File:Schist (Wissahickon Formation, multiply metamorphosed during the Paleozoic; near Philadelphia, Pennsylvania, USA) (15218563954).jpg

Schist from the Paleozoic of Pennsylvania, USA (wet, cut surface; 15.5 centimeters across at its widest).

This is a sample from the Wissahickon Formation of southeastern Pennsylvania. The Wissahickon is the basement unit in the Philadelphia urban area. The unit is named after Wissahickon Creek, a tributary stream of the Schuylkill River. The Wissahickon Formation consists principally of mica schists and quartzites. These are metamorphosed sedimentary rocks (shales and sandstones) originally deposited in a continental slope environment during the Ordovician. The slope deposits were later metamorphosed by multiple tectonic collision events between Laurentia (= the ancient North American paleocontinent) and various landmasses. For example, the Taconic Orogeny was a mountain building event that involved a colliding volcanic island arc. The Acadian Orogeny involved a colliding microcontinent. The Allegheny Orogeny involved Africa colliding with Laurentia. Seven different deformation events have been identified in the metamorphic rocks of the Wissahickon Formation.

The specimen shown above is a schist, an intermediate- to high-grade metamorphic rock. It appears to consist of muscovite mica, biotite mica, chlorite, talc, vermiculite, and small porphyroblasts. In places, weathering and erosion of Wissahickon Formation has produced detrital garnets, which can be found in local streams.

Stratigraphy: Wissahickon Formation, multiply metamorphosed during Paleozoic orogenies (Late Ordovician to Pennsylvanian)