Late Wisconsinan and Holocene history of southwestern Saskatchewan

Abstract

The Late Wisconsinan Laurentide Ice Sheet reached the limit of glaciation along the north-facing slope of the unglaciated part of the Cypress Hills nunatak, but elsewhere around the Cypress Hills and Wood Mountain uplands of southwestern Saskatchewan it did not reach the limit of glaciation. An interval of deglaciation was followed by a readvance of the ice sheet, about 15 ka, to a position near the Late Wisconsinan limit, and was marked by strong flow of Keewatin ice from the north and weaker flow of Hudson ice from the northeast. Final deglaciation resulted in the formation of glacial lakes around the Cypress Hills nunatak. A network of ice-marginal and subglacial trenches, presently occupied by the Frenchman Valley and its tributaries, was formed when these lakes drained along, over, and under ice towards the western part of the Wood Mountain Upland to the east. The main trench joined an ancestral part of Frenchman Valley along the ice-free southern slopes of Wood Mountain Upland. Meltwater from the ice bordering the northern margin of the upland drained southward, excavating trenches across the highest parts of the upland and deepening ancestral valleys across the southern slopes. A succession of proglacial lakes fronted parts of the ice margin as it retreated downslope to the north. The area was deglaciated about 13.5 ka. The southern limit of a north to south forest–grassland transition was established by 13 ka along the regional drainage divide across the study area. Prairie grassland vegetation covered the southern slopes of the uplands and forests of deciduous and coniferous trees covered the highest parts of uplands and drift-mantled parts of the continental glacier to the north. The belt of forest–grassland transition had shifted well to the north of the study area by 9 ka, and a climate that was warmer and drier than at present continued to about 5 ka, when conditions became somewhat cooler.