St Clair PA Fossil Plant Collection
These plant fossils were found in the Llewellyn Formation (300 mya, Pennsylvanian Period) and are one of the few places where one can find these very detailed white (sometimes yellow) ferns on a striking contrast of black shale. The plants died and fell into the swamp, wherein in a low temperature, pressure, and oxygen environment the plant tissue was slowly replaced by pyrite (from sulfides). Pyrophyllite (aluminum silicate, a whitish mineral) is believed to have replaced the pyrite at a later stage as the sediments piled up and the temperature and pressure became greater. The ferns most commonly found are Alethopteris, Neuropteris, Pecopteris, and Sphenophyllum.
