The Tyrrell Sea, named after Canada, Canadian geologist Joseph Tyrrell, is another name for prehistoric Hudson Bay, namely as it existed during the retreat of the Laurentide Ice Sheet. Roughly 8,000 years before present, BP, the Laurentide Ice Sheet thinned and split into two lobes, one centred over Quebec-Labrador, the other over District of Keewatin, 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 isostasy, isostatically depressed the surface as much as 270-280 metres, 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."Geomorphology From Space, Plate C-24: Hudson Bay Shorelines."
(Accessed 3/7/06) It was at its largest at roughly 7,000 years BP. Post-glacial rebound, 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.

References

Historical geology Geology of Ontario Geology of Canada Holocene Hudson Bay {{palaeo-geo-stub