Somes Sound is the only fjord on the east coast of the United States, a long, deep, narrow valley carved by glaciers and flooded by the sea. At the narrow place between Acadia and Norumbega Mountains, a concentrated ice flow cut deep into the granite bedrock one hundred fifty feet below the present water level. At the mouth of Somes Sound, where the glacier deposited much gravel and rocky debris, the water is relatively shallow.