260 ILLINOIS STATE ACADEMY OF SCIENCE 



lobe of the Late Wisconsin entered western Iowa, extend- 

 ing south to Des Moines. Another lobe entered north- 

 eastern Illinois but did not extend as far to the west or 

 south as the Early "Wisconsin. In eastern Indiana and 

 western Ohio, however, it covered nearly all of the Early 

 Wisconsin drift while it overrode all earlier drift in 

 northeastern Ohio. South of the glacial drift the coun- 

 try is badly dissected all the way from eastern Ohio to 

 Missouri. In the Plains Region, however, the relief is 

 moderate. The Late Wisconsin drift has many strong 

 morainal ridges and depressions while the Early Wiscon- 

 sin, although it has some strong moraines, is nearly level 

 over large areas in east central Illinois. All the earlier 

 glaciations left a comparatively plain surface, a large 

 part of which has not been dissected up to the present 

 time. Before the advance of the Late Wisconsin glacier, 

 the Mississippi, the Illinois, and the Rock rivers had cut 

 large valleys and had dissected the adjoining country to 

 some extent. During the retreat of the last ice sheet, 

 Lake Chicago was formed at the south end of Lake Mich- 

 igan basin and Lake Maumee at the west end of the Erie 

 basin. Both of these lakes found outlets to the south- 

 west, the former emptying into the Illinois river and the 

 latter through the Wabash valley into the Ohio river. 



During the advance of each ice sheet, the timberline 

 was depressed and the forest trees pushed farther and 

 farther to the south. In hilly country the conifers proba- 

 bly occupied the hilltops and exposed slopes while the 

 hardwoods were distributed along the protected slopes 

 and in the valleys. Between the timberline and the ice 

 front there was a tundra, whose width depended on the 

 topography. At the time of the maximum advance of the 

 last ice sheet the ice front was against rough hilly country 

 in eastern Ohio, and the area of tundra in this place 

 must have been small just as it is in mountainous Green- 

 land today. In western Ohio and Indiana the tundra 

 probably was wider but still rather narrow. Farther 

 west, where the earlier glaciations left a comparatively 

 flat surface, the tundra must have been very wide as it 

 is in the flat country of north Siberia today. It seems 

 probable that the tundra covered all the region between 



