Smoke Hole is a rugged canyon of 1421 feet elevation formed by the South Branch of the Potomac River and extending eighteen miles to the junction with the North Fork. In Coeymans Limestone of Devonian Age, the river has carved various formations and caves. Big Cave and Old Mines Cave, where saltpeter was made during the War Between the States, are the most noted. Smoke Hole is one of the State's natural wonders.