The Wissahickon Formation underlies 1100 sq. km. in southeastern Pennsylvania and northern Delaware. It includes many different lithologies at metamorphic grades from greenschist to granulite. Lithologies mapped in recent investigations illustrate the variety within the formation.

The Wissahickon Formation is named for alternately pelitic and moderately psammitic schist interlayered with thin calcareous sandstone exposed along Wissahickon Creek in Philadelphia. These rocks can be mapped continuously to the west where, in central Delaware County, they are interlayered with amphibolite resembling back-arc basin basalt. Farther west, near the contact with the arc-related Wilmington Complex, pelitic gneiss is intruded by thin boninitic dikes. All these rocks were metamorphosed at moderate P amphibolite facies conditions during the Devonian, but in Delaware County the Devonian metamorphism overprints Silurian-aged low-P assemblages. Monazite with early Ordovician cores occurs only in rocks nearest the Wilmington Complex, indicating that these rocks may have formed the basement to arc magmas.

Farther west in Chester County, five distinct belts are found in the Wissahickon of the Coatesville and West Grove quadrangles. The northernmost belt is a poorly layered aluminous schist associated with continental rift (CR) amphibolites and marble. This may represent early syn-rift deposits. The next belt consists of sillimanite-bearing aluminous schist interlayered with MORB-like amphibolite. This is followed to the south by qtz-pl-bt gneiss with thin pegmatite layers that appear to be in-situ melts and CR amphibolite. The southernmost belt is aluminous schist associated with abundant map-scale pegmatite bodies that may also be in-situ melts, CR amphibolite, and marble. A well-layered garnet-bearing schist with CR amphibolite is found west of the latter unit.

Although multiple episodes of deformation and metamorphism obscure many primary features of the rocks, lithologic characteristics, metamorphic and structural histories, and geochemistry of interlayered metavolcanic rocks suggest that designation of these varied schist units as members of the same formation is untenable.