FACTORS 47 



The lake again receded, probably to a low level, and readvanced to 

 a 20-foot level known as the Tolleston stage (Fig. 5). Here the develop- 

 ment of beaches continued and the cutting of new cliffs was inaugurated. 

 From these beaches, dunes were developed which are fourth in point 

 of age. The position of these beaches is not indicated on the map. 

 The lake is believed to have fallen after this to a level of 60 feet below 

 the present level of Lake Michigan (60-62), which is known as the Cham- 

 plain stage. At this time the sea came up the Gulf of St. Lawrence as 

 far as Lake Ontario. Since the cliffs and beaches of this stage were 

 again submerged, they are no doubt of some importance to the aquatic 

 life in Lake Michigan, because they affected slope and bottom locally. 

 The water rose again to a level 12 to 15 feet above the present lake, 

 known as the Algonquin or post-Tolleston stage (Fig. 6), which was 

 followed by a retreat to the present level. 



2. THE FORMER CLIMATE AND ANIMALS (66) 



During the ice age, the entire region about Chicago was overridden 

 by the ice, and plants and animals migrated southward. There are at 

 present a few animal species which inhabit glaciers and ice fields, and 

 probably such were the only regular inhabitants at that time. The 

 tundra and coniferous forest were crowded to the southward, and with 

 them the caribou, musk ox, and other northern animals. As the ice 

 retreated north of the southern end of the basin of Lake Michigan and 

 the Lake Chicago stage was inaugurated, a tundra climate no doubt 

 prevailed in the Valparaiso Moraine. It was probably the breeding- 

 place of the present tundra species of birds; the home of the musk ox, 

 the caribou, the snow grouse, and other northern animals. The ponds 

 grew aquatic plants and probably supported hordes of mosquitoes (2) 

 and other aquatic insects in summer. Early Lake Chicago is said to 

 show no evidence of life. If we may judge from Arctic lakes at present, 

 it had a summer fauna, especially of small crustaceans and probably 

 some fishes. 



As the ice retreated still farther northward, the coniferous forest 

 displaced the tundra, and the musk ox and caribou were presumably 

 only winter visitors; the woodland caribou and the moose were probably 

 regular residents. Conditions in the lake were similar to those of the 

 preceding stage. By this time a relatively rich flora and fauna probably 

 existed. Organic material accumulated in the soil, shade was produced, 

 etc. With the further retreat of the ice, the coniferous forest continued 

 for a long time, but the plants and animals became gradually more and 



