File:Crinoid stem columnal in fossiliferous limestone (Upper Mercer Limestone, Middle Pennsylvanian; Symmes Creek at Mollies Rock Road bridge, Muskingum County, Ohio, USA) (23652332522).jpg

Crinoid stem columnal in fossiliferous limestone 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 Member (?).

The Upper Mercer Flint is a moderately laterally persistent chert horizon in the Pottsville Group. 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 (for example, see the above photo), although often 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 whitish structure at the center of the above photo is a single crinoid stem columnal, composed of a single crystal of calcite (CaCO3 - calcium carbonate). Crinoids are essentially a starfish on a stick - they are sessile, benthic, filter-feeding echinoderms. The stem, or stalk, of a crinoid is composed of stacked columnals, similar to poker chips. Upon death, the crinoid skeleton falls apart relatively quickly, resulting in numerous disarticulated elements.

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