Pennyroyal Plateau

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Regions of Kentucky, with the Pennyroyal Plateau shown in light brown (labeled as the Mississippi Plateau) KYphysiography.jpg
Regions of Kentucky, with the Pennyroyal Plateau shown in light brown (labeled as the Mississippi Plateau)

The Pennyroyal Plateau or Pennyroyal Region, often spelled Pennyrile, [1] is a large physiographic region of Kentucky that features rolling hills, caves, and karst 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. [2]

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. [3]

To the south, the Pennyroyal continues as the Highland Rim of Middle Tennessee.

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. [4]

Where the capping sandstone is intact, the land surface is usually forested, rugged hills.

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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 Pottsville Escarpment on the east and terminates at the Ohio River in the west. However, in truth, it continues in Indiana as Floyds Knobs.

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<span class="mw-page-title-main">Geology of Kansas</span>

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<span class="mw-page-title-main">Pottsville Formation</span>

The Pennsylvanian Pottsville Formation is a mapped bedrock unit in Pennsylvania, western Maryland, West Virginia, Ohio, and Alabama. It is a major ridge-former in the Ridge-and-Valley Appalachians of the eastern United States. The Pottsville Formation is conspicuous at many sites along the Allegheny Front, the eastern escarpment of the Allegheny or Appalachian Plateau.

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The Pennington Formation is a geologic formation named for Pennington Gap, Virginia. It can be found in outcrops along Pine Mountain and Cumberland Mountain in Kentucky, Virginia, and Tennessee, where it is the uppermost Mississippian-age formation. The name has also been applied to similar Mississippian strata in the Cumberland Escarpment of eastern Kentucky, though the rocks in that area were later renamed to the Paragon Formation.

The geology of Ohio formed beginning more than one billion years ago in the Proterozoic eon of the Precambrian. The igneous and metamorphic crystalline basement rock is poorly understood except through deep boreholes and does not outcrop at the surface. The basement rock is divided between the Grenville Province and Superior Province. When the Grenville Province crust collided with Proto-North America, it launched the Grenville orogeny, a major mountain building event. The Grenville mountains eroded, filling in rift basins and Ohio was flooded and periodically exposed as dry land throughout the Paleozoic. In addition to marine carbonates such as limestone and dolomite, large deposits of shale and sandstone formed as subsequent mountain building events such as the Taconic orogeny and Acadian orogeny led to additional sediment deposition. Ohio transitioned to dryland conditions in the Pennsylvanian, forming large coal swamps and the region has been dryland ever since. Until the Pleistocene glaciations erased these features, the landscape was cut with deep stream valleys, which scoured away hundreds of meters of rock leaving little trace of geologic history in the Mesozoic and Cenozoic.

The geology of Kentucky formed beginning more than one billion years ago, in the Proterozoic eon of the Precambrian. The oldest igneous and metamorphic crystalline basement rock is part of the Grenville Province, a small continent that collided with the early North American continent. The beginning of the Paleozoic is poorly attested and the oldest rocks in Kentucky, outcropping at the surface, are from the Ordovician. Throughout the Paleozoic, shallow seas covered the area, depositing marine sedimentary rocks such as limestone, dolomite and shale, as well as large numbers of fossils. By the Mississippian and the Pennsylvanian, massive coal swamps formed and generated the two large coal fields and the oil and gas which have played an important role in the state's economy. With interludes of terrestrial conditions, shallow marine conditions persisted throughout the Mesozoic and well into the Cenozoic. Unlike neighboring states, Kentucky was not significantly impacted by the Pleistocene glaciations. The state has extensive natural resources, including coal, oil and gas, sand, clay, fluorspar, limestone, dolomite and gravel. Kentucky is unique as the first state to be fully geologically mapped.

References

  1. "Pennyrile region" at the Kentucky Atlas and Gazetteer. Retrieved 10 March 2022.
  2. "The Mississippian Plateau or Pennyroyal Region , Kentucky Geological Survey site". www.uky.edu. Retrieved 2021-07-05.
  3. "Geology of Kentucky: Chapter 15, Mississippian Plateaus". www.uky.edu. Retrieved 2021-07-05.
  4. "Interior Low Plateaus Province (U.S. National Park Service)". www.nps.gov. Retrieved 2021-07-05.

Further reading

37°30′N86°10′W / 37.500°N 86.167°W / 37.500; -86.167