THE PLEISTOCENE OR GLACIAL PERIOD 881 



Later, the whole Erie basin, and a portion of that of Ontario, 

 was freed of ice, and a lake twice the area of the present Lake 

 Erie (Lake Arkona) developed. An advance of the ice changed 

 the lake and in its changed outline it is known as Lake Whittlesey 

 (Fig. 586). 



With further retreat of the ice, the ponded waters in the Sag- 

 inaw basin became confluent with those in the Erie basin, which 

 had, in the meantime, become extended into the borders of the 

 Ontario basin, but were blocked in that direction by the Ontario 

 ice-lobe. The extensive water-body thus developed is known as 

 Lake Warren (Fig. 587). At first, this lake discharged across 

 Michigan into Lake Chicago, but later, when the Mohawk valley 

 was freed from ice, it offered the lower outlet, and the level of Lake 

 Warren was drawn down, and it was divided into two lakes, Erie 

 and Iroquois (Fig. 588). 



Meantime, the glacial lakes in the basins of Lake Michigan and 

 Superior experienced analogous shif tings of areas and of outlets. 

 While Lake Iroquois was discharging through the Mohawk valley, 

 Lake Algonquin (Fig. 588), formed by the coalescence of the gla- 

 cial lakes of the Superior, Michigan, and Huron basins, was dis- 

 charging its waters eastward. At first the outlet was probably by 

 the St. Clair-Erie route, through Lake Iroquois, to the Mohawk; 

 but later, when the ice had retired farther north, an outlet appears 

 to have been effected from Georgian Bay, via the Trent River to 

 Lake Iroquois. 



W^hen at length the ice withdrew from the Adirondacks so far as 

 to permit the waters of Lake Iroquois to find an outlet lower than 

 that by way of the Mohawk, a new series of lowerings of the lakes 

 followed. At first the outlet seems to have skirted the Adiron- 

 dacks and emptied into a glacially-ponded water-body (glacial Lake 

 Champlain) that occupied the Champlain basin, and discharged 

 southward into the Hudson. Later the outlet was to the Cham- 

 plain arm of the sea presently to be noted. By this time Lake 

 Algonquin had given place to the great Nipissing Lakes (Fig. 589), 

 which had their outlet via Lake Nipissing to the Ottawa, and thence 

 to the Champlain arm of the sea. Subsequently the outlet was 



