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Four zones that are roughly concentric about the Keewatin Ice Divide are defined by landform/sedi­ment assemblages. Zone 1, including the Keewatin ]ce Divide, is characterized by a lack of eskers or oriented landforms and includes significant areas of low relief hummocky moraine. Zone 2 is delimited by the distribution of ribbed (rogen) moraine, but also includes extensive drumlin fields and well deve­loped esker systems. 'Zone 3 is characterized by a fairly continuous, commonly drumlinized, drift cover, that thins outward; Zone 4 comprises large tracts of bedrock outcrop with minimal drift cover. The types or associations of landforms, the presence or absence of landforms, and the configurations of the zones themselves are controlled by a complex interplay of ice dynamics and bedrock geology. 'Zones 3 and 4 are more influenced by the quantity of debris available for construction and hence bedrock geology. Zones 1 and 2 are more influenced by ice dynamics, in particular stagnation of a thin ice sheet during deglaciation. Yet even within 'Zone 2, bedrock geology, which determines the physical properties of the entrained debris, influences the manner of ice deformation and thus the type of landform produced. In general, less deformable sediments form ribbed moraine while more deformable occur in drumlins. Unlike drumlins, ribbed moraine, the morphology of which seems to be a relict of englacial structure, may be preserved only where associated sediment is released from stagnant ice. Hence, the outer edge of 'Zone 2 may mark the limit of widespread stagnation of the ice sheet. The integrated system of trunk and tributary eskers, formed in conduits at the base of the ice by melt­water plunging downward from the surface, probably mirrors the pattern of movement of meltwater on the surface of the ice sheet. This suggests virtual stagnation of the ice sheet while it still covered a large area.

The surficial geology of the Mackenzie valley and adjacent areas is presented in 1:1 000 000 scale compilation maps and is accompanied by geotechnical data from 56 boreholes chosen to represent typical stratigraphy of each major geological unit at several locations along the valley. For each of the representative boreholes, the potential for settlement of the ground surface as frozen ground thaws is determined, based on the thaw strain calculated for each soil layer of the borehole. Those geological materials most sensitive to thaw settlement are lacustrine and morainal, ice-rich, fine-grained sediments, and also the ice-rich and potentially highly compressible peat bogs. Much of the Mackenzie region is underlain by these materials; thus extensive terrain is susceptible to the impact of warming climatic conditions.

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