The Pennyroyal Plateau, or, as it is more commonly called in 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 also called the "Mississippi Plateau".
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. North of the Pennyroyal, the Bluegrass borders the Chester Escarpment. This escarpment, mostly based on weathering-resistant sandstone, is a fairly dramatic one, featuring many cliffs, canyons, waterfalls and rock shelters.
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, a mostly alluvial area. The Pennyroyal is usually thought to include the Western Coal Fields of Pennsylvanian age, located in the northwestern area of the state, and separated from the rest of the Pennyroyal by the Dripping Springs Escarpment, also based on sandstone.
The Pennyroyal is largely in farmland where the bedrock is limestone, and most of the Pennyroyal is based on Mississippian age 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—sandstone over limestone—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.