Sea Over North America

An epicontinental sea is an ocean that sits on top of continental lithosphere. Continental lithosphere is thick but light, riding high on the semi-fluid asthenosphere. Most oceans are on top of oceanic lithosphere, which is thin but heavy and thus sits lower than continental lithosphere. When sea level is particularly high and a large area of continental lithosphere is at a low elevation, the ocean water spreads over the continent and creates an epicontinental sea.

In the past, the Pittsburgh area was often covered by an epicontinental sea. For much of the time before the Pennsylvanian Period, the Pittsburgh area was at a lower elevation than it is today. Sea level began to rise during the Early Cambrian and remained high all the way through the Ordovician, Silurian, and Devonian Periods. During the Pennsylvanian Period, the Pittsburgh area was raised to its current elevation when Africa collided with North America