POSTGLACIAL BIOTA OF THE GREAT LAKES REGION 1 1 1 



tween 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 avail- 

 able colls. 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 innumerable small lakes. But as the ice retreated farther 

 into the basin, the sizes of the lakes tended to increase as their basins were en- 

 larged; 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 formed a complex series of antithetical changes resulting in the 

 making and unmaking of lakes. This continued until the obstructing ice 

 withdrew from the axis of the St. Lawrence 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. " 2 



C. LAKE MICHIGAN BASIN; LAKE CHICAGO 



One of the first lakes to take definite form is known as Lake Dowagiac, 

 which extended northeasterly and southwesterly across southwestern Michi- 

 gan, and drained into the Illinois River via the Kankakee River (Plate XL VII, 

 figure 1). It is not definitely known whether any fife migrated into this region 

 from the Illinois River via the Kankakee River. Many of the small lakes which 

 now occupy the territory once covered by glacial Lake Dowagiac contain marl 

 deposits beneath two or three feet of peat and swamp deposits. These de- 

 posits contain life which could have inhabited a cold temperate region (see be- 

 low under Michigan-Magician Lake). 



As the Michigan lobe retreated into the Michigan basin, a small lake formed 

 behind the Valparaiso moraine, extending an arm northeasterly into Michigan, 

 into which the Paw Paw and St. Joseph rivers emptied. This lake, known as 

 Lake Chicago, drained southward into the Illinois River via the Desplaines 

 River (Plate XLVLT, figure 2). As the Michigan lobe melted back, Lake 

 Chicago extended in area toward the north until it filled the entire lake basin 

 (see Plate LI). These different lake stages are discussed in the previous chap- 

 ter. 



D. LAKE SUPERIOR BASIN; LAKE DULUTH 



As the Superior lobe melted and withdrew into the Superior basin a small 

 lake, known as Lake Upham, formed on the north side. This soon enlarged 

 and formed Lake Duluth, which drained into the Mississippi River via the St. 

 Croix River (Plate LI). Several small lakes, more or less transitory, developed 

 before the formation of Lake Duluth. The ice finally retreated from the Super- 

 ior basin and Lake Duluth coalesced with Lake Algonquin. 



2 Chamberlin and Salisbury, Geology, III, p. 395. 



