Marginal Zones of Vanished Glaciers Reconstructed from the Preconsolidation-Pressure Values of Overridden Silts

Thickness values for vanished glaciers are useful in solving many of the difficult problems of glacial stratigraphy, sedimentation, and geomorphology, and it is expected that a knowledge of the profiles of vanished continental ice margins may eventually enhance our understanding of the climates of the glacial stages of the Pleistocene epoch. Values for the preconsolidation pressures of silts that were overridden by four oscillations of the Wisconsin continental ice margin were determined in the soil-mechanics laboratory at Purdue University. Results of seven consolidation tests were used to reconstruct part of the vanished East-White sublobe of central Indiana. A paleoglacier map that is presented for this sublobe is for the particular advance whose maximum extent is delineated by the "Champaign" drift boundary. The map shows altitudes on the vanished glacier surface and 200-foot contours as much as 25 miles back from the margin. The glacier is determined to have been 1,690 feet thick at a point 27.6 miles from the margin. A log-log plot of glacier thickness at a consolidation-test site versus till-sheet thickness at the site yields a linear relationship. A nearly linear relationship also is obtained if a log-log plot is made of the distance of a sample site from the glacier margin versus the till-sheet thickness at the sample site. Assuming linearity for the former relationship, a straight-line curve was fitted to the datum points. When glacier thickness and till-sheet thickness are expressed in feet, the equation for this function is y=3.15x1.69. This power function may represent a rough "erosion-transportation" rule for the vanished glacier; for example, the thickness of material (in feet) which the glacier eroded and transported in this marginal zone was approximately equal to 0.32 (the glacier thickness [in feet])0.59.