Doughnuts: "Flavours" and Genesis of Hummocky Glacial Terrain, Alberta

Hummocks, of all sizes and geomorphic patterns, occur throughout Alberta and cover about 40% of the land surface. Unlike other hummock-like forms, those in Alberta are often several magnitudes larger in area and relief. They vary from a few metres in diameter and a metre in height to in excess of 500 m in diameter and 25 m in height. The hummocks are composed of glacial sediment, predominately till, although they can incorporate significant quantities of glaciolacustrine and glaciofluvial sediment. Terrain names include: stagnant ice terrain, disintegration terrain and dead ice terrain. The two most common landforms are simple hummocks, an assemblage of equidimensional hills and hollows, and doughnut terrain, composed of circular hummocks with depressed centres. Other geomorphic patterns include plateau-like mounds and brain-like ridge patterns.

This terrain was deposited in areas where the retreating glacial ice became covered by a comparatively thick blanket of supraglacial drift and therefore melted slowly, allowing hummocks to form and be preserved. The melting of the buried glacial ice is known to have take thousands of years in some areas. During the Holocene, this terrain has undergone moderate post-depositional modification with sediment being eroded from the crests and deposited on the flanks, or for doughnuts, within the central depressions. Multiple weathering horizons have been documented within this sediment.

Another hummocky terrain unit is composed entirely of glaciolacustrine sediment. The relatively low relief phases of these landforms are referred to as humpies. Their genesis is generally due to deposition of glacial lake sediment over grounded glacial ice which subsequently melts to produce the hummocky/doughnut terrain. In some areas, it is proposed that they may also be a product of rapid permafrost development in recently drained glacial lakes under periglacial environments, which subsequently disintegrated during the early Holocene.