When a stress is applied to a polycrystalline aggregate of ice with a random distribution of c-axes, greater than 90% of the deformation is accommodated by glide on basal planes (Castelnau et al. 1996). The deformation of any individual grains is restricted by the glide in neighbouring grains (see fig. 1.2.4). Grains rotate relative to each other and the applied stress so that deformation can proceed. The rotation may be accommodated by grain boundary sliding (Goldsby & Kohlstedt 1997) at the same time as internal glide proceeds. Grains tend to rotate such that their c-axes move towards a compressive stress and away from a tensile stress. Thus, ice which has undergone prolonged compression will have c-axes oriented in a solid cone about the compression direction (see fig. 1.4.2), while ice that has experienced prolonged tension will have c-axes oriented in a girdle orthogonal to the tensile direction. The rotation of grains relative to an applied stress, combined with elongation of grains parallel to their basal planes, produces a mechanical anisotropy and in simple shear this is a strong concentration symmetrically related to the new grain shape fabric (see fig. 1.4.3).

Recrystallisation also contributes to fabric development. If strain is sufficiently high then recrystallisation tends to replace grains at 45° to the compressive direction. Wilson & Russell-Head (1982) and Castelnau et al. (1996) suggest that recrystallisation begins with less than 1% shortening. Once new grains are formed their c-axes continuously rotate towards the compression direction leading to a smaller diameter girdle around the compressive axis. If recrystallisation occurs at lower strain then the girdle is thinner and the diameter larger (Van der Veen & Whillans 1994).