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
Lake Champlain 'door of the country' or 'lake to the country'. The lake is an important eastern gateway to Iroquois Confederacy lands. The lake was named after the French explorer Samuel de Champlain, who encountered it in July 1609. While the ports of Burlington, Vermont, Port Henry, New York, and Plattsburgh, New York today are primarily used by small craft, ferries, and lake cruise ships, they were of substantial commercial and military importance in the 18th and 19th centuries. New France allocated concessions all along lake Champlain to French settlers and built forts to defend the waterways. In colonial times, Lake Champlain