GEOLOGICAL AND NATURAL HISTORY SURVEY. 65 



When the ice sheet withdrew from the Ontario basin an east- 

 ward outlet was opened by the Mohawk. The Erie and Huron 

 basins were then made tributary to the Mohawk and Hudson, and 

 it is probable that they never afterwards discharged into the Lake 

 Michigan basin. There are, however, complications not yet fully 

 understood, arising in large part from the differential uplifts of the 

 region, which may have resulted in a brief flow of the waters west- 

 ward along the straits of Mackinac after the ice had withdrawn 

 from the northern end of the Lake Michigan basin. 



The working out of the lake history has also brought evidence 

 that the Lake Michigan basin was for a time free from water in its 

 southern end far within the limits of the present shore of the lake. 

 This low stage of the southern end of the basin, it appears, was 

 due not so much to the shrinking of the lake as to lower elevation 

 in the northern latitudes, for at that time the lake apparently stood 

 above its present level at the north end of the basin. The evidence 

 now at hand points to a general northeastward elevatory move- 

 ment, following more or less closely the retreat of the ice sheet. 

 While, therefore;, the retreat of the ice sheet opened low outlets to- 

 ward the east and thus led to the withdrawal of the water from the 

 southern and western end of the kke basins, the succeeding uplift 

 at the northeast, complicated by conditions which cannot well be 

 entered into here, raised these outlets sufficiently to cause the 

 water to be thrown back and give these lakes their present extent. 



Theintroduction of the name Lake Chicago, for the glacial lake, 

 which was held in the southern end of the Lake Michigan basin, 

 seems convenient, if not necessary, inasmuch as its area was not 

 coincident with that of Lake Michigan, and its outlet was in the 

 reverse direction. It is also in keeping with the custom of students 

 of glacial lakes, who find it advantageous to have a special name for 

 each of the temporary bodies of water in the several basins. The 

 name. Lake Chicago, seems especially pertinent, since the glacial 

 lake extended about as far beyond the present limits of Lake 

 Michigan, in the vicinity of Chicago, as at any part of its border. 

 It is also a name which readily suggests the position of the lake, 

 and it is in keeping with the name which has come into use for the 

 outlet, namely, the Chicago outlet. 



The name Lake Chicago is applied provisionally to all the 

 stages at which there was a southwestward outlet, but it is not yet 

 certain whether they were all formed during the occupancy of a 

 portion of the Lake Michigan basin by the ice sheet. This ques- 

 tion is discussed at some length further on. 



