THE PLEISTOCENE OR GLACIAL PERIOD. 395 



As the ice border withdrew to the north of the divide separating 

 the St. Lawrence basin from the Mississippi basin, the glacial waters 

 were ponded between the ice on the north and the divide on the south. 

 To find escape across the divide, the waters were compelled to rise 

 to the heights of the lowest available cols. At first, nearly every 

 considerable depression in the divide to the south was occupied by a 

 discharging stream, and the ponded water to the north formed innumer- 

 able small lakes. 1 But as the ice retreated farther into the basin, 

 the sizes of the lakes tended to increase as their basins were enlarged; 

 but at the same time the ponded waters tended to unite along the 

 edge of the withdrawing ice, and to utilize only the lower passes across 

 the divide to the south. This tended to lower the lakes, and hence 

 to reduce them. There thus followed a complex series of antithetical 

 changes resulting in the making and unmaking of lakes. This con- 

 tinued until the obstructing ice withdrew from the axis of the St. Law- 

 rence basin. The last of the shifting series of ice-ponded lakes of this 

 basin then disappeared, leaving the present rock-bound lakes as their 

 successors. The full details are too voluminous for introduction here, 

 but a brief sketch of the history of the leading lakes will indicate the 

 nature of the changes which took place. 



When the end of the Lake Michigan ice-lobe withdrew a little within 

 the Lake Michigan basin, a crescentic belt of water formed about its 

 southern extremity, and found a point of discharge into the Illinois 

 valley through a col southwest of Chicago, . which it proceeded to 

 erode to greater depths. This valley has since become the site of 

 the Chicago drainage canal. 2 A glacial lake (the extinct Lake Chicago) 

 was thus initiated, and as the ice-lobe withdrew, the lake gradually 

 extended northward (Fig. 516). 



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

 ice-lobe, and discharged through an outlet at the head-waters of the 

 Brule and St. Croix Rivers to the Mississippi. Another lake of like 

 origin (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. 



1 For local lakes in New York, see Fairchild, Bull. Geol. Soc. Am., Vol. X, pp. 27-68. 



2 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. 



