BED DEFORMATION BY ICE SHEETS: EVALUATING THE ASSUMPTIONS BEHIND THE PARADIGM

By the late 1980s the hypothesis that glaciers can move primarily by shearing their sediment beds was widely embraced as both a mechanism for fast glacier flow and landform development. This model was heralded as a new paradigm in glaciology. Diverse landforms were attributed to bed deformation, including till plains, terminal moraines, drumlins, boulder pavements, Rogen moraines, tunnel valleys, and till deltas. To formulate quantitative models of glacier flow and landform genesis by bed deformation, the following assumptions have been made in over 70 articles: till behaves as a viscous or viscoplastic fluid; basal motion occurs primarily by bed deformation if bed sediments are sufficiently weak; basal shear stress equals bed shear strength, and pore-water pressure is independent of sediment deformation.

Data collected beneath modern glaciers, in laboratory experiments, and from the geologic record indicate that these assumptions are either false or true of only specific conditions. Laboratory studies supported by field data indicate definitively that steady till deformation is not viscous or visco-plastic. Moreover, these studies indicate that pore-water pressure should generally be coupled to deformation during transient till shear, leading to a wide range of apparent rheological behavior. Measurements beneath modern glaciers indicate that if pore-water pressure is high ice slips over the bed rather than shearing it. Thus, basal shear stress may not equal bed shear strength, even if the bed is weak and deformable. The geologic record indicates that bed deformation is common, but primary heterogeneities in till that are deformed but intact preclude the large strains required of the bed-deformation model; micro-structural tools for evaluating strain magnitude in massive tills have only just begun to be applied. Mechanical models of glacial-landform genesis should acknowledge these facts even if they are inconvenient, rather than relying on intuitive conjecture.