ORIGIN OF HUMMOCKY MORAINE IN THE CALGARY AREA, ALBERTA, CANADA

The genesis of hummocky moraine in southern Alberta has been debated in the literature for over 40 years. The prevailing school of thought regards the hummocky topography to result from reworking of superglacial till from stagnant ice blocks; a contrasting view is that the topography is created by squeezing and mobilization of wet subglacial till beneath the ice. Proponents on both sides have applied very little sedimentological or engineering data to the debate. Data we have collected in the area east of Calgary support the subglacial hypothesis.

Regional geomorphology: Air photos and published surficial geology maps show that subglacial flutes occur in close proximity to hummocky moraine. Since at least some of the surficial deposits in the region are subglacial in origin Occam's razor suggests that the adjacent hummocks probably are also subglacial in origin.

Grain sizes: Analyses of East Calgary tills yield a mean grain size composition of 29% sand, 46% silt, and 25% clay. Previously published and unpublished data sets for sub- and superglacial tills from various places suggest that such values are typical of subglacial till, and that superglacial tills tend to be coarser.

Atterberg limits: Limits for the East Calgary samples were used to construct a plasticity plot, showing plasticity index vs. liquid limit. The samples show a linear trend in the region of the graph occupied by subglacial tills in previously published work.

Consolidation: Geotechnical properties from tills sampled in the East Calgary area were analyzed to assess their degree of overconsolidation, an indicator of previous compression of these soils under past effective vertical pressures higher than those of the present. Subglacial tills are expected to be overconsolidated due to the pressure of overlying ice at time of deposition. The combined use of empirical correlations of Atterberg limits, Standard Penetration (SPT) and Cone Penetration (CPT) tests indicate that these tills are overconsolidated, though the degree of overconsolidation could not be determined with confidence. The results of an oedometer consolidation test suggest that the till at one location is lightly overconsolidated, consistent with previously published conclusions that Laurentide subglacial tills are only modestly overconsolidated due to fairly high pore pressures.