The Nashville Group formed 454 million years ago, when the central United States was covered with a very flat, shallow inland sea, already filled with ancient sediment from Cambrian times. Flat layers of limestone settled into place in waters that varied in depth as the ocean flooded and ebbed over millions of years. At times the ocean was a shallow lagoon-like environment carpeted with life. After about 3.5 million years, at the end of the Middle Ordovician, a long ice age sucked water into polar ice caps, and the ocean receded completely. The old ocean beds dried up and slowly began to erode, creating a gap in the geologic record. When the seas finally returned several million years later, the Leipers shale was laid down over the older, layered Middle Ordovician sedimentary rock.