Vol. 6] Beid: The Geomorphogeny of the Sierra Nevada. 147 



The older uplift formed the east scarp, and at the time the vari- 

 ous surrounding plateau areas were all probably one. The sec- 

 ond uplift raised the high west ridge and range crest and formed 

 the structural features underlying Little Valley. The fault 

 through Little Valley is the most important north-south fault 

 east of Lake Tahoe and north of the Genoa topographic area, 

 judging by the physiographic features of the country traversed. 



The next important faulting in order of relative age is that 

 in an east-west direction. As noted before, the presence of 

 north-facing scarps that have caused distinct jogs in the walls 

 of the north-south scarps, the faulted creek gravels in Little Val- 

 ley and to the east, and the jogs in Franktown Creek and the 

 stream in south Washoe Valley, all indicate a movement along 

 an east-west plane later than the two longitudinal faults. A little 

 consideration makes it evident that this east-west motion is of at 

 least two periods. Both in Little Valley and in the valley next 

 east of its south part the prominent east-west scarps and the dis- 

 location of the creek deposits are the results of faulting of two 

 different orders. In Little Valley, were the cross-scarp and fault- 

 ing of the gravels contemporaneous, there must have been a com- 

 pression of the creek deposit south of the cross-fault, with re- 

 sultant deformation by the great narrowing of the valley. From 

 the stream terrace in this part of Little Valley, such deformation 

 did not occur ; in other words, the gravels composing the faulted 

 terrace appear to have been deposited after the narrowing of the 

 valley. This establishes at least two ages of the east-west move- 

 ments. Supplementary evidence is not lacking. In the small 

 fault-block south of the jog in Franktown Creek, on the line of 

 the east-west scarp, a complex of east-west fractures is filled 

 with small andesite dikes, that were intruded at the time of this 

 motion. Likewise on the line on the floor of Little Valley and 

 in the east ridge occur other small patches of the same rock. Frag- 

 ments of this fine-grained and somewhat porous andesite are 

 found, showing strongly grooved surfaces, representing a move- 

 ment later than the time of the intrusion. These grooved frag- 

 ments show no slickensides, the rock appearing unable to take a 

 polish. This characteristic seems to be due both to the inherent 

 porosity and to some slight alteration previous to the initiation of 

 further faulting. 



