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The Pennyroyal Plateau, or, as it is more commonly called in Western Kentucky and adjacent areas, the Pennyrile, is a large area of the state that features rolling hills, caves, and karst topography in general. It is named for a wild mint that grows in the area. It is also called the "Mississippian Plateau," for the Mississippiangeologic age in which it was formed.
The Pennyroyal is bordered by the Pottsville Escarpment in the east. The Pottsville Escarpment is the transition zone from the central part of Kentucky to the higher and geologically younger Cumberland Plateau in the eastern part of the state. The Pennyroyal is bordered on the north by Muldraugh Hill, the geological escarpment that forms the transition from the geologically older Bluegrass to the Pennyroyal. This is a series of knobs and ridges based on hard siltstones.
The Pennyroyal is bordered on the west by the younger Jackson Purchase. The Pennyroyal is often thought by non-geologists to include the West Kentucky Coal Field of Pennsylvanian age, located in the northwestern area of the state, as the southeastern part of the Illinois Basin. The coalfield and the Pennyroyal are of different geologic ages and are separated by the Clifty Region, largely of Pennsylvanian sandstone, occasionally bituminous; the boundary with the Pennyroyal is the Dripping Springs Escarpment.
The Pennyroyal is largely in farmland where the bedrock is limestone, and most of the Pennyroyal is based on Mississippian limestone, particularly the St. Louis Limestone or Ste. Genevieve Limestone. In some areas, the limestone is capped with a soft sandstone. This kind of formation is featured in the Mammoth Cave area, and has enabled the formation of the world's most extensive cave system. Numerous other caves exist in the Pennyroyal, where some of the most intensely cave-forming limestones of the world are to be found.
Where the capping sandstone is intact, the land surface is usually forested, rugged hills.
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