Productid brachiopod in flint-bearing fossiliferous limestone from the Pennsylvanian of Ohio, USA.

Black = flint Brownish = weathered limestone

The Upper Mercer Flint is a moderately laterally persistent chert horizon in the Pennsylvanian of eastern Ohio, USA. It is often black-colored 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 whitish-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.

This chert horizon is actually a chertified fossiliferous limestone. Non-chertified limestone is frequently present, 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 fossil shell seen here is a productid brachiopod. Brachiopods are sessile, benthic, filter-feeding, marine invertebrates. They first appear in Cambrian rocks and were abundant in Earth's oceans throughout the Paleozoic. They were also common in Mesozoic oceans, but are scarce in modern oceanic biotas. Brachiopods have two shells, called valves, that are usually calcareous (made of calcite - CaCO3 - calcium carbonate). Each shell of a brachiopod is bilaterally symmetrical, unlike each shell of a bivalve (clam).

Classification: Animalia, Brachiopoda, Articulata (a.k.a. Rhynchonelliformea), Strophomenida, Productidae

Stratigraphy: Upper Mercer Flint (= chertified Upper Mercer Limestone), upper Bedford Cyclothem, upper Pottsville Group, Atokan Series, lower Middle Pennsylvanian