The success of the Drake well quickly led to oil drilling in other locations in the western Appalachian mountains, where oil was seeping to the surface, or where salt drillers had previously found oil fouling their salt wells. During the American Civil War, the oil-producing region spread over much of western Pennsylvania, up into western New York state, and down the Ohio River valley into the states of Ohio, Kentucky, and the western part of Virginia (now West Virginia). The Appalachian Basin continued to be the leading oil-producing region in the United States through 1904.