GENESIS OF HUMMOCKY TERRAIN IN THE SOUTHWEST SECTOR OF THE LAURENTIDE ICE SHEET: IMPLICATIONS FOR DEGLACIAL RECONSTRUCTIONS

Large belts of hummocky terrain are often cited as evidence of former ice margin positions and used in deglacial reconstructions. An assumption in this linkage is that hummocky landforms were formed along a stagnant ice margin, where either subglacial sediment is remobilized by differential pressures under the ice, or redistributed in a supraglacial position as ice differentially melts. In either of these models, the materials are likely deformed, but they may contain a variety of different glaciogenic sediment types. Although both of these are legitimate models for the formation of hummocky features, numerous sites around central and southern Alberta have characteristics that invalidate these models for large tracts of hummocky terrain.

The sediment contained in hummocky terrain is highly variable and linked with topographic position. By far the most dominant sediment type contained in hummocky landforms is till, with secondary glaciolacustrine and glaciofluvial sediment. However, numerous hummocky landforms do not contain any till. In fact, morphologically indistinguishable hummocks are composed entirely of bedrock. These hummocks may contain highly deformed bedrock or bedrock showing no evidence of deformation. At some locations, bedrock hummocks are interspersed with ones dominated by till. Bedrock hummocks are most commonly found along margins of hummocky terrain tracts, the most highly deformed ones located on upglacier-facing slopes. In both types of bedrock hummocks, bedding is truncated at the surface. Downglacier from these slopes, the occurrences of bedrock in the hummocks decreases, succeeded by deformed and stacked till packages. Although the sediment type changes, truncated beds at the surface of the landforms are common.

These characteristics highlight three important points for understanding the importance of hummocky terrain for reconstructing former ice margins. First, glaciotectonic processes were important for emplacing sediment contained in many hummocks. Second, transitions between the sediment types found in hummocks indicates sediment emplacement during glacial onset. Third, in many locations the hummock shapes are demonstrably erosional.