12 CHICAGO ACADEMY OF SCIENCES. 



11. Late Wisconsin drift sheets. 



Substage i. Great bowlder belts and accompanying 



moraines. 

 Substage 2. Valparaiso morainic system. 

 Substage 3. Lake-border morainic system. 



12. Lake Chicago submergence. 



13. Emergence of plain, covered by Lake Chicago. 



14. Partial resubmergence of plain, covered by Lake Chi- 

 cago. 



15. The present stage of Lake Michigan. 



It appears from the above outline that the late Wisconsin 

 series covers the entire Chicago Area. A brief discussion of the 

 several drift sheets which preceded the Late Wisconsin will be of 

 advantage, however, both for comparison and for the interpreta- 

 tion of the deposits of earlier drift found beneath the late Wiscon- 

 sin within the Chicago Area. 



Explanation of Map of North America. —The position of the 

 four main centers of glaciation is indicated, viz. : The Cordilleran, 

 confined chiefly to the region west of the Rocky Mountains; the 

 Keewatin, extending over the area between the Rocky Mountains 

 and Hudson Bay and reaching Central Missouri at its farthest 

 point south; the Labrador, extending over the peninsula of Lab- 

 rador, and the Great Lakes, with its extreme southwest terminus 

 in Southern Illinois, and the eastern borders of Missouri and Iowa. 

 (The eastern New England, New Brunswick, Nova Scotia and 

 Newfoundland glaciation may be from small independent cen- 

 ters, but this can scarcely be considered as demonstrated.) The 

 fourth center of glaciation^ is the Greenland, which is still largely 

 ice covered. The Driftless Area of Southwestern Wisconsin and 

 border districts stands between the fields covered by the Keewatin 

 and Labrador ice sheets. South from this driftless area is a nar- 

 row belt covered first by the Keewatin and later by the Labrador 

 ice sheet, as indicated in the text. 



The principal glacial lakes are perhaps sufficiently outlined 

 to be deciphered. Lake Agassiz is shown clearly. Lake Iroquois 

 occupies the Ontario basin and border districts. The Warren and 

 Algonquin Lakes are more difficult to decipher. Lake Warren 

 is now restricted by several geologists to a body of water that dis- 

 charged across Southern Michigan from the Erie-Huron basin. 

 At that time Lake Chicago was occupying the south end of the 

 Lake Michigan basin. A small predecessor of Lake Warren, 

 called Lake Maumee, had southwestward outlet to the Wabash. 

 A lake discharging southwestward from the Lake Superior basin 

 was probably a contemporary of Lake Warren. At the Lake 

 Warren stage the ice sheet was apparently covering the region 

 about the straits of Mackinac, much of Georgian Bay, and the 

 eastern end of Lake Ontario. Lake Iroquois, in the Ontario basin, 

 is of later date than Lake Warren, and is thought to have been 

 held in on the northeast by the ice sheet, as shown by studies of 



