Monongahela Series, division of Pennsylvanian rocks in the eastern and southeastern United States. (The Pennsylvanian Subperiod began about 318 million years ago and lasted about 19 million years.) It was named for exposures studied along the Monongahela River in Pennsylvania and is also recognized in Ohio, Maryland, Virginia, and West Virginia. Rocks of the Monongahela Series overlie those of the Conemaugh Series and are the uppermost Pennsylvanian rocks in the region; they consist of coal beds, sandstones, and shales that are as much as 150 metres (500 feet) thick. The Pittsburgh Coal occurs in the Monongahela and is the single most economically valuable coal bed in the world.