FLOW MECHANISM OF ICE SHEETS ON UNLITHIFIED SEDIMENT: PLOWING OF CLASTS AT THE ICE-BED INTERFACE

Large portions of former ice sheets, such as the Laurentide Ice Sheet, moved rapidly over deformable till (soft beds). Fast flow of most glaciers is related directly to conditions at the ice-bed interface. The strength of the ice-till coupling controls whether glaciers slip over their beds or shear their beds pervasively, with major ramifications for the mode of sediment transport and the formation of drumlins, eskers, and moraines. The coupling between ice and till is weakened when clasts gripped by glacier ice plow through the bed surface. If plowing accounts for most glacier motion, models of glacial geomorphic processes that require deforming beds may not be valid. Although there is field evidence for such plowing, there are no data with which to constrain mechanical models of this process.

A ring-shear device was used to study resistive forces on plowing clasts as a function of plowing speed. Hemispheres of different sizes were pushed at steady speeds through water-saturated till. The till was in hydraulic communication with water in an external reservoir at atmospheric pressure. Resistive forces on the hemispheres and pore-water pressures in adjacent till were measured at various plowing speeds ranging from 15 to 380 m/year. Tests were conducted using the basal till of the Des Moines Lobe. Results indicate a progressive decrease in resistive force on the hemispheres with increasing plowing speed—the opposite of that expected if the till behaved as a viscous fluid. At high plowing speeds pore-water pressures well in excess of hydrostatic were generated in front of the hemispheres, which weakened the till and reduced resistive forces. Excess pore-water pressures developed because pore-pressure diffusion could not keep pace with the rate of till compaction in front of the hemispheres. These data will be compared with the results of tests using till from Engabreen, which has a hydraulic diffusivity two orders of magnitude larger than that of the Des Moines Lobe till.

These results indicate that increases in glacier sliding speed should reduce resistive forces exerted by till on plowing clasts, which will tend to decouple ice from the bed and inhibit pervasive bed deformation. This result is consistent with field measurements that indicate glaciers do not shear their soft beds when pore-water pressure and ice velocity are sufficiently high.