SURFICIAL GEOLOGIC MAPPING OF THE BEAVER CREEK QUADRANGLE IN SOUTHERN ILLINOIS: HIGHLIGHTING WELL-PRESERVED ILLINOIS EPISODE (MIS 6) ICE-CONTACT LANDFORMS AND DEPOSITS
In the northwestern part of the quadrangle, a several km long esker or ice-walled channel (SW oriented), contains up to 35 m of gravelly sand and terminates in a partially eroded distributary system, leading into a proglacial outwash plain. Subsurface materials in the esker, draped by last glacial loess, are substantiated from stratigraphic test cores, water well boring logs, engineering boring logs, and two electrical resistivity transects. Regionally, this feature may occur in an interlobate area, where an eastern flank of the Springfield sublobe may have overridden the western side of the Kaskaskia Sublobe. Both sublobes are divisions within a proto-Lake Michigan Lobe (Illinois Episode). Two Illinois Episode glacial till units were delineated, the Vandalia Member (upper) and Smithboro Member (lower) of the Glasford Formation. The more stiff Vandalia till has a higher content of sand, dolomite, Mg, and illite, whereas the softer Smithboro till has more silt and less carbonate, having overridden proglacial fine-grained lacustrine sediment, the Yarmouth Geosol, and pre-Illinois Episode sediments.