FLUCTUATION OF GLACIERS. 267 



glaciers of large size formerly flowed down from the High Sierra, which 

 forms its western border, and deposited moraines of great magnitude, on 

 which the terraces of the Quaternary lake, that formerly filled the basin to 

 the depth of nearly 900 feet, are distinctly traced. The moraines at Mono 

 Lake were carried out into the valley as parallel ridges, or morainal embank- 

 ments as we have found it convenient to call them, which in several instances 

 are prolonged for a considerable distance below the highest of the ancient 

 beaches, and have terraces traced not only on their outer slopes but on the 

 inner sides of the couches formerly occupied by glacial ice. In some in- 

 stances deltas have been formed between the extremities of the morainal 

 embankments. The proof is therefore conclusive that the greatest exten- 

 sion of the glaciers preceded the maximum rise of the lake. How far the 

 glaciers had retreated up the canons befoi'e the lake occupied their former 

 beds it is impossible to determine. It has also been found that the glaciers 

 of the Mono basin had two or more i^ei'iods of maximum exten.sion, sepa- 

 rated by times when the ice withdrew far up the canons through which it 

 flowed. There were at least two well-marked glacial epochs in the Sierra 

 Nevada. The lacustral records of the Mono basin indicate two periods of 

 high water, corresponding, it is presumed, to the two main periods of glacial 

 extension. All the facts known to us are in harmony with the conclusion 

 that the two humid periods recorded in the Bonneville and Lahontan basins 

 were practically synchronous with the two periods of maximum extension 

 of the Sierra Nevada glaciers. The fact that the greatest rise of the Qua- 

 ternary lake occupying Mono Valley occurred after the greatest expansion 

 of the glaciers does not militate against this determination, but indicates 

 that the melting of the snow and ice on the mountains contributed an un- 

 usual supply of water to the lake, which then received its greatest flood. 

 When mountains bordering an inclosed basin are loaded with snow and ice, 

 it is evident that a rise of temperature will cause a flooding of the valleys. 

 The analogy between the glacial climate of the Great Basin and the winter 

 climate of the same region at the present time, thus finds another parallel. 

 The evidence leading to the coirelation of the two high-water stages 

 of Lake Lahontan with the two Glacial epochs of the northern hemis- 

 phere has already been indicated. Should this conclusion be sustained. 



