Nodules of chert, an impure flint, are found in limestone formations. Chert is light-colored and opaque and breaks readily into sharp flakes. It is one of the materials the Indians used to make arrowheads and spearheads. It is found as fist-sized lumps scattered irregularly throughout some limestones, particularly those of Mississippian and Pennsylvanian age. After years of weathering, the nodules stand out as bumps and prickles on the strata, accumulate in the talus below the limestone bluff, or remain defiantly in the soil to annoy the farmer.
Chert must have formed when the limestone sediments were still relatively fresh and unconsolidated. The Burlington limestone of Mississippian age has several prominent chert-bearing horizons in it. The surrounding limestone is coarse-grained, and contains only occasional tough, hardy fossils such as fish teeth, crinoid cups, and heavy brachiopods. The chert nodules, when broken open or sectioned, disclose large, delicate brachio-pod shells which are perfectly preserved. They extend to the edge of the chert nodule but do not continue into the limestone. The protozoa, brachiopods, and delicate, lacy bryozoa that are common in the chert are not found in the limestone, though obviously they must have been there before being dissolved away where not protected by the siliceous chert.
A chance break may expose enough of a fossil to identify it, but full realization of the beauty of chert fossils comes only from thin sections carefully cut, ground, and polished. "Rice agate," neither rice nor agate, is a brownish-gray chert loaded with protozoa found in Pennsylvanian
Chert may contain well-preserved fossils, such as these protozoa Triticites, named "rice agate" because of the shape and size of the protozoa. Pennsylvanian; Red Oak, Iowa.
Ammonites from the Cretaceous of South Dakota. These were found in concretions. The specimen in lower right still has part of the concretion attached. Lower left is a specimen showing suture lines where the mother-of-pearl layer of the shell is missing. (Photo South Dakota Department of Highways)
deposits of western Iowa and occasionally turned into cuff links or ringstones. It polishes well, as will any fossil-bearing chert.
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