348 MAN AND THE GLACIAL PERIOD. 



As we have seen, Lake Agassiz occupied a position 

 quite similar in most respects to Lake Michigan. Its 

 longest diameter was north and south, and the same forces 

 which have eroded the cliffs of Lake Michigan and piled 

 up sand-dunes at its southern end would have produced 

 similar effects upon the shores of Lake Agassiz, had its 

 continuance been anywhere near as loug as that of the 

 present Lake Michigan has been. But, according to Mr. 

 ITpham, who has most carefully surveyed the whole region, 

 there are nowhere on the shores of the old Lake Agassiz 

 any evidence of eroded cliffs at all to be compared with 

 those found upon the present Great Lakes, while there is 

 almost an entire lack of sand deposits about the south end 

 such as characterise the shore of Lake Michigan. " The 

 great tracts of dunes about the south end of Lake Michi- 

 gan belong," as L'pham well observes, " wholly to beach ac- 

 cumulations, being sand derived from erosion of the west- 

 ern and eastern shores of the lake. . . . But none of the 

 beaches of our glacial lakes are large enough to make 

 dunes like those on Lake Michigan, though the size and 

 depth of Lake Agassiz, its great extent from north to 

 south, and the character of its shores, seem equally favor- 

 able for their accumulation. It is thus again indicated 

 that the time occupied by the recession of the ice-sheet 

 was comparatively brief." * 



From Mr. Upham's conclusions it would seem that if 

 ten thousand years be allowed for the post-glacial existence 

 of Lake Michigan, one tenth of that period would be more 

 than sufficient to account for the cliffs, deltas, beaches, 

 and other analogous phenomena about Lake Agassiz. In 

 other words, the duration of Lake Agassiz could not have 

 been more than a thousand years, which gives us a meas- 



* Proceedings of the Boston Society of Natural History, toI. 

 xxiv, p. 454; L T pham's Glacial Lakes in Canada, in Bulletin of the 

 Geological Society of America, vol. ii, p. 248. 



