GLACIAL AND POSTGLACIAL HISTORY OF GREAT LAKES REGION. 319 



and this seems to indicate that if the lake had a northward outlet then as now, the northern 

 part of the basin stood relatively lower than now, for in its present attitude it lacks only 8 feet 

 of overflowing at its southern end. It seems certain that the depressions which constitute the 

 lake basins were involved in each one of the several glacial stages; and yet all the basins, except 

 perhaps that of Lake Superior, retain very distinct characters which belong to stream-eroded 

 valleys. Indeed, except for the drift deposits and the effects produced by tilting, they may 

 almost be said to 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 in the earher glacial stages are matters which would be of great interest 

 and importance if they were accessible; but they seem destined to remain in obscurity, because 

 the record made by the ice at the climax of each minor movement of advance was continually 

 being overridden and obhterated by later and more energetic readvances; and further, in the 

 region of the Great Lakes the drift sheets of the older glacial stages were almost entirely over- 

 ridden by the later ones. 



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



COMPLEXITIES OF THE HISTORY. 



The retreat of the ice in the last or Wisconsin stage of glaciation marks the beginning of 

 that later phase of the lake history whose records are so clearly and completely preserved in the 

 present surface deposits. This part of the history is spread like an open book upon the surface 

 of the lake region. An immense body of facts bearing upon it and giving a fairly full knowledge 

 of its details has already been gathered, and continued exploration will afford many more. 



This part of the lake history has its own complexities, arising from several causes: (1) The 

 oscillating manner of the retreat of the ice border, which was accompanied by periodic minor 

 movements of retreat and readvance; (2) the irregularities of topography which characterize 

 the lake region; (3) the variation in the direction of the general retreat of the ice limit across 

 the lake basins; and (4) the differential elevation of the land during and after the ice occupation, 

 the maximum elevation occurring in the north and producing several changes of outlet. 



The combined effect of these and other less important factors produced a complex history, 

 not only as expressed by the distribution and relations of the various drift forms, but also by 

 the remarkable effects of the ice sheet on the associated drainage. It determined the location 

 of great rivers which flowed only temporarily from the ice or along its border and it caused 

 remarkable shiftings of their courses. Its most noteworthy effect, however, was the production 

 of a complex succession of shifting and changing lakes — enlarging, falling, shrinking, combining, 

 dividing, and rising — of large extent and frequent changes of outlet, coming at last to the lakes 

 as they are to-day. As investigation has inclined more and more to details and has covered 

 an increasing area it has been found that the succession of changes involved in the later lake 

 history is much more complex than was formerly supposed. 



SHRINKAGE OF THE ICE SHEET INTO THE LAKE BASINS. 



In one of its earher stages (the Illinoian) the ice sheet covered the entire region of the 

 Great Lakes, the only exception being the well-known Driftless Area which lies chiefly in western 

 Wisconsin. This area is in the angle between Lake Michigan and Lake Superior, but does not 

 comprise any part of the drainage basin of either one of them. As the ice sheet moved south- 

 ward the lake basins naturally offered the easiest lines of flow and the high lands between the 

 basins were areas of greater resistance and slower flow. But when the ice attained its maximum, 

 reaching nearly to Cairo in Illinois, about 10 miles beyond Ohio River south of Cincinnati in 

 Kentucky, and to Beaver Falls in Pennsylvania, the lake basins became relatively unimportant 

 in their effect upon the ice movement, for at that time the ice overwhelmed them all, including 



