Interbedded shale and limestone in the Mississipipian of Kentucky, USA.

Western Kentucky's Mammoth Cave is the longest known cave on Earth, with 412 mapped miles, as of March 2019. The cave is developed in three Mississippian-aged limestone formations. Common lithologies include lime mudstones (micritic limestones), fossilferous wackestones, oolitic limestones, and intraclastic limestones. Other lithologies include dolomitic limestones, dolostones, chert, argillaceous limestones, and shales.

Seen here is a thin interval of interbedded limestone and shale along Houchins Narrows, which is the first cave passage after entering the mouth of the cave (Historic Entrance). This is the Bethel Member of the Girkin Limestone. Apart from this unit, the Girkin consists of relatively pure limestones.

Based on regional and intercontinental correlation, deposition of the Bethel Member's shales represents the onset of glaciation in Gondwana on the old South Pole. Glaciations result in global sea level drops (regressions). The limestones of the Mammoth Cave area were deposited in a warm, shallow ocean environment, so a sea level drop will result in an influx of siliciclastic sediments - in this case, fine-grained muds.

Stratigraphy: Bethel Member, Girkin Limestone, lower Upper Mississippian