The lowlands adjacent to the Red River Valley were flooded whenever Quaternary glaciation dammed the northward-draining river systems of the region. The most recent impoundment, referred to as Lake Agassiz, began shortly before 13 500 years BP, as late Wisconsinan ice retreated northward in the Valley for the last time. In southern Manitoba, three fine-grained lacustrine units, numerous beach deposits, and an extensive area of fluvio-lacustrine (deltaic) sediment were deposited in and around the main depositional center of the Lake Agassiz basin during the life of the lake.The oldest offshore deposit of Lake Agassiz (Unit 1) is a silty clay containing ice-rafted clasts of till, clayey silt, and rock. Most of the silty and sandy sediment of the Assiniboine Delta also was deposited at this time. A readvance of ice into the northern and eastern Lake Agassiz basin, about 9800 years ago, caused a new influx of ice-rafted sediment into the offshore silty clays of southern Manitoba. Clast-rich Unit 2 was deposited at this time along the northern and eastern margins of the basin. When the ice retreated from the area shortly after 9800 years BP, lake levels dropped, and siltier, better laminated, and relatively clast-free Unit 3 was deposited. Units 1, 2, and 3 in southern Manitoba are correlated, respectively, with the Brenna Formation, lower part of the Sherack Formation, and upper part of the Sherack Formation of Lake Agassiz in North Dakota and Minnesota. Lake Agassiz deposition continued in southern Manitoba until after 8700 years BP.

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