<p>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.
</p><p>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).
</p><p>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
</p><p>Stratigraphy: lower Delaware Limestone, Eifelian Stage, lower Middle Devonian
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Locality: roadcut along the southern side of Emerald Parkway, immediately east of Rt. 257 intersection, Dublin, northwestern Franklin County, central Ohio, USA (40° 06' 34.79" North latitude; 83° 06' 35.73" West longitude)
<a rel="nofollow" class="external text" href="https://www.flickr.com/people/47445767@N05">James St. John</a>
CC BY 2.0
<a rel="nofollow" class="external text" href="https://www.flickr.com/photos/jsjgeology/42189091231/">Pyrite atop chert nodule (Delaware Limestone, Middle Devonian; Emerald Parkway roadcut, Dublin, Ohio, USA) 4</a>