Hudson Bay was the center of the great Laurentide Ice Sheet, which was almost 2 miles thick. Stack two of the new World Trade Center towers, each 1,776 feet tall, and they still fall short of that ice.
Randall asks the listener to imagine what that weight does to the planet. It pressed the crust down, probably between 1,000 and 2,000 feet. Remove the ice in a geological instant, and the land starts to rebound.