Paleontology and geology
Sea level fluctuated during the Ordovician, leaving behind evidence of both marine conditions and intervals of erosion of exposed land surfaces. Limestones, formed during times when the sea flooded South Dakota, contain fossils of straight cephalopods and the gastropod, Maculurites, as well as abundant trace fossils. A zone of vertical burrows of the trace fossil, Scolithus, can be seen in the rocks that represent the boundary between the Cambrian and the Ordovician.
Although this map only indicates the presence of undifferentiated rocks, there are exposures of Ordovician rocks in the Black Hills region of western South Dakota.