Sand‐wedge and ventifact palaeoenvironmental indicators in North‐West Saskatchewan, Canada, 11 ka to 9.9 ka BP

Exhumed sand wedges and ventifacts in the upper Churchill watershed of north‐western Saskatchewan indicate that aeolian and periglacial processes were dominant from c. 11 to c 10.5 ka BP . Nineteen sand wedges were identified that display vertical foliation, form polygonal nets, and penetrate either till, glaciolacustrine sediment or glaciofluvial diapiric sand. The sand wedges, and sand lenses within the till and glaciolacustrine sediments, indicate permafrost aggradation after glaciation and lake drainage. Ventifacts that overlie the sand wedges have preferred keel orientations that correlate with palaeo‐katabatic wind directions interpreted from stabilized parabolic dune orientations. Rat‐tail lineaments on faceted surfaces of coarse‐grained, granitic ventifacts also indicate palaeo‐wind flow direction. Strong and weakly developed lineaments on the same einkanter are explained by opposing flow from prevailing winds, katabatic winds and anticyclonic air flow from the adjacent Laurentide Ice Sheet, or by flow separation at the ventifact keel under unidirectional air flow. Two geomorphic surfaces are identified: (1) an older surface with sand wedges and weakly developed ventifacts indicative of permafrost and brief aeolian activity; and (2) a lower and younger segment of the former Lake Agassiz basin, without permafrost indicators but with well‐developed ventifacts. The upper surface, developed in association with the Beaver River Moraine and glacial Lake Meadow‐McMurray, is 11–10.5 ka BP in age, synchronous with the Younger Dryas cold period. The younger, lower surface was last modified beginning at 9.9 ka BP , when the north‐west outlet of Lake Agassiz opened, was susceptible to greater aeolian reworking than the upper surface, and postdates the Younger Dryas. © 1996 John Wiley & Sons, Ltd.