Abstract

The mean annual temperature during a minor glacial readvance prior to deposition of the Pinnacle Hills moraine in western New York State is inferred to have been −5 °C. This conclusion is based upon an analysis of a glacially induced bedrock thrust exposed in the bank of Densmore Creek, 5 km northeast of Rochester. Deformation could have occurred only if the glacier were frozen to the bed and excessive pore pressures existed. This condition was created by a lack of drainage of pore water from overridden sediment and bed rock, reducing the normal stresses produced by the overlying ice. Only when the glacier was frozen to the substrate could the basal shear stress of the ice exceed the shear strength of the bed rock. Using a conservatively high basal temperature of−1 °C and a thickness of ice based upon its terminal position, coupled with genetic data from the thrust, an estimated surficial temperature of−5 °C is calculated.

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