Acknowledgements

This work was supported by the Départements Sciences Pour l'Ingénieur and Sciences de l'Univers du CNRS. We are very grateful to V. Lipenkov for valuable suggestions during the preparation of the manuscript.

attributed to a single deformation mechanism, dislocation creep, deemed independent of grain size and characterized by a stress exponent n of ca. 3. Despite the widespread adoption of the Glen flow law in glacier and ice-sheet mechanics over the past 50 yr, however, numerous laboratory studies on relatively coarsegrained (>1mm) samples have suggested that a transition to a creep mechanism characterized by n < 2 occurs at stresses lower than investigated in Glen's experiments and relevant to glacier and ice-sheet flow (o <0.1 MPa) (e.g. Steinemann, 1954, 1958a; Mellor & Smith, 1966; Mellor & Testa, 1969; Colbeck & Evans, 1973; Pimienta & Duval, 1987). Unfortunately, the transition to values of n < 2 observed in these previous studies occurs at

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