Conclusions

The thrust of this paper is to point out that there are striking differences in glacial landforms and sediments along the southern margin of the LIS. We believe that these differences are significant from a palaeo-glaciological point of view, and we encourage further research on the genesis of glacial landsystems and their significance to reconstructions of former ice sheets and ice lobes.

It is clear that climate and bed geology were important controls on the landforms that developed. Another important control was temperature at the base of the ice and in particular whether the bed was frozen or unfrozen. We postulate that there was an area (landsystem A) along the southernmost margin of the ice sheet where wet-based ice advanced into a spruce forest, and that permafrost was probably discontinuous and formed later than the maximum advance and then quickly disappeared. This area was dominated by basal melting, and broad end moraines were developed almost entirely of basal till. Behind this, subglacial sliding, ploughing and deformation produced a flat till plain with only small-scale flutes on the surface. Subglacial sediment was brought to the ice surface in interlobate areas and a few locations where bed slope was upward toward the ice margin, particularly along deep basin margins. This was particularly true during the short-lived readvances out of Great Lakes basins when large volumes of lake sediment were moved short distances into end moraines.

Further north in landsystem B, we believe ice advanced over permafrost that was tens if not hundreds of metres thick and extended laterally tens of kilometres under the advancing ice margin. The thickness of the permafrost probably increased to the north. This permafrost wedge must have had a major influence on subglacial drainage, on subglacial pore pressures, and on subglacial processes in general. In the marginal zone in the southern part of landsystem B, where the subglacial permafrost zone was fairly narrow (tens of kilometres), significant upward thrusting and stacking of material took place. Further north, where the frozen zone was wider, an extensive area of high-relief hummocky topography now marks the location of that frozen bed. Tunnel channels, which probably formed by drainage of subglacial water dammed behind this frozen wedge, are common in landsystem B and northward, but are absent in landsystem A. Likewise thrust masses and drumlins also appear to require the presence of a permafrost zone near the ice margin for their formation. There are thousands of drumlins in landsystem B, but drumlins are absent in landsystem A and commonly palimpsest in landsystem C. Most eskers in the northern landsystems appear to be later features, formed when climate warmed and water was present at the bed in marginal areas.

To the west, where regional stagnation of lobe margins followed surges, vast areas of ice-stagnation topography were produced (landsystem C). Low topographic relief, the presence of large shallow lake basins, and the fine-grained nature of tills in the area may have predisposed these lobes to unstable dynamic behaviour, particularly after the LGM when the climate warmed and subglacial water was available. Similar features and behaviour seem also to have occurred around all the Great Lakes basins during deglaciation.

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