A good grade of hematite ore was discovered in the Catoctin Mountains in the 1770s by Thomas Johnson Jr. The Catoctin Furnace started producing pig iron in 1776. This furnace was fueled by charcoal from the Catoctin forest. The original furnace continued in blast intermittently until 1890, when it was dismantled. In 1856 a new owner erected a second furnace, charcoal fired and using a steam powered hot blast, which operated until 1893. The remains of this furnace are visible today. A third furnace, fired with coal was erected in 1873, but was shut down and dismantled in 1903. The remains of these iron works are found at the base of the Catoctin Mountains in Cunningham Falls State Park.

http://www.dnr.state.md.us/publiclands/cunninghamhistory.html