Glacial Lake Nantucket Sound was a glacial lake that formed during the late Pleistocene epoch inside modern Nantucket Sound. After the Laurentide ice sheet retreated, glacial ice melt washed over the terminal moraine of Cape Cod and the glacial meltwater settled in the modern day sound, creating the lake.
Map of Glacial Lake Nantucket Sound, United States
References
Glacial Lake Nantucket Sound Wikipedia(Text) CC BY-SA