Theories of glacier flow are based commonly on the assumption that ice is not a newtonian fluid, but has a non-linear stress-dependent viscosity1–5. Here we re-examine the spreading of Antarctic ice shelves and suggest that the data cannot define a unique flow law. Tilt measurements in four boreholes in both the Arctic and Antarctic seem to show that a linear flow law ε̇ = A1τ (where A1∼10−15 s−1 Pa−1 at −28 °C), where ε̇ is the effective strain rate and τ the effective shear stress, may be just as appropriate for describing the flow of polar ice sheets.