along the old fault planes east 

 and west of Lake Tahoe on the Truekee-Carson quadrangle, 

 though further north such movement has been recognized. If 

 this be so, the whole of the Sierra block from the Great Valley 

 to the Carson Range has moved as a unit since the eruption of 

 the andesitic lavas, concentrating the faults in the Carson 

 Range. 



In a cross-section of the Sierra through Lake Tahoe there 

 are two summits, separated by the down-dropped block of the 

 Tahoe Moat and Truckee Valley. The west summit forms the 

 Great Western Divide, and is the true axis of the range. The 

 east crest is formed by the Carson Range and carries the two 

 highest peaks of this part of the mountains, Mt. Rose northeast 

 of the lake, and Freels Peak to the southeast. The structure of 

 that portion of the range east of the Great Western Divide is 

 complicated by the down-dropped portion underlying Lake Tahoe, 

 otherwise it might be regarded as a single crust block tilted to the 

 west. A knowledge of the configuration of the floor of Lake 

 Tahoe is greatly needed in this connection. This east-west sec- 

 tion across the range can be discussed more fully after an ex- 

 amination of the diastrophic record of the lake. Genetically the 

 Virginia and Pine Nut ranges are a portion of the Sierra Nevada, 

 and records of orogenic movements are preserved not only in the 

 Carson Range but also in the Nevada valleys and ranges. 



