Penobscot Bay originates from the mouth of Maine's Penobscot River. 11,000 years ago, at the beginning of the Holocene era, the Gulf of Maine's sea level fell as low as 180 feet (55 m) below its present height. Penobscot Bay was then a continuation of Penobscot River that meandered through a broad lowland extending past present day Matinicus Island. Penobscot Bay is between Muscongus Bay and Blue Hill Bay.