Mississippian. Pierson chert is found in the St. Joe formation which lies just below the Boone formation. Thin-bedded, coarse-crystalline bioclastic limestone with ubiquitous 3–6-mm-diameter crinoid fragments. Limestone is commonly pink to red due to hematite staining, but vary with location. Chert nodules are uncommon but, where present, are tabular and reddish. Base of unit is a 1.5–3-ft- (0.5–1-m-) thick sequence of phosphate-pebble-bearing tan sandstone and overlying greenish-gray shale that, although thin, is laterally persistent throughout much of northwestern Arkansas (McKnight, 1935). The member is approximately 30–50 (9–15 m) ft thick (Hudson 2003).
