Paleontology and geology
Warm, shallow seas covered the state during the Early and Middle Ordovician. Brachiopods, clams, snails, trilobites, and bryozoans lived on the quiet sea floor, and nautiloid cephalpods swam through the water. During the Late Ordovician, plate movement caused the North American continent to collide with a volcanic island arc that had formed to the east of what is now Pennsylvania. This mountain-building event, the Taconic Orogeny, created a mountain range along the ancient continental margin in approximately the same area that we see the Appalachians today.