Washington County, New York is part of the Great Appalachian Valley (also known simply as the 'Great Valley') which is a long narrow valley strip often between tall mountain ranges. The county transitions from the Taconic Mountains to the Adirondack Mountains, and from the Lake Champlain Valley to Hudson River Valley. Much of the county is part of the slate valley of the Upper Taconic Mountains (Taghkanic, meaning 'in the trees'). The eastern boundary of Washington County is the New York–Vermont border, part of which is Lake Champlain. This is also the border with New England proper. The northern end of the county is
New York (state) New York and Ontario sharing the Thousand Islands archipelago within the Saint Lawrence River, while most of its border with Quebec is on land; it shares Lake Champlain with the New England state of Vermont; the New England state of Massachusetts has mostly a land border; New York extends into Long Island Sound and the Atlantic Ocean, sharing a water border with Rhode Island, while Connecticut has land and sea borders. Except for areas near the New York Harbor, New York has a mostly land border with two Mid-Atlantic states, New Jersey and Pennsylvania. New York is the only state