Roughly 8,000 years BP, the Laurentide Ice Sheet thinned and split into two lobes, one centered 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. 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. Indeed, in some places the shoreline was 100 to 250 km farther inland than at present. It was at its largest at roughly 7,000 years BP.
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. The rate of uplift decreased with time however, and in any event was nearly matched by sea-level rise from the melting ice sheets. When the Tyrrell Sea "became" Hudson Bay is difficult to define, as Hudson Bay is still shrinking from isostatic rebound.