Composition, Internal Structures, and an Hypothesis of Formation for Drumlins, Waukesha County, Wisconsin, U.S.A.

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Abstract
Late Wisconsin-age drumlins west of Milwaukee are clustered atop uplands bounded by discrete scarps. Gravel operations within the uplands have exposed drumlin cores of undeformed till and outwash beds which are truncated at the sides of the drumlin. In a few areas, clastic dikes, faults, overturned bedding and shear folds disturb the original layering. The drumlin shapes are blanketed with a basal till 1–3 m thick (“retreat” till) which truncates all internal structures. This till covering cannot be distinguished from the youngest truncated till (“advance” till). Truncation of all of these structures shows that these drumlins are erosional forms carved from pre-existing drift. Apparently, during erosion under compressive flow, some beds became unstable and initiated large-scale movements of material into the drumlin from beneath the drumlin form. Failure and mobilization of the finer-grained beds may have been due to increased pore pressures, differing stages of dilatancy, or differing bulk densities. A growth relationship for drumlins may exist that is based upon the rates of material moving into the drumlin from below and rates of erosion of the drumlin form.

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