Coal from southwest Pennsylvania, including the Youghiogheny River Valley, helped fuel America’s industrial revolution and filled the railroad cars that once ran along this path. The husbands and sons of many local families rode deep underground each day to dig tons of coal from ancient deposits. Coal beds in the hills along the Youghiogheny date from the Carboniferous Period, about 300 million years ago, when tropical seas and wetlands covered southwest Pennsylvania. Small coal seams can still be seen compressed between layers of other rock, and Pennsylvania miners often worked in cramped underground shafts while low-slung rail cars, powered by electricity in the 20th century, carried coal through mine tunnels to the surface.