The reason is a phenomenon called "post-glacial rebound". Essentially, land areas in the Northern Hemisphere that once were covered by mammoth ice sheets during the last Ice Age, such as Canada and parts of the Northeast U.S., were weighed down like a trampoline with a boulder on it. At the same time, the land around the periphery of the ice sheets, along with the U.S. mid-Atlantic coast, for example, rose up.
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