The Massachusetts coastline is deeply indented with bays, coves, and estuaries, separated by narrow promontories. Some of these form natural harbors that gave rise to the state's historic ports, including Newburyport, Gloucester, Salem, Boston, and New Bedford. The state has a few small barrier islands, the largest of which is Plum Island. The state's largest promontory is the Cape Cod peninsula. Its backbone is formed by glacial moraines, but much of its coastline has been shaped by the longshore drift of coastal sand, which forms many of its famous sandy beaches. To the south of Cape Cod, glacial moraines rise above the ocean surface to form the state's largest islands: Martha's Vineyard, Nantucket, the Elizabeth Islands, and Monomoy Island.