RECENT GEOMORPHIC, GEOPHYSICAL, AND LITHOLOGIC DISCOVERIES ON THE ILLINOIS EPISODE (MIS 6) GLACIAL TERRAIN OF SOUTHERN ILLINOIS

Lidar digital elevation maps, combined with field observations, soil maps, archived field notes, subsurface investigations and laboratory data compilations are providing a better understanding of glacial history and stratigraphy in Illinois during the Illinois Episode (MIS 6). About 90 % of Illinois was covered by an ancestral Lake Michigan Lobe that advanced southward and westward during the Illinois Episode. New observations and compilations are part of an effort to remap the Quaternary Geology of the state at 1:500,000 scale. Lidar maps viewed at about 1:80,000 scale portray a glacially scoured landscape in many regions of southern Illinois, particularly in areas with shallow bedrock, draped by 1 to 5 m of loess and thin till deposits. Some hills in south-central Illinois, previously thought to be underlain by ice-contact glacial deposits are now known, by drilling and geophysics, to be cored by Pennsylvanian sandstone. Glacial borders and flow directions of the penultimate glaciation are also becoming more evident with lidar maps, although many questions and curiosities do still remain. In Clay County and Christian County, areas of crevasse-squeeze ridges are newly discovered from lidar data and help to outline the former shape of sublobes of the Illinois Episode ice sheet. Such features may reflect a surging ice sheet in areas of soft substrate and high water tables. Electrical resistivity transects are also helping to decipher the location of glacial meltwater channels that were previously suspected but not proven. Lastly, analytical data for till units, such as percent sand and water content, are helping to subdivide till units into mappable units. Some lithologic units represent distinct glacial advances, whereas others reflect changing substrates that were incorporated into a thinning ice sheet during its advance.