A distortion within a material, such as glacier ice: individual ice crystals react to stress by elongating, or by melting and recrystallizing, and ice may also shear along separate shear planes; see Hambrey et al. (2005) J. Geophys. Res. 110, F01006 and Mair et al. (2001) ESPL 23. See Lewkovicz and Harries (2005) Geomorph. 69, 1–4 on internal deformation in permafrost.