34 GEOLOGICAL HISTORY OF LAKE LAHONTAN. 



cut through by a stream tliat once flowed into Lahontan basin through the 

 channel that opens opposite Old Camp Churchill. The lake which occu- 

 pied Mason and Walker Lake valleys cut down its point of overflow about 

 85 feet, and discharged its waters northward. The bottom of the channel, 

 thus formed, where it leaves Mason Valley is between 60 and 70 feet below 

 the level of the Lahontan beach, as determined by measurements of level 

 connected with the profile of the Carson and Colorado Railroad. These 

 measurements indicate that Lake Lahontan did not extend into Mason 

 Valley until after the channel cut by the overflow from that basin was 

 formed. We know, however, that there has been considerable post- 

 Lahontan orographic movement in this region, and it seems not unlikely 

 that the relative height of Mason Valley and the Lahontan beach along 

 the Carson River, may have been changed since the evaporation of the 

 former lake. It is, therefore, possible that Lahontan during its highest 

 stage extended through the pass connecting Churchill and Mason vallej's, 

 before the present channel was excavated, and occupied Mason and Walker 

 Lake valleys. The tufa deposits about Walker Lake, as will be explained 

 in a future chapter, are of the same nature as the similar formations in the 

 main areas of Lake Lahontan, and indicate that the}- were precipitated 

 from waters of the same character. 



After determining that Lake Lahontan did occupy Walker Lake 

 Valley, we explored its ancient beaches, and found that the former lake 

 extended only a few miles southward of the one which now fills the bottom 

 of the basin. Well preserved gravel bars sweep around the southern end 

 of the valley but do not reach the level of the pass leading south into Soda 

 Springs Valley; at the end of this basin there is also a low pass that is 

 uncut bv stream erosion. 



As the localities noticed above are the only ones on the southern 

 border of the Lahontan basin that would suggest a possible outlet, the con- 

 clusion that the former lake did not overflow in that direction is positive. 



In the northern part of the basin all the passes leading to the valleys 

 draining into the Owyhee, one of the tributaries of the Columbia, were 

 specially examined, as well as the divide between the northern end of the 

 Black Rock Desert and Alvord Valley; at none of these places are there 



