Studies of superglacial debris on valley glaciers

Abstract

On the stagnant ice of Wolf Creek Glacier, in the Canadian Yukon, fully 90 per cent of the superglacial debris is derived from an englacial source. A mantle averaging two feet thick has developed by ablation of ice containing but a fraction of I per cent debris. This mantle is a heterogeneous mixture ranging from large angular blocks 15 to 20 feet in diameter to well-bedded pond deposits of clay ·and silt. With increasing time in the superglacial environment size and angularity of fragments are reduced, sorting is improved, and mean grain size is increased through removal of fine material by melt water. On areas of ice long stagnant, 50 to 60 per cent of the superglacial debris has been reworked by running water. A heterogeneous and irregularly distributed superglacial mantle promotes differential ablation which is a major factor in producing and maintaining the irregular topography of stagnant ice. Layers of fine debris, even though moist, provide insulation equivalent to a foot or two of coarse debris owing to the ease with which air and water circulate through the larger openings of the latter. Ice well insulated by superglacial detritus forms irregular mounds and ridges which increase in size until progressive thinning of the debris mantle, or its removal by some independent agent, permits more rapid ablation, eventually resulting in a complete inversion of topographic relief. Under conditions now existing in the Wolf Creek area, decades are probably required for such an inversion in the areas of heavy superglacial mantle. Glacial tables, moraines, mud ridges, and other features of the superglacial mantle are also described briefly. Dust wells and dust basins form on relatively clean ice where indirect ablation exceeds direct ablation. In this region much of the bottom silt is probably not dust. Continued deepening of wells with shaded bottoms is attributed chiefly to direct absorption of diffused radiation and partly to transmission of direct and diffused radiation through the ice. Dust basins are more largely the product of direct radiation, and their consistent east-west elongation is ascribed to greater combined retreat of the east and west walls than of the north wall alone. The division of these features into alpine- and polar-type Kryokonitlöcher does not hold on Wolf Creek. The complete wasting of stagnant ice leaves an accumulation of debris reproducing on reduced scale the irregular topography of the stagnant ice and having the constitution of the superglacial mantle.