Paleontology and geology
During the Ordovician, Maryland lay much closer to the equator than it does today. A shallow sea covered much of the state and limy sediments accumulated on the sea floor. Exposures of these Ordovician limestones occur in Frederick and Washington Counties in central Maryland and contain fossils of trilobites, brachiopods, and molluscs. 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 Maryland. 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.