Wissahickon schist

Wissahickon Formation Stratigraphic range: Ediacaran to Early Cambrian (Depositional)
A hand sample of the type Wissahickon from the banks of the Wissahickon Creek
Type	Geological formation
Unit of	none
Sub-units	Mt. Cuba, Doe Run schist, Laurels schist, Greystone schist
Thickness	unknown
Lithology
Primary	Schist
Other	Gneiss and Quartzite
Location
Region	Piedmont of eastern North America
Extent	Southeastern Pennsylvania, northern Delaware, Northeastern Maryland
Type section
Named for	The Wissahickon Creek

The Wissahickon Formation is a mapped bedrock unit in Pennsylvania, New Jersey, and Delaware. It is named for the Wissahickon gorge in Fairmount Park, Philadelphia.

In Maryland formations, the term "Wissahickon" is no longer used. Rocks in this classification have since been divided into several units, such as Lower Pelitic Schist and Prettyboy Schist.

The Wissahickon is described as a pelitic schist and gneiss with interlayers of quartzite. Color is highly variable as is the mineralogy. A general description for the unit is described as a silver to brown garnet mica-schist.

The highly variable nature of this rock type is also why the metamorphic grade is also complex. The existence of the minerals biotite, garnet, staurolite, and kyanite all imply a low-intermediate to high metamorphic grade. The metamorphic facies, which is described as lower to upper amphibolite facies, implies a moderate to high metamorphic temperature and a moderate pressure.

The age indicated on the most recent geologic map of southeast Pennsylvania shows the Wissahickon being Ediacaran to Cambrian in age. This age is a relative date since the sediments that created the Wissahickon are highly deformed and went through several deformation events.

The sediments of the Wissahickon were altered during the Taconic orogeny and most dates do not place the deformation older than the Silurian. Although there is some evidence of Devonian-aged deformation.