Research into glacier flow brings together scientists with varied experience, knowledge and interests. Work in the field, laboratory experiments and theoretical analyses go, or should go, hand in hand. It is therefore important that the scattered literature on the subject should from time to time be sifted and critically reviewed. Professor Sharp’s review is written “to bring the geologist up to date on current work, to survey recent progress, and to define some of the problems ahead”. It would very well serve to bring all glaciologists up to date for it is an admirably lucid and compact statement of the position.

The review begins, logically, with a section on the physical properties and behaviour of ice and gives a fair summing up of the plastic versus viscous argument, now, one hopes, substantially resolved. It is pointed out that the relation between plasticity and hydrostatic pressure at the pressure melting temperature needs thorough investigation because of its bearing on the velocity distribution within a glacier. Such an investigation could presumably not be entirely experimental for one would have to consider at the same time the heat flow problem of the moving glacier. A survey of studies on crystal sizes and glacier petrofabrics is followed by a section on the various mechanisms by which glaciers may move. The title of this section, “Mechanics of glacier flow”, seems unfortunate, and it would be a pity if this usage became general. Would not “mechanisms” be a better word, allowing us to keep the traditional meaning of “mechanics”, that is, statics and dynamics? The mechanisms discussed are: (1) slipping on the floor, (2) slipping along large-scale discrete shear planes with the glacier, (3) the transfer of material resulting from changes of state, (4) intergranular shifting, and (5) intragranular yielding. While all these may occur to some extent the evidence points to (1) and (5) as being dominant. As regards (4), which many consider to be a strong candidate, Professor Sharp cogently remarks that if intergranular shifting is predominant in glacier ice there seems little reason why a preferred crystallographic orientation should develop, and every reason why it should be destroyed. One might suggest as an alternative that glacier ice deforms mainly by crystallographic slip on the basal plane, and that this is accompanied by a grain boundary migration which permits the crystals to maintain fairly constant shapes. But the observed crystal orientations do not support such a view. We are surely forced to put the blame either on recrystallisation from new nuclei or, perhaps to a lesser extent, on the operation of a hitherto unseen slip system.

Professor Sharp goes on to review measurements of velocity distributions in both space and time and gives useful discussions of seasonal and diurnal variations of velocity. He emphasizes the need for more study of the movement of waves and bulges through glaciers. Ogive-type bulges at the foot of icefalls, known for so long, remain as mysterious as ever.

In describing the theoretical attempts to explain the velocity distributions Professor Sharp does the best that can be done for the doctrine of extrusion flow, but can find little justification for clinging to it in face of the adverse theoretical analyses and field tests. He is sympathetic to the present writer’s ideas on glacier flow although, as he says, they need further examination, analysis and test. It might be added that these calculations can only deal as yet with differential motion within the ice. This is really only one half of the problem. We still lack any convincing analysis of the other major part of glacier motion, namely slipping on the floor.