26 GEOLOGICAL HISTORY OF LAKE LAHONTAN. 



It is not to be understood, however, that the old lake basin was formed 

 l)y a single, simple displacement; on the contrary, it is the result of exceed- 

 ingly complex faulting- that affected the entire region included between the 

 Wasatch and the Sierra Nevada mountains. The time when these movements 

 began is unknown, but they antedate the Quaternary, were in process during 

 the existence of lakes Bonneville and Lahontan, and probably have not yet 

 ceased, as will be shown in Chapter X. The old lake basin, instead of 

 Ijeing a simple orographic valley, is composed of a large number of separate 

 and independent depressions of the Great Basin type, which are united with 

 one another directly, or by the intervention of narrow passes, and so 

 nearly coincident in level that a single lake 900 feet deep in the lowest 

 depression could flood them all. It is to the union of these various, inde- 

 pendent, monoclinal valleys that the extremely irregular outline of Lake 

 Lahontan is due. 



Nearly all the ranges of northwestern Nevada are rugged and form 

 serrate crests having an approximately north and south trend, and, as 

 already stated, as a nearly universal rule they are monoclinal. An older 

 structure, however, as first recognized by King, '^ is frequently apparent, in 

 which a folding of the rocks into anticlinal and synclinal may be traced. 

 In the older deformation the rocks were crumpled and contorted as in the 

 Alleghanies and the Alps, but during the later disturbances they were broken 

 without being folded. Tlie monoclinal blocks resulting from the second 

 disturbance are the elements giving character to the present topography; 

 the surface features due to the former structure having been rendered 

 inconspicuous by the later movements. The trend of the fault lines, and 

 consequently of the mountain axes, is in general nearly north and south, 

 but in the central part of the Great Basin, north of latitude 37°, it is more 

 nearly north-northeast and south-southwest. 



At present we can only call attention to a few characteristic examples 

 of the displacements that gave origin to the Lahontan basin; these may be 

 taken as types of the prevailing structure of the region. 



In the Santa Rosa Mountains, in northern Nevada, the fault determining 

 the treud of the range follows its western base and has a throw of not less 



I' U. 8. Geological Exploration of the Fortieth Parallel, Vol. I, p. 735. 



