File:Fossiliferous flint mass in limestone (Upper Mercer Limestone, Middle Pennsylvanian; Rock Cut railroad cut, south-southeast of Dresden, Ohio, USA) 2 (33303148216).jpg

This is the Upper Mercer Limestone in the Pottsville Group, 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 Member (?).

The Upper Mercer Limestone is a moderately laterally persistent, chertified limestone horizon in the Pottsville Group. It is often composed of black-colored chert/flint but can be dark bluish to bluish-black colored as well (the latter colors are referred to as "Nellie Blue Flint"). Upper Mercer Flint has light-colored fossils and fossil fragments that include fusulinid foraminifera, crinoid ossicles, and other Late Paleozoic normal marine fossils. Apparent phylloidal algae can also be present as squiggly lines.

Non-chertified limestone is frequently present in the Upper Mercer horizon, although minor in volume. Limestone usually occurs along the outside portions of chert masses, but also in relatively small patches within the chert.

In places, the Upper Mercer Flint/Limestone horizon is missing, usually removed by paleoerosion.

American Indians sometimes used Upper Mercer Flint to make arrowheads and spear points and knife blades. "Flint Ridge Flint" (= Vanport Flint) was the most desirable source rock for these objects, but other chert horizons also attracted attention.

The black material in the photo is a fossiliferous, black flint nodule. The surrounding grayish-brown material is fossiliferous limestone.

Stratigraphy: Upper Mercer Limestone, upper Pottsville Group, Atokan Stage, lower Middle Pennsylvanian