Ice-Wedge Casts and Relict Patterned Ground in Central Illinois and their Environmental Significance

Sediment features and soil patterns indicate that permafrost developed beyond the late Wisconsinan (Woodfordian) ice margin in Illinois. Vertical wedge-shaped bodies in Woodfordian glacigenic deposits and buried by Woodfordian loess are interpreted to be ice-wedge casts. Large-scale patterned ground, common on the late Wisconsinan drift plain and on the Illinoian drift plain to the south and west, also is interpreted to be the result of ice-wedge formation. Permafrost extended to about 38° 30′ N latitude and formed during the interval from about 21,000 to 16,000 yr B.P. Formation and degradation of permafrost was time-transgressive, directly related to the ice-marginal climate, and probably restricted temporally to less than 1000 years at a site. The following environmental conditions are suggested near the ice margin: mean annual ground temperature −3°C; mean annual air temperature −6 ± 2°C; less precipitation than modern, particularly in winter; strong and likely dry wind systems; and relatively open vegetation with few woody plants.