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Glacier motion and sediment transport are attributed commonly to pervasive shearing of subglacial till to high strains (the bed-deformation hypothesis). However, the processes that control bed deformation are difficult to observe beneath modern glaciers, and observations of the geologic record have not been sufficient to determine whether tills have been sheared to the high strains required by this hypothesis. The shear strength of the ice-till interface controls whether a glacier slips over its bed or shears it pervasively over much of its thickness, with major ramifications for the flux of sediment transported subglacially. When clasts gripped by basal ice plow through the bed, the ice-till interface is weakened. A ring-shear device was used to study resistive forces on plowing clasts as a function of plowing speed. Results of experiments in which hemispheres were pushed through water-saturated till indicate a progressive decrease in resistive force on hemispheres with increasing plowing speed. This effect is due to the generation of high pore-water pressures in front of the hemispheres at fast plowing speeds (e.g. 380 m y"1), which weakens till and reduces resistive forces. These results indicate that increases in glacier sliding speed will reduce resistive forces exerted by till on plowing clasts, which will promote decoupling of the ice from the bed and inhibit pervasive bed deformation. In addition, this positive pore-pressure feedback may constitute a flow instability that promotes fast glacier flow. The ring-shear device was also used to develop criteria at microscopic scales for identifying highly sheared tills. Results of experiments indicate that the alignment (fabric) of elongate, sand-sized particles becomes strong at high strains. Furthermore, microshears

Laboratory studies of till deformation with implications for the motion and sediment transport of the Lake Michigan Lobe