Weathered bituminous coal in the Pennsylvanian of Ohio, USA.

The Pottsville Group is a Pennsylvanian-aged cyclothemic succession containing nonmarine shales, marine shales, siltstones, sandstones, coals, marine limestones, and chert ("flint"). The lower Pottsville dates to the late Early Pennsylvanian. The upper part dates to the early Middle Pennsylvanian. The Lower-Middle Pennsylvanian boundary is apparently somewhere near the Boggs Limestone horizon (?).

The photo shows weathered, loose talus pieces of bituminous coal eroded from the Middle Mercer Coal. Stratigraphically, this coal horizon occurs immediately below the Lower Mercer Limestone, a widespread marine fossiliferous limestone unit. The Middle Mercer Coal usually consists of bituminous coal, but in places it is a cannel coal. At this locality, the Middle Mercer is bituminous. Portions of the horizon at this site are remarkable because the coal contains marine fossils (brachiopods, crinoids, gastropods) [DON'T tell the creationists!]. Applying Walther's Law (conformable facies now in vertical succession were originally next to each other), the coal swamp that formed the Middle Mercer Coal was near a marine shoreline. A storm likely washed in shells and fragmented skeletons from the shallow ocean into the swamp. Swamps directly adjacent to the sea can be observed in many places on modern Earth.

Stratigraphy: Middle Mercer Coal, Pottsville Group, lower Atokan Stage, lower Middle Pennsylvanian