Pennsylvania's Coal Seams

Most people were taught in elementary or perhaps middle school science class that coal was formed from plant material that grew in vast forest-swamps on Earth eons ago. But, like the subject itself, there are more layers to this story than may first meet the eye. Dig.

The lush vegetation grew and died in a continuing cycle that accumulated into a thick layer of decaying material that was eventually buried by sediments as geologic forces and weathering transformed Earth. Three hundred million years ago, a massive coal-swamp forest covered much of western Pennsylvania. Drastic changes in weather patterns caused the swamp-forest to die off and re-grow numerous times. During that same period, the Appalachian Mountains eroded away and sediments covered the dead vegetation over and over again. Over time, many different layers of coal, or “seams,” were formed from the dead vegetation buried beneath the weight of the sediments, which themselves turned to rock, shale, clay and soil. Eventually, some of those coal seams and the overlying rock were moved as mountain building continued. Today, coal seams that lie horizontally hundreds of feet below ground in one area have been shoved upwards more than a thousand feet in an area affected by mountain building forces. Often, those uplifted coal seams have completely weathered away. As an example, the “Pittsburgh” coal seam, a seam often ten feet thick in the lowlands west of Chestnut Ridge in Fayette and Westmoreland Counties, was shoved upward and is now non-existent on the ridge to the east.

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