GLACIAL LAKES TAYLOR. 295 



THE GREAT LAKE BASINS IN THE SUCCESSIVE GLACIAL AND INTER- 

 GLACIAL EPOCHS. 



The glacial ]:)eriocl as a whole hns ])een found to be made up of 

 four (or possibly five) distinct epochs of glaciation separated by inter- 

 vening warm periods when the ice sheet either slirank to relatively 

 small proportions or disappeared altogether. The last ice sheet 

 deposited what is known as the Wisconsin drift. It seems certain 

 that the depressions which constitute the lake basins were involved 

 in each one of the several glacial epochs, and ^-et all the basms, ex- 

 cepting perhaps that of Lake Superior, retain very distinct characters 

 which belong to stream-eroded valleys. Indeed, except for the 

 drift deposits and effects produced by tiltmg, it may almost be said 

 that they show no other characters. All the changes produced by 

 the several glacial invasions have not destroyed these characters nor 

 obliterated them to any great extent. In fact, when the last ice 

 sheet crept from the north down into the lake basins it appears 

 to have found them in almost every detail the same as they are to-day. 



No doubt the events of the lake history which occurred during the 

 advancing phase of the last ice sheet, as well as m the earlier glacial 

 epochs, are matters which would be of great interest and importance 

 if they were accessible; but thej seem destined to remain in obscurity, 

 because the record made by the ice at the climax of each mmor move- 

 ment of advance was continually being overridden and obliterated 

 by later and more energetic readvances; and, further, in the region 

 of the Great Lakes the drift sheets of the older glacial epochs were 

 almost entkely overridden by the later ones. 



THE GREAT LAKES DURING THE RETREAT OF THE LAST ICE SHEET. 



The foregoing is a brief outlme of the complex history of the 

 Great Lakes do^vn to the time of the maximum extension of the last 

 or Wisconsin ice sheet. It is only when we begm to follow the retreat 

 of this ice sheet across the lake region that we come upon that later 

 phase of the lake history which is so clearly and completely recorded 

 in the present surface deposits. This part of the history is spread 

 out upon the surface of the lake region like an open book. An 

 immense body of facts has been gathered bearing upon it and this 

 gives us a fairty full knowledge of its details, with the promise, 

 through continued exploration, of still more detailed knowledge. 



This i)art of the lake history has its o^vn complexities and these 

 arise from several different causes. Fkst, from the oscillating man- 

 ner of the ice retreat, which was accompanied by many periodic 

 minor m.ovements of retreat and readvanco; second, from the irreg- 

 uliiritics of topograph}^ which characterizes the lake region; third, 

 from the direction of the general retreat of the ice across the lake 



