Abstract

Fabric studies suggest that an up-glacier thinning wedge of deformed superimposed ice is present beneath glacial ice at the margin of the Barnes Ice Cap. Where the glacial ice contains debris, the contact between the two is approximately marked by a ridge-shaped ice-cored moraine, 100 to 150 m from the margin. Where the glacial ice contains little or no debris, the surface profile is smooth and convex upward. Measurements of surface flow and ablation rates on both types of margins suggest that a general advance of the glacier is necessary to incorporate such superimposed ice into the margin, and hence, to instigate development of this type of ice-cored moraine. However, once a moraine forms, it may continue to grow in height and move outward without further advance of the glacier as a whole. As the moraine moves outward, the depression up-glacier from it becomes deeper, and the moraine is gradually separated from the glacier.

Debris that slumps from the moraine onto forelying superimposed ice may become buried by new superimposed ice and be reincorporated into the margin. During a subsequent advance, this debris layer will be deformed, possibly overturned, and finally brought back to the surface where it will melt to form a new ridge. Such recycling of debris may result in very complex structures in an ice-cap margin.

Measurements in boreholes and in a 125-m ice tunnel do not support the hypothesis that discrete shear zones occur at or near the contact between the dirt-bearing glacial ice and the superimposed ice.

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