GLACIAL AND POSTGLACIAL HISTOEY OF GREAT LAKES REGION. 321 



what similar origin that were formed along the east side of the Lake Michigan basin (pp. 225-227). 

 Winehell has described a series of small lakes that preceded the larger lakes in the western end of 

 the Lake Superior basin. 1 



When the ice front had retreated to a position a little east of the watershed at Fort Wayne 

 Ind., a long, narrow, crescent-shaped lake, known as glacial Lake Maumee, was formed between 

 the ice barrier and the land. Similarly, when the ice front had withdrawn a few miles north of 

 the watershed at the south end of Lake Michigan and was building the later members of the Lake 

 Border morainic system, another long, slender, crescent-shaped lake was formed between the ice 

 barrier and the land; this was the beginning or first stage of glacial Lake Chicago. If the mo- 

 raines have been rightly interpreted Lake Maumee came into existence shortly before Lake 

 Chicago. 



A third lake of the same kind (Lake Duluth) was formed in the same way at the western end 

 of the Lake Superior basin, when the Lake Superior ice lobe finally shrank within the line of the 

 watershed west of Duluth. Mr. Leverett's work in Minnesota in 1910 and 1911 inchoates that 

 the beginning of tins lake was considerably later than that of the first lakes at Fort Wayne and 

 Chicago. He makes a tentative correlation of the Port Huron morainic system (formed at the 

 time of Lake Whittlesey) with a moraine which encircles the west end of Lake Superior and 

 antedates Lake Duluth, thus making Lake Duluth younger than Lake Whittlesey. 



Two small lakes, formed probably a little later than Lake Chicago, gathered in front of the 

 Green Bay lobe of the ice sheet in Wisconsin and discharged first southward to Rock River and 

 then southwestward to Wisconsin River. These lakes, however, had a relatively short inde- 

 pendent existence, for they soon combined into one lake and when the retreating ice opened a 

 passage eastward to the basin of Lake Michigan their waters merged with those of Lake Chicago. 



From these four relatively small beginnings there grew a series of glacial lakes, the like of 

 which for size and complicated history is not known in any other part of the world. The total 

 area covered by their waters from first to last was much greater than the entire area of the present 

 Great Lakes, but the whole area was not covered at any one time. Only one glacial lake of larger 

 size is known to have existed; this is Lake Agassiz, which overspread northwestern Minnesota, 

 northeastern North Dakota, and a great area in Manitoba and Saskatchewan. But the history 

 of Lake Agassiz appears to have been very simple in comparison, for the basin which it occupied 

 was simpler in form and its relations to the ice sheet were less complicated. 



GLACIAL LAKES IN THE HURON-ERIE BASIN. 



COMPLEXITIES OF DEVELOPMENT. 



The lowland which stretches from Lake Huron southward to Lake Erie was abandoned by 

 the ice sheet in the middle stages of the development of the glacial lakes, but continued to be 

 covered by the lake waters, so that the waters in the southern part of the Lake Huron basin 

 were joined with those in the basin of Lake Erie as one, and at a later stage this extensive lake 

 was further expanded so as to cover the western part of the basin of Lake Ontario. 



The succession of lakes in tins basin is more complex than in any other part of the lake 

 region. These complexities grew out of several different causes: (1) The configuration of the 

 chief elements of relief — deeper basins separating higher lands and ridges, and the position and 

 varying altitude of the watershed south of the lakes; (2) the general direction of the glacial 

 retreat and the trend of the ice front with reference to these features; (3) the oscillations of the 

 ice front during retreat, comprising not only periodic movements of retreat and halt, but also 

 alternating movements of readvance over relatively wide intervals of space; and (4) (especially 

 in the later stages) the tilting or northward differential elevation of the land. The readvances 

 of the ice introduced the greatest element of complexity and produced their effects chiefly by 

 closing outlets and raising the level of the waters. This occurred repeatedly shortly after the 

 waters had been lowered by a movement of retreat. 



i Winehell, N. H., Bull. Geol. Soc. America, vol. 12, 1900, pp. 121-122. 

 34407°— 15 21 



