THE PLEISTOCENE OR GLACIAL PERIOD 879 



drift, for example, the several sheets are more commonly wanting 

 than present. 



X. The glacio-lacustrine substage. In the course of the re- 

 treat of the ice of the Wisconsin epoch, a complex series of lakes 

 arose between the ice border on the one hand, and the higher 

 land fronting it on the other. Many of these lakes were temporary 

 and shifting, and had shifting outlets. Thus as the ice border 

 receded north of the divide separating the St. Lawrence basin from 

 the Mississippi basin, glacial waters were ponded between the ice 

 on the north and the divide on the south. This gave rise to many 

 lakes, the history of which cannot be given here; but a brief sketch 

 of the history of the Great Lakes will indicate the nature of the 

 changes which took place. 



When the end of the Lake Michigan ice-lobe (Fig. 584) with- 

 drew a little from the southern end of the Lake Michigan basin, a 

 lake formed about its southern end, and found a point of discharge 

 into the Illinois valley southwest of Chicago. The outflowing 

 waters eroded'its channel to greater depths, and it has since become 

 the site of the Chicago drainage canal. 1 The glacial lake (Lake 

 Chicago) thus initiated was gradually extended northward (Fig. 

 585) as the ice-lobe was melted. 



A similar lake was formed about the head of the Lake Superior 

 ice-lobe, and discharged to the Mississippi. Lake Maumee was 

 formed about the end of the Erie ice-lobe, and discharged its waters 

 by way of Fort Wayne into. the Wabash, and thence to the Gulf. 

 A later stage of Lake Chicago and Lake Maumee is shown in 

 Fig. 585, when, finding a lower outlet as the ice melted back, 

 Lake Maumee sent its outflow across southern Michigan to Lake 

 Chicago. 



Somewhat later, Lake Saginaw developed about the end of the 

 Saginaw ice-lobe, and discharged by way of Grand River into Lake 

 Chicago, and thence to the Mississippi. Lake Maumee then dis- 

 charged into Lake Saginaw. 



1 This valley appears to have served a similar function in earlier stages 

 of glacial retreat; but it was not the preglacial outlet of the Lake Michigan 

 basin, as there are much lower channels (now buried) both north and east 

 of it. One of these must have been the preglacial outlet of the Lake Michi- 

 gan basin, 



