ThePennyroyal Plateau orPennyroyal Region, often spelledPennyrile,[1] is a largephysiographic region ofKentucky that features rolling hills,caves, andkarst topography in general. It is named forHedeoma pulegioides (the American pennyroyal), a wild mint that grows in the area. It is also called the "Mississippian Plateau," for theMississippian geologic age in which it was formed.[2]
The Pennyroyal is bordered by thePottsville Escarpment in the east. The Pottsville Escarpment is the transition zone from the central part of Kentucky to the higher and geologically youngerCumberland Plateau in the eastern part of the state. The Pennyroyal is bordered on the north byMuldraugh Hill, the geological escarpment that forms the transition from the geologically olderBluegrass to the Pennyroyal. This is a series of knobs and ridges based on hardsiltstones.
The Pennyroyal is bordered on the west by the youngerJackson Purchase. The Pennyroyal is often thought by non-geologists to include theWest Kentucky Coal Field ofPennsylvanian age, located in the northwestern area of the state, as the southeastern part of theIllinois Basin. The coalfield and the Pennyroyal are of different geologic ages and are separated by the Clifty Region, largely of Pennsylvanian sandstone, occasionallybituminous; the boundary with the Pennyroyal is the Dripping Springs Escarpment.[3]
To the south, the Pennyroyal continues as theHighland Rim ofMiddle Tennessee, to the north it continues as theIndiana Uplands ofSouthern Indiana.
The Pennyroyal is largely in farmland where the bedrock islimestone, and most of the Pennyroyal is based on Mississippian limestone, particularly theSt. Louis Limestone orSte. Genevieve Limestone. In some areas, the limestone is capped with a soft sandstone. This kind of formation is featured in theMammoth Cave area, and has enabled the formation of the world's most extensivecave system. Numerous other caves exist in the Pennyroyal, where some of the most intensely cave-forming limestones of the world are to be found.[4]
Where the capping sandstone is intact, the land surface is usually forested, rugged hills.
