AKA Coshocton Flint
Upper Mercer flint is a high-quality, microcrystalline variety of chert found in Eastern and Central Ohio. Named after its occurrence within the Upper Mercer member of the Pottsville Formation, this flint was one of the most critical lithic raw materials utilized by Native American populations in the Ohio Valley for over 13,000 years, spanning from the Paleoindian period through the Historic contact era.
Known for its deep black, dark blue, and mottled grey varieties, Upper Mercer flint is widely recognized for its fine grain, predictable conchoidal fracture, and high utility in tool production.
Geological Occurrence and Distribution
Upper Mercer flint occurs stratigraphically within the Pottsville Group (Lower Pennsylvanian System). It typically manifests as bedded limestone replaced by silica, appearing either as continuous marine limestone/flint strata or as bedded nodular zones directly above the Upper Mercer coal seam.
Spatial Extent
The primary outcrop and quarry zone runs along a broad belt through east-central and northeastern Ohio. The most significant historical quarrying complexes are concentrated in three key counties:
- Coshocton County: Home to the famous Nellie and Warsaw deposits along the Walhonding River Valley. This area represents the heaviest concentrations of prehistoric quarrying for this material.
- Muskingum County: Significant exposures occur along northern tributaries of the Muskingum River.
- Perry and Hocking Counties: Southern extensions where the member outcrops alongside other Pottsville cherts.
Physical Characteristics and Varieties
Upper Mercer flint is structurally dense and brittle, exhibiting a vitreous (glass-like) to sub-vitreous luster on freshly knapped surfaces.
Color Spectrum
- Coshocton Black / Nellie Gray: The most iconic varieties. Ranging from jet black to dark coal grey, often featuring lighter grey clouds, mottling, or concentric banding.
- Blue-Black: Deep navy or indigo tones, frequently found in unweathered bedded layers.
- Light Gray / Tan: Common near exposure edges or weathered rind margins where iron leaching or alteration has occurred.
- White Inclusions: Often peppered with light-colored streaks, small quartz crystalline pockets (vugs), or fossil fragments.
Fossil Content & Identification
Unlike the fossil-poor chalcedony of Flint Ridge, Upper Mercer flint frequently retains visible, replaced marine fossils. Under magnification—and often to the naked eye—it contains:
- Crinoid stem fragments
- Spicules and brachiopod shell pieces
When fossils are present, they frequently appear as distinct, light-colored or white spicules, crinoid ossicles, brachiopod fragments, or small inclusions scattered across the matrix.
Prehistoric Exploitation and Archaeology
Upper Mercer flint was actively quarried and traded across vast distances of Eastern North America. Its high quality made it a prime resource for bladelet, projectile point, and formal biface production across multiple cultural traditions.