Basal Sliding and Conditions at the Glacier Bed as Revealed by Bore-hole Photography

Abstract

Bore-hole photography demonstrates that the glacier bed was reached by cable-tool drilling in five bore holes in Blue Glacier, Washington. Basal sliding velocities measured by bore-hole photography, and confirmed by inclinometry, range from 0.3 to 3.0 cm/d and average 1.0 cm/d, much less than half the surface velocity of 15 cm/d. Sliding directions deviate up to 30° from the surface flow direction. Marked lateral and time variations in sliding velocity occur. The glacier bed consists of bedrock overlain by a ≈ 10 cm layer of active subsole drift, which intervenes between bedrock and ice sole and is actively involved in the sliding process. It forms a mechanically and visibly distinct layer, partially to completely ice-free, beneath the zone of debris-laden ice at the base of the glacier. Internal motions in the subsole drift include rolling of clasts caught between bedrock and moving ice. The largest sliding velocities occur in places where a basal gap, of width up to a few centimeters, intervenes between ice sole and subsole drift. The gap may result from ice—bed separation due to pressurization of the bed by bore-hole water. Water levels in bore holes reaching the bed drop to the bottom when good hydraulic connection is established with sub-glacial conduits; the water pressure in the conduits is essentially atmospheric. Factors responsible for the generally low sliding velocities are high bed roughness due to subsole drift, partial support of basal shear stress by rock friction, and minimal basal cavitation because of low water pressure in subglacial conduits. The observed basal conditions do not closely correspond to those assumed in existing theories of sliding.

Direct Measurement of Basal Water Pressures: Progress and Problemss

“…Recent work on Blue Glacier, Washington, using bore-hole photography, has yielded important information on basal sliding and on the character of the ice-rock interface at the glacier bed (Engelhardt and others, 1978). I t is useful to compare and interpret the drilling results in the light of their findings.…”

“…I t is useful to compare and interpret the drilling results in the light of their findings. Engelhardt and others ( 1978) found a layer of gravel-like material about 10 cm thick between the bedrock and glacier sole, which is ~ctively involved in the sliding process. The voids in this "active sub-sole drift" , as they term it, are only partly filled with interstitial ice, the remainder being saturated with water at a pressure near the ice overburden pressure.…”

“…* The word "active" used by Engelhardt and others (1978) in "active sub-sole drift" refers to the fact that the material is actively involved in the sliding process. The word will be omitted to avoid confusion with the hydraulically "active" water system referred to in this paper.…”

Transfer of Basal Sliding Variations to the Surface of a Linearly Viscous Glacier

“…Although the bases of glaciers can be reached by various means for direct observation of sliding speed and factors affecting it (e.g. Engelhardt and others , 1978;Vivian, 1980), these techniques are difficult and usually very expensive.…”

Stress-Gradient Coupling in Glacier Flow: I. Longitudinal Averaging of the Influence of Ice Thickness and Surface Slope

“…The disc repancy here probably reflects increasing contributions from basal sliding as the terminus is approached. It is known that the sliding component is less than about 10% of the total annual motion in the middle part of the glacier, x --<l.8 to -Hl.8 km (Engelhardt and others, 1978;Echelmeyer, unpublished), whereas it increases both in absolute and relative amount near the terminus as indicated by measurements of marginal sliding there (Meier and others,197 4,p. 198 ).…”