The high specific gravity of the replaced fossils allows the collector to use placer-mining techniques in the creeks of some areas. Shovel some gravel onto the bank and wash it down by splashing water onto the gravel with the shovel. The gravel will wash away leaving the fossils behind. The Mississippian-age portion of the Cane Hill Member outcrops in limited areas of Searcy and Van Buren Counties. It contains an abundant, diverse, and well preserved fauna including ammonoids, nautiloids, bivalves, brachiopods, bryozoa, plant material, blastoids, crinoids, gastropods, shark teeth, and arthropods.
Limestones dominate the rest of the Mississippian-age deposits of northern Arkansas.
Fossil collecting is not as easy in these units as it is in the shales, but it can be just as rewarding. Some fossils can be found weathered out and lying on the surface of an outcrop, but most have to be broken out of the rock. The lower Pennsylvanian-age rocks of the Boston Mountains are much like the Mississippian rocks in the variety of fossils they produce. They consist of limestones, sandstones, siltstones, and shales. In the Arkansas River Valley, Pennsylvanian shales and siltstones associated with coal seams often have plant fossils in abundance.