These are shards of shale with embedded fossil fern that have turned white in color. According to fossilguy.com, who leads fossil collecting trips, this formation is unique to the St. Clair, PA area, and can be collected at the many abandoned strip mines in that area. He reports:
“Many of these excavated strip mines are now used for ATV’s, hunting, and shooting ranges by locals. The area was almost turned into a land fill. I was not going there for any of those reasons, especially the land fill! I was here because of the unique fossils found in the Llewellyn Formation, which surfaces in a few abandoned strip mines in the area.”
“The Llewellyn Formation:
“This is a Pennsylvanian age formation (precisely 300 to 308 myo) that contain sequences of Sandstone, Siltstone, Shale, Conglomerate, and Anthracite Coal (according to the USGS). Each layer represents a different depositional environment. In particular, the shale was probably formed from sedimentation of a muddy bottomed coastal lagoon. During storms, plant material in the coastal bogs (which were abundant during this time period) would be washed offshore into the lagoon. Many of these plant fragments settled onto the low oxygen muddy bottom of the lagoon and were covered by a rain of fine black sediment. The ferns became fossilized in the black shale layers.
“Fern fossils in shale near coal seams are incredibly common, and expected. However, what makes this site unique is what happened later. During fossilization, Pyrite replaced allot of the organic plant material, giving the fossils an orange color. However at some later time, through oxidation and replacement reactions, the Pyrite was replaced by a white substance called Pyrophyllite in sections of the fern laden shale. This caused many of the orange fern fossils to turn white. This is a very rare occurrence. So, today, the Llewellyn formation contains unique fossils, white ferns preserved on black shale. Anytime you see a white fern on a slab of black shale in a natural history museum, chances are, it’s from St. Clair.”