The Pennyroyal Plateau or Pennyroyal Region, often spelled Pennyrile, is a large physiographic region of

Kentucky Kentucky (, ), officially the Commonwealth of Kentucky, is a landlocked U.S. state, state in the Southeastern United States, Southeastern region of the United States. It borders Illinois, Indiana, and Ohio to the north, West Virginia to the ...

that features rolling hills,

cave Caves or caverns are natural voids under the Earth's Planetary surface, surface. Caves often form by the weathering of rock and often extend deep underground. Exogene caves are smaller openings that extend a relatively short distance undergrou ...

s, and

karst Karst () is a topography formed from the dissolution of soluble carbonate rocks such as limestone and Dolomite (rock), dolomite. It is characterized by features like poljes above and drainage systems with sinkholes and caves underground. Ther ...

topography in general. It is named for '' Hedeoma pulegioides'' (the American pennyroyal), a wild mint that grows in the area. It is also called the "Mississippian Plateau," for the Mississippian geologic 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. To the south, the Pennyroyal continues as the Highland Rim of Middle Tennessee, to the north it continues as the Indiana Uplands of Southern Indiana. The Pennyroyal is largely in farmland where the bedrock is

limestone Limestone is a type of carbonate rock, carbonate sedimentary rock which is the main source of the material Lime (material), lime. It is composed mostly of the minerals calcite and aragonite, which are different Polymorphism (materials science) ...

, 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

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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