Abstract

Ice-wedge casts in Late Wisconsinan glaciofluvial sand and gravel deposits in southern Ontario taper downwards to form a V- or funnel-shaped morphology, ranging from 1 to over 3 m deep and from 1.0 to 2.5 m wide at the top. Pressure-derived primary structures, caused by repeated thermal contraction and expansion, include upturned strata, realigned stones, and small folds in the enclosing sediments. Although paleotemperature is difficult to estimate, the presence of ice-wedge casts testifies to the former presence of permafrost under prevailing periglacial conditions over this area. The regional till stratigraphy constrains the timing of ice-wedge growth to ca. 15 000 to ca. 13 000 BP. At this time, newly exposed terrains would have occurred in an interlobate area where severe climate created favourable conditions for permafrost to develop due to the close proximity to the receding ice. Inactive areas in braided rivers such as raised topographic surfaces and sand-filled channels with limited vegetation cover and minimal capability to trap drifting snow would have greatly enhanced ice-wedge growth. Rapid climatic amelioration and vegetation colonization across this area beginning ca. 13 000 BP probably slowed or stopped the growth of ice wedges. This limited window of opportunity, both geographically and in time, probably accounts for the limited occurrence of ice-wedge casts in southern Ontario.

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