Geology

There are four distinct areas of geology, when symbolized by primary rock type, within the Susquehanna River Basin. The northern area of the Basin, primarily in New York, is composed mostly of shale with some small sections of limestone and sandstone in the far north. Geology across north-central Pennsylvania is almost completely sandstone or sandstone with interbedded shales. These areas in the northern half of the Basin contain large glacial deposits that are hydrologically important.

Significant folding is apparent where the Appalachian Mountains cross the Basin from the eastern-middle to the southwest. This region is also predominantly shale and sandstone, but carbonate rocks, such as limestone and dolomite, are present in many of the valleys. Carbonates dominate the Great Valley region south of Blue Mountain through all are parts of Cumberland, Lancaster, York, and Dauphin Counties. Rock types are much more diverse south of the Great Valley: the only outcrops of igneous and metamorphic rocks, including a large area of schist, exist here.