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

physiographic Physical geography (also known as physiography) is one of the three main branches of geography. Physical geography is the branch of natural science which deals with the processes and patterns in the natural environment such as the atmosphere, h ...

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 ''Hedeoma pulegioides'', also known as pennyrile, American pennyroyal, or American false pennyroyal,USDA Plants Profile''Hedeoma pulegioides'' Accessed June 19, 2007. is a species of '' Hedeoma'' native to eastern North America, from Nova Scotia ...

'' (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 The Pottsville Escarpment is a resistant sandstone belt of cliffs and steep sided, narrow crested valleys in eastern Kentucky, USA. It features rock shelters, waterfalls, and natural bridges. It is also called the Cumberland Escarpment and forms ...

in the east. The Pottsville Escarpment is the transition zone from the central part of Kentucky to the higher and geologically younger

Cumberland Plateau The Cumberland Plateau is the southern part of the Appalachian Plateau in the Appalachian Mountains of the United States. It includes much of eastern Kentucky and Tennessee, and portions of northern Alabama and northwest Georgia. The terms " Al ...

in the eastern part of the state. The Pennyroyal is bordered on the north by

Muldraugh Hill Muldraugh Hill is an escarpment in Bullitt, Hardin, Jefferson, and Nelson counties of central Kentucky separating the Bluegrass on the north and north-east from the Pennyrile on the south and south-west. This escarpment fades into the Pottsvi ...

, 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

siltstone Siltstone, also known as aleurolite, is a clastic sedimentary rock that is composed mostly of silt. It is a form of mudrock with a low clay mineral content, which can be distinguished from shale by its lack of fissility. Although its permeabil ...

s. The Pennyroyal is bordered on the west by the younger

Jackson Purchase The Jackson Purchase, also known as the Purchase Region or simply the Purchase, is a region in the U.S. state of Kentucky bounded by the Mississippi River to the west, the Ohio River to the north, and the Tennessee River to the east. Jackson's P ...

. The Pennyroyal is often thought by non-geologists to include the

West Kentucky Coal Field The West Kentucky Coal Field, alternatively The North Pennyrile or simply Northwest Kentucky, comprises an area in the west-central and northwestern part of the state, bounded by the Dripping Springs Escarpment and the Pennyroyal Plateau and the O ...

of Pennsylvanian age, located in the northwestern area of the state, as the southeastern part of the

Illinois Basin The Illinois Basin is a Paleozoic depositional and structural basin in the United States, centered in and underlying most of the state of Illinois, and extending into southwestern Indiana and western Kentucky. The basin is elongate, extending a ...

. The coalfield and the Pennyroyal are of different geologic ages and are separated by the Clifty Region, largely of Pennsylvanian sandstone, occasionally

bituminous Bitumen ( , ) is an immensely viscous constituent of petroleum. Depending on its exact composition, it can be a sticky, black liquid or an apparently solid mass that behaves as a liquid over very large time scales. In American English, the m ...

; the boundary with the Pennyroyal is the Dripping Springs Escarpment. To the south, the Pennyroyal continues as the

Highland Rim The Highland Rim is a geographic term for the area in Tennessee, North Alabama, and Kentucky which surrounds the Central Basin. The Central Basin is a geological dome which has subsequently fractured and eroded to produce a basin. The Highland R ...

Middle Tennessee Middle Tennessee is one of the three Grand Divisions of Tennessee that composes roughly the central portion of the state. It is delineated according to state law as 41 of the state's 95 counties. Middle Tennessee contains the state's capital an ...

, to the north it continues as the

Indiana Uplands The Indiana Uplands or the Hoosier Uplands are a geographical region in south-central Indiana. On a topographical map the Indiana Uplands begin slightly north of the city of Martinsville, Indiana and continue south to the Ohio River. The descripti ...

Southern Indiana Southern Indiana is a geographic and cultural region that generally comprises the southern third of the U.S. state of Indiana and borders the states of Illinois to the west, Kentucky to the south, and Ohio to the east. Spanning the state's southe ...

. 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 The St. Louis Limestone is a large geologic formation covering a wide area of the midwest of the United States. It is named after an exposure at St. Louis, Missouri. It consists of sedimentary limestone with scattered chert beds, including the ...

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 Mammoth Cave National Park is a List of national parks of the United States, national park of the United States in south-central Kentucky. It encompasses portions of Mammoth Cave, the List of longest caves, longest known cave system in the worl ...

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.

References

Further reading