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Glacial Erosion in Tilted Rock Layers

James H. Zumberge
The Journal of Geology
Vol. 63, No. 2 (Mar., 1955), pp. 149-158
Stable URL: http://www.jstor.org/stable/30080875
Page Count: 10
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Glacial Erosion in Tilted Rock Layers
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Abstract

Isle Royale in Lake Superior and the Rove area of northeastern Minnesota have valley and ridge topographies controlled by bands of massive, poorly jointed rock, alternating with highly jointed rock units. The ridges are asymmetrical in cross section, with gentle dip slopes on one side and precipitous antidip slopes on the other. The valleys contain glacially quarried bedrock lakes which are asymmetrical in cross section because of the same structural elements that control the ridge shapes. The last Wisconsin ice crossed the Rove area at right angles to the regional strike but invaded Isle Royale, moving parallel to the regional strike. In spite of this one difference in geomorphic history, the resultant terrains are nearly the same in appearance. This leads the writer to conclude that the direction of ice movement with respect to preglacial valleys is of little or no importance in the glacial quarrying process but that the structural and lithologic elements of the preglacial terrain are far more potent in controlling the course of glacial denudation. Factors controlling the process of glacial denudation are divided into two categories: (1) the applied force and (2) the resisting force. Glacial abrasion is controlled primarily by factors of the applied force, such as ice thickness, velocity and direction of flow, and amount and kind of debris in the basal ice. Glacial quarrying is controlled primarily by factors of the resisting force, especially structural and lithologic elements.

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