270 GEOLOGICAL HISTORY OF LAKE LAHONTAN. 



The greatest expansion of the waters of the Mono basin, occurred sub- 

 sequent t<? the last extension of the Sierra Nevada glaciers. Although this 

 is the only instance known where the relation of the former lakes and gla- 

 ciers of the Great Basin is clearly determinable, yet it seems a necessary 

 inference that the other lakes of the same region attained their maximum at 

 the same time. As the formation of glaciers and the extension of lakes in 

 inclosed basins necessarily result from similar climatic changes, we corre 

 late the two flood periods of Lake Lahontan with the two periods of maxi- 

 mum extension of the Sierra Nevada glaciers. Again, from similarity of 

 phenomena, the two periods of glaciation on the mountains of the Far West, 

 are correlated in time with the two glacial epochs of northeastern America, 

 as recognized by certain geologists. If this determination is correct, it fol- 

 lows that the last great expansion of the lakes of the Great Basin occurred 

 during the close of the Glacial period, and may be considered as contem- 

 poraneous with the Champlain epoch of the eastern StatesT] 



That the valleys of the Great Basin held lakes, at least at intervals, 

 throughout the Quaternary, is not only probable, a priori, but is indicated 

 by the great thickness of marls, clays and gravels that fill these depressions. 

 In the Bonneville basin these deposits have been penetrated to a depth of 

 over 1,500 feet without reaching the underlying rock. That the lower 

 portion of the material tilling these depressions may be of Tertiary age, is 

 certainly possible, but the records of the passage of the Tertiary into the 

 Quaternary are so obscure and so little known that it is at present impossi- 

 ble, at least in the lake-beds of the Far West, to say where the former ends 

 and the latter begins. When Lake Lahontan began its existence will prob- 

 ably never be known, except in a general way; but that it reached its 

 greatest extension in late Quaternary times and was approximately synchro- 

 nous in its fluctuations with the advance and retreat of the Sierra Nevada 

 glaciers during the Glacial epoch is a fair deduction from the evidence re- 

 corded in the present volume. 



In regard to the time, as measured in years, that has elapsed since the 

 events described in this report took place, we have but shadowy evidence to 

 offer. It has been estimated by James Croll,^^ from astronomical data, that 



» Climate and Time, New York, 1875. Cliap. XIX. 



