SECTIONS EXPOSED IN WALKER CASON. 141 



with the siiiiihir beds in other parts of the Lahoutan basin, but they contain 

 the same species of fossils. Their tufa deposits in various parts of the old 

 lake basin record similar chemical conditions. 



The following section observed in the left bank of the Walker River, 

 about two miles above the gravel embankment shown on Plate XXVIII, 

 represents the prevailing character of the exposui'es to be seen in this re- 

 gion: 



Feet. 



fjEolian deposits forming desert surfiice 1 to 10 

 Light-colortd marly clays, passing into ferrugi- 

 -11 .. ^ nons sandy beds, sometinjes contorted, contain- 

 I ing detached mammalian bones; changing to 

 (, marly clays at the base 40 to 50 



Contact unconformable. 

 Medial gravels Gravels aud loam, colored with iron 25 to 30 



Contact unconformahle. 

 Lower lacnst ral clays . . . Light-colored marly clay ; to river 75 



The sections taken at various points along the Walker River show 

 great variation, but the differences are caused almost entirely by the want 

 of constancy in the medial gravels; the upper and lower members of the 

 series are remarkably uniform throughout. In the majority of cases where 

 the upper or lower contacts of the medial gravels could be seen they were 

 found to be unconformable with the adjacent beds. 



The most difficult problems presented by the superficial geology of 

 the Walker River Valley are in connection with the orographic disturb- 

 ances that have affected the region in post-Lahonta ntimes. The valleys 

 occupied by Walker Lake and Walker River are of the Great Basin type, 

 and owe their formation to pre-Lahontan faulting; the main displacement 

 that gave origin to the depressions — which are structurally a single basin — 

 follows its western border and determines the extremely precipitous eastern 

 face of the Walker Lake or Wassuck Mountains. Other faults, less plainly 

 distinguishable, occur on the eastern border of the valley, especially near its 

 northern end, and connect with the displacements to be seen in Mason Valley. 

 Some of these ancient fault lines, including the largest of all — that following 

 the western border of the valley — appear at the surface within the basin of 

 the former lake; in such instances a post-Lahontan movement of the ancient 



