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Delaware's Geologic Units

The geology of Delaware includes parts of two geologic provinces: the Appalachian Piedmont Province and the Atlantic Coastal Plain Province. The Piedmont occurs in the hilly northernmost part of the state and is composed of crystalline metamorphic and igneous rocks. These include a variety of rock types that were formed deep in the earth by metamorphic processes, mostly in the early part of the Paleozoic Era (app. 400-500 million years ago), and later uplifted. The Coastal Plain, a flatter area that comprises most of the state, is underlain by a series of younger layers of sediments, ranging in age from the Cretaceous Period (app. 120 million years ago) to relatively recent. These have been slightly tilted through geologic time, with very minor faulting or folding in places.

The rocks and sediment layers of both of Delaware's geologic provinces can be subdivided into geologic units called lithodemic units (for the crystalline rocks) or lithostratigraphic units (for the sedimentary units). These bodies of rock are identified by distinctive geological characteristics and are sufficiently thick and areally extensive to be mapped at the earth's surface and/or in the subsurface. The chart below summarizes the age and distribution of the geologic units that are recognized in the state by the Delaware Geological Survey.