Introduction

Permafrost was a dynamic and prominent element in the forefield of and under the southern margin of the Laurentide Ice Sheet (LIS) during the last glacial cycle. As it waxed and waned in response to changing climate, it had an impact on groundwater flow and the subglacial thermal regime and, consequently, the extent, form, and dynamics of ice lobes and the landforms they produced (Mickelson et al., 1983).

In one of the first examinations of time-dependent groundwa-ter-permafrost-glacier interactions, Cutler et al. (2000) investigated the influence of permafrost on subglacial conditions using a coupled numerical ice-flow-permafrost model. The model was applied to the Green Bay Lobe, which flowed into the Great Lakes region and terminated in Wisconsin. Experiments focused on conditions during ice advance, as it is during such conditions that subglacial permafrost was likely most extensive and most influential to subglacial drainage, ice motion and landform evolution.

We know little about the time-transgressive nature of ice-permafrost interaction and the likely thickness and horizontal extent of pro- and subglacial permafrost. Consequently, Cutler et al. (2000) addressed three fundamental questions:

1 How deep was ice-marginal permafrost under the Green Bay Lobe?

2 How wide was the subglacial frozen zone (measured from the ice margin and extending upglacier) during ice advance?

3 How quickly does subglacial permafrost thaw once overridden?

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