Lock Ridge, representative of the many iron furnaces in the Lehigh Valley, was constructed here in 1868, operated until 1921, and was the last furnace in the U.S. to solely use anthracite coal as fuel. By the late 19th century, the Lehigh Valley was the state’s largest producer of pig iron. Among the only remnants of Pennsylvania’s anthracite iron industry, the partially restored complex is listed on the National Register of Historic Places.