Rotational Movement in Cirque and Valley Glaciers

The concept of rotational slip in glaciers, previously introduced by Lewis, must now be considered in the light of recent theoretical work by Orowan and Nye, based on the assumption that ice is an ideally plastic material and that rate of stress is a function of the shear stress. This paper is chiefly concerned with the formation of shear surfaces in "passive" flow. Possible inferences of Nye's work are discussed. If substantial differential movements develop along the surfaces of maximum shear stress of the passive state, we may have found an important part of the explanation of the variation of intensity of glacial erosion. Previous work relating to rotational movement is surveyed; and important pioneer work by Gibson and Dyson is discussed. Results of systematic studies of banding of various cirque glaciers in the Jotunheimen area of Norway are presented. The remarkable systematic behavior of the dips of the bands, which increase steadily away from the head wall and decrease steadily again toward the terminal moraine, is most easily attributable, at present, to some form of rotational movement. Surface features of the ice below the Mount Collon icefall are discussed in relation to subglacial conditions exhibited in a tunnel dug through to the rock wall. Possible implications of a set of data concerning ice movement and distribution of banding are discussed. We may have here the first evidence of rotational movement beneath an icefall.