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A time-dependent, flow-line ice-sheet model is used to explore interactions between last glacial climate, the Laurentide ice sheet, and the Great Lakes. We seek to understand perturbations in the ice-flow field that caused three neighboring lobes to leave different geomorphic and sedimentary records. Driven by reconstructed air-temperature variations from 65 to 18 ka, the simulated lobe dynamics are consistent with constraints from loess and till chronologies, till stratigraphy, and ice-surface profiles. Contrasts in lobe dynamics are best explained by the geometry of lakes along each flow line. As ice entered these lakes, calving influenced glacier mass balance. In particular, deep water in Lake Superior likely delayed ice advance into northern Wisconsin by promoting large calving losses, whereas lobes to the east that encountered shallower water were less affected by calving. The Driftless Area of Wisconsin may thus owe its existence, at least in part, to the presence of Lake Superior. Our results suggest that morainal-bank evolution should be treated in ice-sheet models in lacustrine and shallow-marine settings. Determining the sediment flux to the morainal bank remains a difficult task. For example, ice advance across central Lake Superior was probably sediment-flux limited. Otherwise, the Driftless Area would have been glaciated. Keywords: Laurentide ice sheet, numerical model, Great Lakes, glacial geology, mass balance.

We used a numerical ice-sheet model to reconstruct the North American Laurentide Ice Sheet during the last glacial maximum. Our model simulates ice-sheet conditions that can be specified experimentally as either a rigid substrate (hard bed) or a wet, deformable till (soft bed); basal sliding is excluded. We use geologic records of former basal ice-sheet processes to prescribe the distribution of hard and soft beds. Our reconstruction of the Laurentide Ice Sheet is significantly lower in ice-surface height and contains less ice volume than the CLIMAP (maximum) reconstruction. In contrast, our reconstruction agrees well with the ICE-4G reconstruction, both in height and volume. Because the ICE-4G reconstruction is based on the inversion of relative sea-level data, whereas our reconstruction is based on glacial geology and ice mechanics, this agreement suggests that soft beds provide a glaciological mechanism to explain the shape and volume of the Laurentide Ice Sheet that is most consistent with observations of relative sea-level change and other geodynamic considerations.

D.H. Mooers claims that the ice margin used for Minnesota which was taken from studies done by Dyke and Prest in 1987 was incorrect. He believes that the ice dome southwest of Hudson Bay may have been smaller. A reconstruction of the Laurentide ice sheet was done again using 21,000-22,000 14C year B.P. as suggested by Mooers. The results obtained agrees with Mooers' contention that an ice dome southwest of the Hudson Bay is 600 meters shorter than what it would have been had it been reconstructed using Dyke and Prest's ice margin.