Geology of Pennsylvania - Atlantic Coastal Plain

Atlantic Coastal Plain

One of the smallest provinces in the state and is confined to Philadelphia, Delaware, and Bucks counties along the Delaware River. Local relief is less than 200 feet (61 m) and much of the bedrock is buried under recent alluvial deposits. On the geologic map, "Trenton Gravel" is used to describe most of these sediments. However, much of the alluvial sediments that exist here are sand, silt, and clays. The traditional boundary of the coastal plain is the fall line. The coastal plain in Pennsylvania was once home to thousands of acres of fresh water tidal marsh. This was important in the early development of Philadelphia and Chester. Many of the small tributaries to the Delaware have cut small but impressive gorges into the bedrock including the Ridley Creek, the Chester Creek, and the Wissahickon Creek. Flash floods are becoming a local problem in the province.