Outwash Plains


Sand dunes
 Large areas within the former Lake Agassiz basin are covered by thick layers of clay and sand that settled out from the water of the lake

The lower clay unit is 20 to 40 feet thick in the Winnipeg area, and increases to the south to over 50 feet thick; it consists of a lower grey silty massive clay and an upper brown thinly laminated clay. The upper silt unit rests unconformably on the clay unit, and in places is separated from it by sand or gravel. At Winnipeg, the silt unit is up to 15 ft. thick, and increases to the south to 35 feet thick. Varved clays, consisting of alternating thin layers of dark winter clay and light summer clay are common in the northern clay areas.

Some areas of the lake basin are not covered by clay deposits. A large area in the interlake region is almost devoid of clay, because of being deglaciated under subaerial conditions or because of remoteness from sources of sediment, or by erosion of a thin veneer of clay.

A huge delta formed where the ancestral Assiniboine River emptied into Lake Agassiz east of Brandon. A great volume of sand and gravel was deposited. The Assiniboine delta covers an area of 2000 sq. miles and has an average depth of at least 50 feet. This represents a volume of 20 cubic miles of material derived mainly from the melting ice. The sandy delta surface was heaped up by wind into dunes 25 to 100 feet high soon after the withdrawal of the glacial lakes.

Another deltaic deposit formed at Bird's Hill, when the ice-sheets had retreated as far as Winnipeg. These deposits are stratified, indicating that the material issuing from the stream was dropped where the fast-moving water of the stream emptied into the still water of Lake Agassiz, which at the time was about 400 feet deep in the Bird's Hill area. The deposit is thus of a delta type, with the coarse material concnetrated at the west end. The finer gravel and sand were carried farther out, and the clay remained in suspension until it had travelled far into the lake.

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