Paleontology and geology
There was no tectonic activity in Arkansas during the Ordovician. Shallow marine environments covered the northern part of the state, and layers of carbonate sediment built up on the sea floor. These are preserved today as limestone. Brachiopods, corals, gastropods, crinoids, bryozoans, and trilobites thrived in the warm waters, and their fossils can be found in these limestones. Sands and muds accumulated in the deeper waters that covered the southern part of the state. The shale that formed from the deep sea mud is black and contains fossils of graptolites, along with rare fossils of inarticulate brachiopods and sponges.