Paleontology and geology
During the Cambrian, Pennsylvania lay much closer to the equator than it does today. A shallow sea rose to cover the state, and sediments eroding off the land formed a sandy sea floor inhabited by trilobites, brachiopods, and other marine organisms. Limy sediments accumulated father offshore. Cambrian rocks containing stromatolites typical of a shallow-water environment can be found in Valley Forge National Historical Park west of Philadelphia. Simple, tube-shaped, vertical trace fossils called Skolithos can be found in York County in central Pennsylvania. Although not shown on this map, there are extensive exposures of Cambrian rocks in southeastern and south-central Pennsylvania.