Throughout this period, vast low-lying coastal swamps and deltas covered much of West Virginia and the eastern and midwestern U.S., creating conditions ideal for coal formation. In West Virginia seams with names like Pittsburgh, Pocahontas, Eagle, and Sewell, were deposited. The geologic units and coal seams listed below are nomenclature currently used in West Virginia. During the Pennsylvanian Period the continental plates of Africa, Europe, and North America were in collision, forming volcanic mountain ranges along their margins as oceanic crust was subducted under the relatively lighter continental crust. This process goes on today around the perimeter of the Pacific Ocean in the volcanically and seismically active belt known as the "ring of fire", where volcanos including Mt. St. Helens, Mt. Ranier, Mt. Fuji, etc. belch every once and a while, but for the most part are relatively well-behaved. Sometime during the Triassic Period, the collision was complete, and the supercontinent of Pangea was formed. The Pangean mountain- building supplied sediments which began forming deltas in southeastern West Virginia during the early Pennsylvanian Period, and through continual shoreline accretion gradually pushed the coastline northwestward into northern West Virginia by the end of the Pennsylvanian Period.
In North America, where the early Carboniferous beds are primarily marine limestones, it is treated as a full fledged geologic period between the Mississippian and the Permian. In Europe, the Mississippian and Pennsylvanian are one more-or-less continuous sequence of lowland continental deposits and are lumped together as the Carboniferous period.
The term "Carboniferous" is used throughout the world to describe this period, although this period has been separated into the Mississippian (Lower Carboniferous) and the Pennsylvanian (Upper Carboniferous) in the United States. This system was adopted to distinguish the coal-bearing layers of the Pennsylvanian from the mostly limestone Mississippian, and is a result of differing stratigraphy on the different continents.