The basal ice of sliding glaciers contains rock debris that is held in contact with the rock bed, resulting in abrasion that erodes the bed and friction that slows sliding. Abrasion rates and the magnitude of this friction are controlled by contact forces that clasts exert normal to the bed surface. These contact forces are dependant on the drag exerted on clasts by ice as it flows toward the bed. Recent subglacial measurements indicate that debris-bed friction has been underestimated by at least an order of magnitude. This underestimate may be the result of enhanced drag on particles and associated contact forces, due to the proximity of the bed as a boundary condition that perturbs ice flow.

To test this hypothesis, laboratory experiments were performed to measure drag forces exerted by moving temperate ice on an idealized clast (50 mm sphere) both isolated from and resting on the bed. A stress of 1000 kPa was applied to the top of a confined cylinder of ice 0.2 m in diameter. The ice cylinder was brought to the melting temperature and then melted preferentially at its base, so that ice moved toward the underlying flat bed. The resultant drag force on the sphere (577 - 2112 N) was measured in experiments in which the ice velocity (0.38 - 2.3 mm d^-1) was incremented or decremented after a steady drag force was attained.

The drag enhancement caused by the proximity of the bed, in contrast to the results of numerical models, is small: the drag force on a sphere on the bed is only 1.23 - 1.28 times larger than that on an isolated sphere. This small drag enhancement is due to a water-pressurized cavity that develops beneath the sphere. This cavity prevents low ice pressures from developing there, which are required to achieve a large drag enhancement. These results indicate that theory describing the drag on idealized, isolated clasts provides a reasonable estimate of contact forces. The rheological parameters of the synthetic ice used in these experiments--derived from the measured drag forces and ice velocities for the case of the isolated sphere--were n = 1.3 and B = 9.64y10¹⁰ Pa^-1.3 s^-1, where n is the stress exponent and B is the fluidity parameter in the ice flow rule, as expressed by Lliboutry (1979). If the ice rheology is approximated as being linear (n = 1), then B = 5.35y10^-12 Pa^-1 s^-1.