Roughly 8,000 years BP, the Laurentide Ice Sheet thinned and split into two lobes, one centred over Quebec-Labrador, the other over Keewatin.
This drained Glacial Lake Ojibway, a massive proglacial lake south of the ice sheet, leading to the formation of the early Tyrrell Sea.
[1] The weight of the ice had isostatically depressed the surface as much as 270-280 m below its current level, making the Tyrrell Sea much larger than modern Hudson Bay.
[3] Isostatic uplift proceeded rapidly after the retreat of the ice, as much as .09 m per year, causing the margins of the sea to regress quickly towards its present margins.
[1] The rate of uplift decreased with time however, and in any event was nearly matched by sea level rise from the melting ice sheets.