Commonly in models of both soft-bedded ice streams and subglacial bedform genesis, rates of till deformation, basal slip, or associated rates of sediment transport are assumed to increase with shear stress, consistent with a fluid rheology for till. Although this assumption is convenient, it has led to the sustained misperception that fluid rheological behavior regulates till deformation. In contrast, data from laboratory experiments with till, the sedimentary record, and modern soft-bedded glaciers indicate that bed hydrology—specifically basal water pressure and its control of frictional slip both within the bed and at the glacier sole—is the dominant influence on both till shear resistance and deformation kinematics.

A compilation of laboratory data from experiments on basal tills indicates that resistive stresses associated with till shear are more than two orders of magnitude more sensitive to effective pressure (total stress minus pore-water pressure) than to deformation rate. Moreover, depending upon the till studied, the correlation between deformation rate and stress can be positive or negative, such that a fluid rheology, which can accommodate only a positive correlation, provides a poor characterization of the data. This conclusion is reinforced by stick-slip behavior of soft-bedded glaciers (e.g., Whillans Ice Stream), which requires a negative correlation (rate-weakening). Studies of magnetic fabrics of basal tills of lobes of the southern Laurentide Ice Sheet indicate generally shallow and highly heterogeneous deformation of the bed. Such heterogeneity, which likely plays a major role in bedform development, results from steep hydraulic gradients both in the bed and along its surface that regulate till strength and local shear stresses at the ice-till interface. This inference agrees with observations beneath modern soft-bedded glaciers and contradicts recent models of subglacial bedform genesis that neglect bed hydrology.