158 



University of California Publications. 



[Geology 



causes that produced the Marlett Lake graben acted to form the 

 present Little Valley. 



"With the adoption of this origin of the valleys, the age of the 

 northeast-southwest faults becomes fixed, and was contemporan- 

 eous with the first period of east-west movement, which produced 

 Slide Mountain and Snow Valley Peak. No other hypothesis to 

 account for the present valley appears at all able to fulfill the 

 conditions. The character of the valley previous to the last 

 important faulting is, however, not so clear. The structure was 

 comparatively simple. The drainage was probably consequent, 

 although some features of subsequent streams may have been 

 developed by the coalescence of the several lateral creek branches 

 flowing at the base of the high west scarp. But in any case, with 

 the valley structure produced by the last faulting, the drainage 

 became consequent, and Franktown Creek is an example of a 

 longitudinal consequent stream in a young mountain range. 



Summary op Sequence op Faults. 

 In the Little Valley area, then, where the relative ages of 

 faults can be ascertained, the following sequence obtains. First 

 and oldest was a period of north-south movements, which was 

 surely accompanied by some secondary east-west faulting. The 

 Nevada diastrophic valleys were formed at this period. Second, 

 there was a period of north-south faulting, with secondary 

 movements at right angles. The first Little Valley was formed 

 at this time, though it may have been little more than a level 

 area about a mile wide and six miles long at the base of the high 

 west scarp. Third came a period of intense east-west faulting, 

 with much superinduced motion along older fault-planes, and 

 the formation of several marked northeast-southwest faults along 

 which a shear occurred. The present Little Valley was estab- 

 lished at this time. Fourth, there occurred further slight move- 

 ment along the east-west planes, with probably some induced 

 motion along other lines as well. The floor of Little Valley was 

 somewhat modified, as a result. Warping and tilting in Washoe 

 Valley likewise resulted from these latest differential rock move- 

 ments. Still later but small movements along the old fracture 

 planes have produced tilting of Washoe Valley to the east. 



