This formation of upland and valley extends to the shore-line of Sound and ocean, the broad coastal plain, which is so marked a feature from New Jersey southward, being almost wholly absent in New England. This is probably due to a subsidence of the shore, which allowed the ocean to flow back over part of the land, and which also explains the many hundred islands off the coast of Maine, and the drowned river valleys along the Sound. So numerous are the islands, bays, and headlands of the rugged coast north of the Isles of Shoals, that they expand the two hundred and thirty miles of shore to nearly three thousand, if all are included in the measurement. In this section, also, there are many good harbors, particularly that of Portland, but the coast is so greatly dissected as to make land communication along it very difficult; while the small boats, which partially served the needs of commerce and travel in early days, were seriously interfered with by the great rise and fall of the tides. Both these conditions tended to isolate the colonial settlements and hinder their development. The upland country, with its poorer soil and more difficult conditions of life, also approaches nearer to the sea in Maine and New Hampshire than farther south, so that, although Portsmouth, too, has a fine harbor, those states have always been more thinly settled than the others. The coast of Massachusetts is less rugged, but more varied. South of the granite headland of Cape Ann, the shores of Boston Bay are still rocky and irregular; but both shores of the great sandy curve of the Cape Cod peninsula, which, with Cape Ann, encloses the waters of Massachusetts Bay, are smooth and moulded by wind and wave. The coast again becomes rough around Buzzard's Bay, while the almost land-locked waters of Rhode Island have drowned the old river-system of that state. Opposite Connecticut lies Long Island, the only island of any considerable size along the entire Atlantic coast; so that the Sound, or inland sea, thus formed between it and the mainland, gives to Connecticut the advantage of a quiet, protected waterway for all its ports.