Limestones & sandstones in the Mississippian of Kentucky, USA.

The brown rocks in the upper half of the picture are quartzose sandstones of the Big Clifty Sandstone (lower Upper Mississippian). This unit forms the "cap rock" or "roof" of Mammoth Cave in Kentucky, the longest cave on Earth. As of fall 2017, 412 miles of cave passages have been explored and mapped. The sandstone is not readily soluble in water, so it protects the underlying cave passage systems from erosion.

The cave system itself is developed in limestone - see the gray-colored rocks in the lower half of the photo. Mammoth Cave is developed in three limestone units. From top to bottom (youngest to oldest), they are: Girkin Limestone, Ste. Genevieve Limestone, and St. Louis Limestone. The Girkin is lower Upper Mississippian. The Ste. Genevieve and St. Louis are upper Middle Mississippian.

This outcrop exposes the upper Girkin Limestone. Fine-grained limestones (micrites, or lime mudstones) and fossiliferous limestones (wackestone) of the upper Girkin Ls. can be closely examined along the Dixon Cave Trail. Obvious macrofossils in these rocks include solitary rugose corals ("horn corals") (Animalia, Cnidaria, Anthozoa, Rugosa).