The Editor,

Journal of Glaciology

Sir, Depth hoar on Arctic glaciers

A coarse-grained, low-density layer of snow forms in late summer or early winter in both the accumulation and the ablation zones of Arctic glaciers and on glaciers elsewhere; it usually contains depth-hoar crystals, and forms a convenient stratigraphic annual marker (Reference BensonBenson, 1959, p. 51−52). Benson attributes the formation of the layer to a combination of strong wind and steep temperature gradient in the autumn, causing loss of mass by sublimation and evaporation. He uses the discontinuity between this layer and a finer-grained, denser and harder layer above as his annual boundary in pit profiles in the accumulation zone; in other words, the depth-hoar Iayer is the top component of his annual accumulation.

The purpose of this letter is to point out that in the ablation zone of a glacier the depth-hoar layer which forms immediately above the ice surface in, say, the 1962 spring is naturally regarded as the bottom component of the 1961–62 accumulation; no one would regard it as the accumulation of the 1960–61 budget year deposited after the end of the ablation season. In the accumulation zone, therefore, consistency and convenience in making computations of mass balance require that the bottom, not the top, of the depth-hoar layer be regarded as the annual boundary. Although the same processes operate, the depth-hoar layer does not of course form simultaneously on all parts of the glacier; it forms earliest in the highest accumulation areas and latest near the glacier terminus. It follows that the budget year should be considered as ending on different dates at different altitudes, which is not the same as saying that the budget year ends at the time when the snow-line on the glacier has reached its maximum recession.

11 December 1961