Massachusetts Bay

Massachusetts Bay, also called Mass Bay, is a bay on the Atlantic Ocean which forms the distinctive shape of the coastline of the U.S. state of Massachusetts. It extends from Plymouth Harbor on the south to Cape Ann on the north, a distance of 42 miles (68 km). It is triangular in shape, the north and south shores inclining toward each other until at the entrance to Boston Harbor they are about five miles apart. The depth from the base of the triangle to Boston Harbor is about 21 miles (34 km). The bay's westernmost point is situated at Boston.

The northern shore is rocky and irregular, the southern low, marshy, and sandy. Along the shores are a number of capes and headlands, and off the coast a number of small islands, especially in the entrance to Boston Harbor. The principal inlets are: on the north coast, Gloucester Harbor, Nahant Bay, Salem Harbor, Marblehead Harbor, and Lynn Harbor; on the west, Boston Harbor, Dorchester Bay, and Quincy Bay (the two latter being part of the Outer Boston Harbor); on the south coast, Hingham Bay. Massachusetts Bay is itself part of the Gulf of Maine, which reaches from Cape Cod up to Nova Scotia.

Just south is Cape Cod Bay. Cape Cod Bay is sometimes construed as part of Massachusetts Bay; under this interpretation, the name "Massachusetts Bay" denotes the entire rectangular area of ocean between Cape Ann and Cape Cod.

The bay gave its name to the Massachusetts Bay Colony, one of the two predecessor colonies of the current state of Massachusetts. Massachusetts Bay, along with Quincy Bay, Narragansett Bay, Buzzards Bay and Cape Cod Bay, is also one of the bays adjacent to Massachusetts that give the commonwealth the nickname "the Bay State".