This project involved the construction of a surficial (Quaternary) geologic map of part of McLean County, Illinois. The Saybrook 7.5-minute Quadrangle (scale 1:24,000) was the focus area. The quadrangle largely covers rural agricultural land, but also contains the small town of Saybrook. Multiple, major episodes of glaciation over the past 1,200,000 years have controlled the regional geologic history. The most recent (Wisconsin Episode) glaciation almost exclusively shaped the observed surficial geologic and geomorphic features in the quadrangle. Field observations, USDA soil survey data, private water well data, sparse stratigraphic borehole information, and LIDAR elevation data were used to construct the map. Computational methodologies included ACD Systems Canvas™ and Adobe Acrobat™ software. The surficial geologic map includes four lithostratigraphic units. In stratigraphic order (from oldest to youngest) are the Delavan
Zircons extracted from 567 granitic cobbles, in middle to late Pleistocene tills of the Lake Michigan Lobe in Illinois, provide a remarkably consistent Archean age of ∼ 2.7 Ga, with 87% dating between 2.6 and 2.8 Ga. This finding suggests a persistent glacial flow path of the southern Laurentide ice sheet from the Superior Province into the Lake Michigan basin during Marine Isotope Stage 6 (Illinois Episode) and Marine Isotope Stage 2 (Wisconsin Episode). Based on published crystalline bedrock ages in the Canadian Shield, these cobbles are interpreted to have been transported as much as ∼ 2000 km southwestward from the Quebec–Labrador ice dome, east of Hudson Bay, to the ice-sheet terminus in central to southern Illinois, USA. Some of the glacial flow likely skirted eastern Hudson Bay (source of Omar erratics) and southern James Bay, and traversed outcrops of Huronian jasper conglomerate and diamictite along the north shore of Lake Huron. Transport across the Paleozoic strata may have been enhanced, in part, by an ice stream that advanced across relatively soft and water-saturated sediments that underlie the Lake Michigan basin. The Lake Michigan basin, although present earlier in some form, was likely significantly eroded and overdeepened by accelerated glacial flow and erosion during MIS-6, further constraining the southern Laurentide Ice Sheet (LIS) flow path and influencing its subsequent flow during the last glaciation. As the Lake Michigan Lobe thinned and radiated out from the Lake Michigan basin, topographic effects led to separation of sublobes during the LIS advance to its southernmost extent.
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