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The Gulf of Maine's coastline has shifted significantly landward and seaward through geologic history. This is because sea level has risen and fallen through time, mostly because of the freezing and melting of glaciers. Also, the "land level" has risen and fallen. This is because as glaciers form, they "press down on" or depress the land; as glaciers melt, the land "bounces back" up.
From Virtual Vacationland, http://www.bigelow.org