Geotechnical model for great plains hummocky moraine formed by till deformation below stagnant ice

Hummocky or “disintegration” moraine is a widespread late Wisconsin glacial terrain type across the glaciated Great Plains of mid-continent North America. This terrain consists of chaotic hummocks and enclosed “kettle” hollows that are widely underlain by clay-rich till. The prevailing interpretation of this terrain is one of supraglacial deposition from debris-covered stagnant ice and is based largely on geomorphologic comparisons with modern valley glacier margins. Recently, in contrast, it has been argued that hummocky moraine can form subglacially by the “pressing” of fine-grained deformable till under the overburden pressure of stagnant ice. In this paper, we test the postulated subglacial origin of hummocky moraine using a geotechnical model of a deforming soil (till). The model confirms that hummocky topography can be the result of failure and deformation of fine-grained till by differential loading below stagnant ice. Differential pressures at the ice/bed contact arise from the complete or partial perforation of thin stagnant ice sheet lobes by numerous water-filled, doline-like depressions. The growth and decay of ponds as ice down-wastes result in repeated cycles of subglacial till failure and landform development resulting in the observed random, linear, circular, elongate, and other patterns of hummocks, ridges, and hollows. Formerly streamlined, preferentially oriented subglacial landforms, such as drumlins and Rogen moraines, can also be overprinted forming “washboard moraine”.