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
Champlain and St. Lawrence Railroad line. The line was extended south along the Richelieu River valley in 1851 to Rouses Point, New York and the following year saw a more direct routing built from the St. John-Laprairie line to St. Lambert, directly opposite Montreal to avoid the indirect journey up and down the St. Lawrence River from Laprairie. This latter line effectively reduced the line into Laprairie to branch line status and it would be later abandoned. The C&SL merged with the Montreal and New York Railroad in 1857, formerly known as the Montreal and Lachine Railroad, with the new company being named the Montreal