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Abstract. Based on the results of our studies of the physical conditions beneath Ice Stream B, we formulate a new analytical ice stream model, the undrained plastic bed model (henceforth the UPB model). Mathematically, the UPB model is represented by a non-linear system of four coupled equations which express the relationships among ice sliding velocity, till strength, water storage in till, and basal melt rate. We examine this system of equations for conditions of ice stream stability over short timescales that permit holding ice stream geometry constant (less than hundreds of years). Temporal variability is introduced into the UPB model only by the direct dependence of till void ratio changes ( b = c3e/c3t) on the basal melting rate m,.. Since till strength 'cb{e} and ice stream velocity Ub{'Cb} change as long as till void ratio varies, the first condition for ice stream stability is that of constant till water storage b = 0. The second condition for ice stream stability arises from the feedback between ice stream velocity, till strength, and the basal melting rate which depends on shear heating m,.{ U6'c6}. This is the "weak till" condition which requires that in a steady state till strength is a small fraction of the gravitational driving stress 'c6<(n + 1)-•'ca. The salient feature of the UPB model is its ability to produce two thermo mechanically controlled equilibrium states, one with a strong bed and slow ice velocities ("ice sheet" mode) and one with a weak bed and fast ice velocities ("ice-stream" mode). This bimodality of basal conditions is consistent with the available observations of subglacial conditions beneath slow and fast moving ice in West Antarctica. Basal conditions that do not correspond to these two steady states may occur transiently during switches between the two stable modes. The UPB model demonstrates that ice streams may be prone to thermally triggered instabilities, during which small perturbations in the basal thermal energy balance grow, leading to generation or elimination of the basal conditions which cause ice streaming.

Ice observed on the surface of Blue Glacier is classified texturally into three types: coarse bubbly ice, coarse clear ice, and fine ice. The three types occur intercalated to form the observed foliated structure of the bulk glacier ice. Petrofabric study of fine ice reveals consistently a broad maximum in the density of c‐axis orientations, centered about the pole of the foliation plane. This single‐maximum fabric is in some respects similar to fabrics of stressed ice from polar glaciers, and the textures of fine ice and polar ice are similar. The fine‐ice layers, also resemble layers that have recently been produced by rapid shearing deformation of glacier ice in the laboratory. It is inferred that the fine‐ice layers in the glacier constitute zones that are undergoing (or have recently undergone) rapid mechanical plastic flow, and that the adjacent coarse‐ice layers originate by recrystallization from fine ice and are not now deforming rapidly by mechanical plastic flow (basal glide). Whether the fine‐ice layers have predominately a tectonic origin or whether they originate predominately as in‐fillings of snow in crevasses in the icefall is not known for certain. Coarse bubbly ice fabrics generally show more than one maximum in the density of c‐axis orientations. The statistical significance of multiple‐maximum fabrics is tested by a comparison of several independent fabrics from given stress situations, and it is shown that the basic four‐maximum pattern is reproducible, though subject to unexplained fluctuations in orientation. The ‘diamond‐shaped’ four‐maximum pattern is characteristic of ice subjected to long‐continued shear stress of persistent orientation, and the long axis of the ‘diamond’ is (approximately) parallel to the direction of the stress vector that acts across the persistent plane of maximum shear stress. It is inferred that the basic features of the pattern develop at some depth within the glacier, and that subsequent deformation has affected the pattern to some extent. The results of recent experimental studies of the origin of ice fabrics are in moderately good agreement with the Blue Glacier observations. Recent theoretical treatments are in sufficient disagreement to be ruled out. A new method of presenting orientation data allows statistical inferences to be drawn directly from the fabric diagrams.

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