THE LAURENTIDE ICE-SHEET THROUGH THE LAST GLACIAL CYCLE - THE TOPOLOGY OF DRIFT LINEATIONS AS A KEY TO THE DYNAMIC BEHAVIOR OF FORMER ICE SHEETS

Study of satellite images from most of the area of the Canadian mainland once covered by the Laurentide ice sheet reveals a complex pattern of superimposed drift lineations. They are believed to have formed subglacially and parallel to ice flow. Aerial photographs reveal patterns of superimposition which permit the sequence of lineation patterns to be identified. The sequential lineation patterns are interpreted as evidence of shifting patterns of flow in an evolving ice sheet. Flow stages are recognised which reflect roughly synchronous integrated patterns of ice sheet flow. Comparison with stratigraphic sections in the Hudson Bay Lowlands suggests that all the principal stages may have formed during the last, Wisconsinan, glacial cycle. sheets permits reconstruction of patterns of ice divides and centres of mass which moved by 1000-2000 km during the glacial period. There is evidence that during the early Wisconsinan, ice sheet formation in Keewatin may have been independent of that in Labrador-Quebec, and that these two ice masses joined to form a major early Wisconsinan ice sheet. Subsequently the western dome decayed whilst the eastern dome remained relatively stable. A western dome then re-formed, and fused with the eastern dome to form the late Wisconsinan ice sheet before final decay. Because of strong coupling between three-dimensional ice sheet geometry and atmospheric circulation, it is suggested that the major changes of geometry must have been associated with large scale atmospheric circulation changes. Lineation patterns suggest very little erosional/depositional activity in ice divide regions, and can be used to reconstruct large scale patterns of erosion/deposition. The sequence of flow stages through time products provides an integrative framework allowing sparse stratigraphic data to be used most efficiently in reconstructing ice sheet history in time and space.