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.[1] Indeed, in some places the shoreline was 100 to 250 km farther inland than at present.[2] It was at its largest at roughly 7,000 years BP.[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.[2] When the Tyrrell Sea "became" Hudson Bay is difficult to define, as Hudson Bay is still shrinking from isostatic rebound.[3]