Abstract

Examination of several drumlins in the Caledonia drumlin field of southern Ontario reveals inverse-graded units occurring from the surface to near bedrock. These units were found within a two-till, conformable sequence. Evidence indicates that the two tills, on the basis of provenance and geochemical and geotechnical characteristics, are subfacies of a single depositional phase. Fine subunits within each inverse-graded unit have above average calcium and magnesium contents and exhibit pore-wall cutan formation. Below average calcium and magnesium contents occur within coarse subunits, and no cutans were observed. In explaining this differentiation and the cause of inverse grading, a hypothesis is suggested of pore-water migrating contemporaneously with localized and episodic till "lodging" or deposition from a mobile, basal tractive load. For the drumlins revealing these inverse-graded units, it is thought that an accretion–depositional model is most likely for formation from an initiating nucleal patch of immobilized tractive load. It is suggested that subglacial environments similar to that in the Caledonia area must have existed over widespread lowland areas beneath the Quaternary ice sheets.

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