Geomorphic expression of the Illinois episode glaciation (marine isotope stage 6) in Illinois: Moraines, sublobes, subglacial lineations, and possible ice streaming

In the central United States, the Laurentide ice sheet advanced considerably farther south and west during the Illinois Episode (marine isotope stage [MIS] 6) in Illinois than during the Wisconsin Episode (MIS 2). The Illinois Episode landscape, beyond the last glacial margin, is thus relatively undisturbed from its original form, with only a drape of last glacial loess on uplands, resulting in some of the best preserved geomorphic features of the MIS 6 Laurentide ice sheet. Recent field observations and high-resolution digital elevation maps have led to new ideas about how an ancestral Lake Michigan Lobe reached its southernmost Pleistocene extent (ca. 150- 140 ka) and about the region's deglacial history. Illinois Episode moraines are notably more narrow and discontinuous than last glacial moraines in northeastern Illinois. Subglacial lineations in Illinois, formed during the Illinois Episode, include a continuum from drumlins and megaflutes to megascale lineations. Crag-and-tail forms are most apparent in southeastern Illinois, influenced by buried Paleozoic bedrock obstacles. In north-central Illinois, megaflutes and drumlins occur in an area of thick glacial drift (>20 m). During deglaciation, an MIS 6 Lake Michigan Lobe likely separated into sublobes as the ice sheet thinned and basal ice conditions became warmer and wetter. Ice streaming into the Kaskaskia River Basin, southwestern Illinois, is envisioned during this period. Factors that likely contributed to faster glacial flow in the basin include the regional topography, a relatively soft and fine-grained substrate, and the subglacial hydrology.