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.