Numerical stress maps derived from deformation experiments (see fig. 1.2.4), and the use of such relations to plot deformation mechanism maps, assume that ice is homogeneous and mechanically isotropic. However, an initially isotropic polycrystalline aggregate will develop a fabric after prolonged deformations. In addition to basal glide, deformation is generally accompanied by grain boundary migration, recrystallisation and rotation of crystals (see fig. 1.1.3). All these contribute to a dynamic recrystallisation processes where there is a change in the mechanical properties of a deforming polycrystalline aggregate with time. These processes produce strong mechanical anisotropies in polycrystalline ice and result in the foliation we see in glaciers.

In an ice sheet gravity forces induce internal stresses which drive deformation and glacial flow. Nye (1952) demonstrated that flow is driven by a shear stress acting down the glacier and is related primarily to the surface slope at any point on the sheet and the depth of a column of ice directly below that point according to:

----------------------------------------equation (10)

where is the surface slope measured from the horizontal, g is accleration due to gravity, h is the depth and is the density of ice. At the base of the glacier the shear stress is assumed to be balanced by friction acting in the opposite direction such that there is no basal sliding. This assumption is true for much of the east Antarctic ice sheet where the ice is generally frozen to its base. Nye (1952) also showed that the shear stress at the base was independent of the slope of the base, so long as the difference in slope between the base and the surface was not large.

Within real ice sheets, the shear stress produced by surface slope is not the only differential stress acting on the ice mass. Changes is surface slope, rapid changes in the bed-rock topography and variations in the accumulation and ablation rates, all produce longitudinal and transverse differential stresses. However, equation 10 can be used to derive a first order estimate of stress conditions in an ice body. For example the surface slope of the section of outlet glacier in the Framnes Mountains, east Antartica investigated by Marmo & Wilson (1998) is ~0.02 Rad and depth of over 800m (see fig. 1.6.1). If equation 10 is applied to this generalised column of ice within this outlet glacier, then the shear stress increases linearly with depth from zero at the surface to 0.17 MPa at a depth of 800m (see fig. 1.6.2).

The basal ice, and ice close to the margins on the stream, have a strong single maximum due to simple shearing. In outlet glaciers the strain rate is sufficiently high that recovery via dynamic recrystallisation becomes an important process. Recrystallisation tends preferentially to produce new grains at 45° to the compression direction. With continued deformation the c-axes of ice crystals rotate towards the compression direction to form a tight girdle in ice at depth. The strong girdle fabric in outlet glaciers tends to produce deformation closer to steady state flow than is observed in other parts of large ice sheets. The flow parameter n=3 (see equation 1) is consistent for most outlet glaciers, except for the uppermost parts where grain growth may still occur reducing n to between 1 and 3 (Alley 1992).

The development of fabrics can reduce the resistance of the polycrystals to creep for a given applied stress promoting an increase in strain rate. This presents a feedback loop as a strain-rate increase leads to polycrystalline aggregates accumulating more strain, leading to stronger fabrics. It is possible that this process results in bifurcation, as strain is localised into thin layers that are less resistant to glide. Hudleston (1980) noted the development of thin shear zones in the margins of glaciers. The shear zones initially developed as lenses tens of millimetres thick and attained shear strains greater than . Higher strains were accommodated by the growth and coalescence of these thin shear zones to distribute strain more uniformly through the ice. The continuous development of fabrics and strain localisation adds further complexity to the deformation processes occuring in glaciers and many natural bodies of rock.