Patterns of dispersal of distinctive Proterozoic and Paleozoic erratics across terrain formed on Archean and Aphebian crystalline rocks indicate that (1) ice never flowed from Hudson Bay into Keewatin in the region from the Manitoba border (lat 60°N) northward at least to lat 65°N; (2) westward-southwestward flow out of the bay, probably from a Labradorean dispersal center, interfaced with southward-and southeastwardflowing ice from the Keewatin dispersal center somewhere between Nelson River and Churchill; and (3) at least 300 km of dispersal of distinctive erratics observed from the vicinity of the last position of the Keewatin Ice Divide to the present coast of Hudson Bay required considerably more time than the 1,000 to 3,000 yr that the divide has traditionally been thought to have existed. In fact, the Keewatin Ice Divide and its precursors represent the centers of an independent, land-based ice sheet that probably existed throughout the period of Wisconsin Glaciation.

The Canadian Shield* (Fig. 1) is a geologically complex terrain that makes up about one-third of the North American landmass. Its rocks were deposited, formed, and deformed over a time span encompassing about three-quarters of the earth’s known geologic history. Consequently, it includes lithologies, structural elements, and topography as varied as those found on the North American continent as a whole. Because of the complexity of geomorphic features that reflect this geologic diversity, we have decided to focus this chapter on the most obvious geomorphologic elements of its modern landscape, the various landforms produced by the latest geologic events to affect the area, the Pleistocene glaciation. Virtually all of the distinctive landscape elements commonly associated with the Canadian Shield were formed or noticeably modified as a result of the passage of glaciers that coalesced to form the last great continental ice mass, the Laurentide Ice Sheet (Prest, 1970). Before discussing the glacial geomorphology of the Shield, it is appropriate to make some general observations of the geologic factors that have influenced the nature and distribution of glacial landforms and sediments. Because of the antiquity of the rocks that comprise this “stable” core of the continent, original lithologies have been modified by numerous orogenic and metamorphic events as the earth’s crust evolved and as crustal elements shifted through the agency of plate tectonics. The composition of the oldest rocks, in fact, reflects both sedimentation in a primitive, pre-oxygen atmosphere and igneous events that took place as the

Zonal stagnation is an important element in the regional style of deglaciation in areas of moderate bedrock relief (50–150 m), such as the shield terrane of southern Ontario. Bedrock topography played a major role in the stagnation process, as ice blocks were trapped within bedrock basins of all sizes. Stagnation occurred subglacially as the ice sheet thinned and trapped basal ice became increasingly isolated from its source. Sediment–landform relationships support this model. Bedrock lakes are floored by highly faulted laminated sediment, often displaying large ice-block casts. The dominant depositional landforms are ice-contact terraces, which flank valley walls. Sediment forming the terraces is variable, depending upon ice-marginal depositional environment.Models for two depositional systems are developed: (1) localized ice-marginal and subglacial sedimentation in upland terrain, and (2) thick proglacial basin-fill sequences developed in major structural valleys. In upland areas, sediment cover is thin and discontinuous and is composed primarily of complex sediment-flow assemblages. Basal till is rare but occurs as two distinct lithofacies representing (1) meltout from beneath wholly stagnant ice, and (2) deposition by basal melting from thin slabs of stagnant, debris-rich ice lodged beneath an active shear zone. Thick proglacial basin-fill sequences exhibit complex facies relationships. Ice-contact lacustrine terraces comprise fining-upward subaqueous outwash sequences, developed as density underflows were funnelled between rock knobs and blocks of stagnant ice. Kettle lakes developed within large basins where ice blocks persisted for the longest period of time.

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