240 THE GLACIAL LAKE AGASSIZ. 



most; and they even suggest that remnants of the continental ice-sheet 

 may have hngered there considerably later than the time, computed to be 

 six to eight thousand years ago, when its southern portion retreated. 



SHOET DURATION OF LAKE AGASSIZ. 



The foregoing measures of time, surprisingly short, whether we com- 

 pare them on the one hand with the period of authentic human history or 

 on the other with the long record of geology, carry us back to the date 

 when the ice-sheet was melting a.way from the- basins of the Upper Mis- 

 sissippi, of the Red River of the North, and of the Laurentian lakes. If 

 the postglacial epoch has been so short, we must infer that the final reces- 

 sion of the ice was very rapid and that its barrier between the Red River 

 Valley and Hudson Bay was soon mel.ted away. Though Lake Agassiz 

 attained vast areal extent, its duration or extent in time was geologically 

 brief, as is shown by the small volume of its beach deposits and lacustrine 

 sediments in comparison with the Pleistocene lakes of the Great Basin 

 and with the amount of postglacial erosion and deposition on the shores 

 of the great lakes tributary to the St. Lawrence and Nelson rivers. The 

 geologic suddenness of the final melting of the ice-sheet, proved by the 

 brevity of existence of its attendant glacial lakes, presents scarcely less diffi- 

 culty for explanation of its causes and climatic conditions than the earlier 

 changes from mild or warm preglacial conditions. to prolonged cold and ice 

 accumulation. 



Comparison with postglacial lakes. — The disappearance of the greater 

 part of the vast North American ice-sheet probably occupied not more 

 than two or three thousand years ; and half of this time may measure the 

 duration of Lake Agassiz, with the formation of its beaches marking more 

 than thirty successive stages in the concurrent subsidence of its surface 

 and rise of the earth's crust. But even these short estimates may be too 

 long. Tlie shoi-es of Lake Michigan, similar with those of Lake Agassiz, 

 in the drift of Avhich they are formed, in their north and south trends, and in 

 the adjoining depths of water, have suffered an amount of erosion by the 

 lake waves during postglacial time which very far exceeds the total erosion 

 that was effected upon the shores of Lake Agassiz during all its stages, the 



