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Illinoian Glaciation in Illinois

1961; University of Chicago Press; Volume: 69; Issue: 1 Linguagem: Inglês

10.1086/626713

1537-5269

Morris M. Leighton, John A. Brophy,

Cryospheric studies and observations

Present knowledge of the Illinoian glaciation in Illinois supports the following interpretations: (1) the invading Illinoian glacier was one of great vigor; (2) its limit was determined by a change in its climatic controls which halted its advance and initiated the retreat of its margin, during which moderately thick submarginal drift was deposited but no continuous terminal moraine or recessional moraines were built; (3) northeast-southwest trending ridges of the Kaskaskia River basin are stagnation features, recording the fact that the glacial margin had retreated little more than 25 miles before the glacier became stagnant; (4) many crevasse fillings were made and after some crevasses, both longitudinal and frontal, had reached through the ice, eroding streams confined along them cut straight shallow depressions, some of them crossing divides and forming trellis drainage; (5) numerous beautiful conical and ellipsoidal moulin kames, some compound and mammoth in bulk, were made helter-skelter along and between crevasse fillings as the glacial ice became decadent; (6) the small Mendon recessional moraine on the higher Galesburg Plain north of Illinois River records a somewhat longer glacial activity for a brief time, but its construction was soon followed by stagnancy when crevasse traces were eroded on flat surfaces back of it, causing many of the present linear lines of drainage; (7) the Buffalo Hart Moraine, continuous north of Illinois River for but little more than 60 miles records a brief time of renewed snowfall and glacial activity; (8) the Illinoian Glacial Lobe at its maximum development had the full deployment suggested by Leverett as covering northwestern Illinois at the time that it reached southeastern Iowa and its maximum limit in southern Illinois; (9) the dominant dolo-mitic bedrock of the northern counties and the greater relief of the region provided conditions for different physical and geochemical weathering than those of the coal measures area to the south; (10) the glacier was as vigorous when it transgressed the northern area as it was elsewhere, and it became stagnant when the rest of the glacial lobe became stagnant; (11) during the Sangamon interglacial interval, weathering and erosion modiaed the Illinoian drift everywhere, the profiles of weathering assuming characteristics according to the physical and chemical conditions; (12) no loess was deposited anywhere on the Illinoian drift because there were no valley trains during the wasting-away of the glacier, the glacier melting down in place; (13) the only loess deposit genetically connected with the Illinoian glaciation is the pro-Illinoian Loveland loess that is found over wide areas beneath and beyond the Illinoian drift; (14) Farmdale loess and the superjacent Peorian loess (Iowan and Tazewell) form a widespread mantle over the weathered Illinoian and other deposits of much of the midwest.