CHAPTEE V. 



HISTORY OF LAKE AGASSIZ. 



TWO CLASSES OF PLEISTOCENE LAKES. 



Among the most important geologic records of the Pleistocene period 

 in America are the sediments and shore-lines of former lakes of great extent 

 which are now represented by lakes that occupy, excepting within the 

 basin of the St. Lawrence, only a small part of their ancient area. Lake 

 Bonneville, in the basin of Great Salt Lake, Utah, and Lake Lahontan, 

 in the basin of the Humboldt River and Pyramid Lake, Nevada, are con- 

 spicuous examples of one class of these Pleistocene lakes, formed by 

 increased rainfall, where now an arid climate limits the lakes to small areas, 

 with their surface far below the watersheds across which they would out- 

 flow to the sea. These are south of the glaciated area of the continent, 

 but they appear to have owed theii' existence to the changes of climate 

 by which the ice-sheet of the Glacial period was formed. Lake Agassiz 

 belongs to another class of these lakes, caused directly by the barrier of 

 the ice-sheet where this was accumulated on a northwardly sloping land 

 surface. Such glacial lakes were developed on a vast scale in the basins 

 of Lake Winnipeg and the Laurentian lakes during- the recession of the 

 ice border, when it was being gradually melted away by a wai'mer climate ; 

 and it is also evident that many smalllakes of the same kind then flowed 

 southward over the lowest points of the present watersheds. Examples of 

 this class now existing are the little Merjelen See, pent up in a tributary 

 valley on the east side of the Great Aletsch glacier in the Alps, and similar 

 ice-dammed lakelets in Greenland. 



LAKES BONNEVILLE, LAHONTAN, AND OTHERS IN THE GKKAT BASIN. 



Twice during the climatic changes of the Glacial and post-Glacial 

 periods. Lake Bonneville, described by Gilbert,^ and Lakes Lahontan and 



'Lake Boimevillc. By G. K. Gilbert. Monographs of the U. S. Geol. Survey, Vol. I. 

 192 



