Paleontology and geology
Continental collisions began to form the supercontinent of Pangea during this time. This tectonic activity caused mountains to rise to the east of the state in an event referred to as the Alleghenian Orogeny. Rivers eroded these rising mountains and carried sediment westward into a shallow sea that covered much of Maryland. Early Carboniferous (Mississippian) rocks can be seen in the Sideling Hill road cut along Interstate 68 in the western part of the state. Fossil plants and some thin coal beds are evidence of the vast swamps that began to develop in low coastal areas at this time. Rare fossils of brachiopods and bivalves found in a black silty shale at Sideling Hill tell us that a shift occurred in the shoreline, and the sea returned for a time to cover this part of the state.
Late Carboniferous (Pennsylvanian) rocks can be found only in the far western end of the Maryland panhandle. At this time, large rivers flowing from the rising Appalachians deposited sandstones, siltstones, and shales in great deltas that spread westward into a shallow sea. Extensive coal-forming swamps formed in this wet tropical environment. These cycles of deposition produced alternating beds of coal, sandstone, mudstone, and freshwater limestones in the region. Brachiopods and bryozoans were abundant in the sea, while horsetail rushes and scale trees thrived in the warm, humid climate on land.