396 Adventures in Scenery 



Donner Lake in Granite Basin, 



Water Held by Glacial Debris 



Donner Lake lies in a basin in the granite rock. The basin 

 was evidently once filled with ice of a glacier. Bare granite 

 cliffs form the walls of the upper basin. Below, at the east end 

 of the lake, a moraine deposited by the ancient glacier holds 

 back the waters of the lake. The lake is drained by Donner 

 Creek, which has cut a channel through glacial debris and 

 lava rocks (basalt), and discharges into Truckee River. The 

 valley of Donner Creek is broad and U-shaped, scoured by 

 glacial ice. Moraines are numerous. Large boulders of granite, 

 relics of the ice of the Glacial Period, strew the surface on all 

 sides. 



Eastern Sierra Slope Marked by 

 Moraines 



The eastern Sierra slope down to an elevation of 5,000 feet 

 was long buried under ice. The grinding of this mass of moving 

 ice had the effect of widening the valley bottoms and steepening 

 the sides, tending to change the cross-sections of the valleys 

 from V-shaped to U-shaped. Moraines composed of rough and 

 angular boulders of all sizes, and gravel and sand, were de- 

 posited by the melting ice at the lower ends of ice tongues and 

 along the sides of the valleys. Truckee is at the foot of the 

 eastern (ice covered) slope. The higher part of the city is 

 built upon deposits made by glacial ice. The canyon of Truckee 

 River, between Truckee and Lake Tahoe, has evidently never 

 been glaciated. 



Due to the relatively great precipitation on the Sierra crest 

 (officially reported from 58 to 74 inches annually) Lake Tahoe 

 overflows. The lowest point of the rim of its basin is at Tahoe 

 City on the northwest shore of the lake. From here water 

 escapes through Truckee Canyon and finally disappears by 

 evaporation from Pyramid Lake in the desert of northwestern 

 Nevada. 



