Sir,

The observations made by McKenzie and Peterson are particularly interesting and underline, if that were necessary, the complexity of glacier dynamics.

The description these authors give draws our attention to three points: first the small scale of the forms described (15 to 20 cm), then there is the very heavy debris content of the contorted ice (64% of till), finally the basal-sliding velocity, much smaller at Casement Glacier than at the Glacier d’Argentière (factor of 1: 25).

At Argentière, at the level of the resumption of contact between the ice vault and the bedrock or of the pad of ice which accompanies this resumption of contact (at the location where we reported the uphill movement of ice wedges), we have sometimes been able to observe distorted ice very similar to that reported beneath Casement Glacier; the similarity is indeed amazing. Melting by hot water of these natural formations has shown that—at Argentière—they constitute a minor phenomenon, local and temporary. This ice, equally heavily laden with sand, takes up in turn distorted, folded or simply expanded forms belonging more to the phenomena of restriction or contrariwise of compression at the level of the subglacial cavities than to a type of non-laminar flow.

In our opinion this localized phenomenon is explained: (a) by the nature of the ice which, heavily laden with solid debris, has less internal cohesion than pure ice; but especially (b) by the effects of compression on this ice at the level of resumption of contact of ice masse:; endowed with differing movements due to differing local topographic conditions; and (c) by the very jerky regime of glacier sliding velocities which favours the development of these forms.

At Argentière the distorted ice had not held our attention much, as the large glacier velocity (averaging 3 cm h⁻¹ with extreme values exceeding 10 cm h⁻¹) did not leave it time to develop. The plastic deformation of the ice was thus less apparent, often ephemeral, and was on the whole insignificant in comparison with the sliding displacement over the rock bed which is, under this glacier and in this location, the dominant process.

29 May 1974