Although most Ordovician rocks are sparsely fossiliferous, some beds contain abundant fossils. Expect to use your hammer and chisel to remove the fossils from most of these rocks. Silurian and Devonian strata are limited in their outcrop area. The Silurian rocks are generally limestones and some strata contain abundant trilobite fragments. Crinoids and brachiopods are also frequently found.
Devonian rocks are normally unfossiliferous shales, cherts, and sandstones.
Corals, bryozoa, brachiopods, crinoids, bivalves, gastropods, cephalopods, and plant fossils are common in some intervals throughout the sequence. The Mississippian rocks are limestone, shale, chert, and some sandstone. The Pennsylvanian rocks are sandstone, siltstone, shale, and some limited sections of limestone. Mississippian-age shales offer the easiest collecting of the rocks of north Arkansas. Shale weathers rapidly to mud which washes away, leaving the fossils behind and allowing the collector to pick them up off the surface of the outcrop.