GLACIGENIC UNDULATING TERRAIN ON THE SOUTHEASTERN PLAINS OF CANADA: EVIDENCE OF GLACIOTECTONISM OR SUBGLACIAL MELTWATER EROSION?

Previously glaciated regions of the Southeastern Plains of Canada exhibit tracts of undulating terrain. These sets of broad, subparallel ridges separated by depressions can be identified on digital elevation models and Landsat images of southern Manitoba and southeastern Saskatchewan. Spacing between ridge crests is typically greater than 1 km and the general relief is between 2 and 10 m. Undulating terrain of this type has not been discussed or identified in previous research on the Southeastern Plains. Commonly ridges in glaciated landscapes have been attributed to brittle or ductile deformation either proglacially or subglacially, but have rarely been attributed to meltwater erosion or deposition. This research was undertaken to determine the sedimentary composition of undulating terrain and to infer the process(es) responsible for its formation. By doing so this research advances our knowledge of Laurentide Ice Sheet processes on the Southeastern Plains.

Sedimentary exposures in undulating terrain reveal highly variable compositions including: diamicton, deformed sand and gravel, stacked and folded shale and diamicton beds, and fractured shale. Evidence of substrate deformation could support a glaciotectonic origin; however, structural measurements from deformed beds indicate the deforming force acted at an oblique angle to the ridge axis. It is therefore suggested that deformation was not the sole formative agent of undulating terrain. The nascent glaciotectonic and bedrock structures, however, may have provided favourable conditions for the development of undulating terrain.

Compositional differences between tracts of undulating terrain suggest an erosional origin for this landform. Drumlinized bedrock ridges and flutings overlying and within the undulating terrain, in conjunction with subglacial channels and eskers, suggest a dynamic subglacial hydrological system. It is therefore proposed that following glaciotectonism, undulating terrain was sculpted by subglacial meltwater erosion. Earlier research on the Southeastern Plains examined the role of meltwater in the formation of proglacial spillways, but has not discussed the contribution of subglacial meltwater flows in the development of the present landscape.