The Pennsylvanian Period lasted from 320 to 286 million years ago. During the Pennsylvanian Period, widespread swamps laid down the thick beds of dead plant material that today constitute most of the world's coal. The term Pennsylvanian is a U.S. coinage based on the frequency of rocks of this period in the state of Pennsylvania; internationally, the terms late Carboniferous Period or Silesian Period are preferred.

Although most artist's conceptions of the Pennsylvanian Period emphasize its prolific swamps, these were characteristic only of the equatorial regions. The Southern Hemisphere, which was dominated by the huge continent Gondwana, underwent a series of ice ages during this period. These ice ages sequestered water in times of ice growth and released it in times of melting, causing the ocean to cyclically regress (uncover coastal lands) and transgress (cover coastal lands) around the world. Repeating sequences of sedimentary rock layers record...