CONTRASTING RECORDS OF THE WISCONSIN AND ILLINOIS GLACIAL EPISODES IN ILLINOIS

The Lake Michigan lobe flowed into Illinois from the northeast during both the Wisconsin and Illinois glacial episodes. Although the lithology of the tills of the two episodes is similar, the landforms and sediment sequences from the two episodes vary markedly. These differences are consistent with interpreted differences in ice sheet dynamics during the two episodes.

The glaciated landscape of the Wisconsin Episode is confined to the northeastern 30% of Illinois and is characterized by end moraines (more than 30), till plains, and lake plains. Classic ice contact landforms, like eskers, kames, and kettles, are rare, except in extreme northeastern Illinois. The Wisconsin drift averages 15-20 m thick and ranges up to 100 m thick. At many locations, the Wisconsin drift consists of multiple glacigenic sequences containing till intertongued with proglacial sorted sediment. Till beds consist predominantly of uniform diamicton. This landscape and the sediment sequences are interpreted to reflect a fluctuating ice margin and predominantly active ice deposition as the glacier receded from its late Wisconsin maximum position in Illinois.

In contrast to the Wisconsin Episode landscape, that of the Illinois glacial episode covers more than 90% of the state and is characterized by till plain. Only a few fragmented end moraine segments are mapped. The drift is relatively thin, averaging less than 3 m thick and ranging up to 25 m thick. On the Illinoian till plain, ridges of variable drift up to 18 m high parallel regional ice flow and are interpreted to be ice contact ridges. Descriptions of classic sections document some weak paleosols but little interbedded proglacial sorted sediment between the tills. Sorted sediment is common within till beds. The landscape and sediment of the Illinois glacial episode appear consistent with multiple surge events and ice stagnation during deglaciation.