The Delaware Limestone is a significant carbonate unit in the Devonian of central and northern Ohio. It's actually part of a much more widespread sheet of Devonian carbonates that extends from New York State to the Midwest. The Delaware Limestone represents deposition in a subtropical, shallow-water, carbonate platform environment. The rocks are principally micritic limestones, fossiliferous wackestones, and fossiliferous packstones. Fossils are typical Paleozoic shallow marine invertebrates.

The thin-bedded Delaware Limestone is underlain by the thick-bedded Columbus Limestone (also Devonian). The contact is a prominent disconformity (a type II sequence boundary). Biostratigraphic studies have shown that one conodont biozone is missing at the Columbus-Delaware contact in central Ohio, probably representing ~1 to 3 million years. The Delaware Limestone is overlain by soft gray clayshales of the Olentangy Shale (lower Upper Devonian).

At the center of the photo is a chert nodule, hosted in limestone. Directly above the chert nodule is a mass of pyrite ("fool's gold") - FeS2 (iron sulfide). Oxidative weathering of the pyrite has resulted in orangish-brown, iron oxide weathering stains

Stratigraphy: lower Delaware Limestone, Eifelian Stage, lower Middle Devonian