Ancient Sea Level - Pennsylvanian rocks of western Missouri and eastern Kansas record dozens of deepening and shallowing episodes of the ancient seas that resulted from glacial episodes 300 million years ago. These repeated changes in sea level left a stack of recurrent rock called cyclothems. While this stack of rock in the mid-west is almost all marine in origin, equivalent rocks of the Pittsburgh area are nearly almost all nonmarine. (Lewis and Clark, 7/18/1804)