Western Kentucky’s Mammoth Cave is the longest known cave on Earth, with over 400 miles of passages currently explored & mapped. If the 2nd & 3rd longest caves in the world were put together (Mexico’s White Cave-Two Eyes System & South Dakota’s Jewel Cave), Mammoth Cave would still beat them out by about 50 miles. In a good year, 5 to 6 miles worth of new Mammoth Cave passages are found and mapped. Cave mapping has shown that Mammoth Cave passages are less than 1200 feet away from caves at Fisher Ridge, where 119 miles worth of passages are known. Mammoth Cave’s total known length will jump to >500 miles if a connection is made (both sides have “agreed” to not look for a connection, despite what the Fisher Ridge Cave people and website say). It’s been estimated that, in total, Mammoth Cave may have between 600 to 1000 miles of passages. Casual visitors are only allowed access to about 12 miles worth of Mammoth Cave’s passages.
The name “Mammoth Cave” refers to the immense size of many of the rooms and passages. The name does not refer to its world-record length, nor to the early discovery of mammoth fossils here (actually, fossil proboscidean skeletal remains have been reported from this locality).
The cave is developed in Middle & Upper Mississippian limestones. Partial dissolution of these limestones appears to have started during the late Cenozoic, since at least the Pliocene (the last ~5 million years). Limestone dissolution results from the presence of slightly acidic groundwater. Groundwater originates as rain, snowmelt, or runoff. It percolates through joints and bedding planes of the limestones and slowly dissolves them.
Why is the groundwater slightly acidic? Rainwater is slightly acidic due to the presence of carbonic acid (H2CO3), the result of water-atmosphere interactions (H2O + CO2 --> H2CO3). Once in the ground, water becomes more acidic by picking up complex organic acids from soils and additional carbonic acid. Air occupying spaces in soil is enriched in carbon dioxide gas, so groundwater will end up having more carbonic acid than it did as rain.
Groundwater dissolution has produced a vast network of passages in Mammoth Cave that vary in height, width, and cross-section shape. Horizontal cave passages that are wider than they are tall and have an ~elliptical shape in cross-section view are called tubular passages. They originally dissolved out at or below the water table (in the phreatic zone).
Tubular passage showing the classic, elliptical cross-section shape. This is Cleaveland Avenue in Mammoth Cave.
Vertical passages, steeply tilted passages, and tall & narrow, subhorizontal passages generally dissolved out above the water table. These are known as vadose passages. One type of vadose passage is the canyon passage, which is taller than it is wide. Canyon passages form as water downcuts through limestone while draining toward the water table.
Canyon passage showing the classic, irregularly narrow & high cross-section shape. This is Boone Avenue in Mammoth Cave.
The elevation of the water table in the Mammoth Cave area has been dropping over the past 5 million years. Currently, the water table in the cave is almost at the same level as the Green River, the main drainage in the area. So, the lowest passages of Mammoth Cave are partially to fully flooded. All the passages above that are dry to mostly dry. The upper dry levels indicate where the water table used to be. Depending on how one counts “levels”, Mammoth Cave has 6 or 7 levels (named Levels A to F).
Mammoth Cave, Historic Entrance (looking SE) - many of the currently-offered cave tours enter here. The rocks here represent the Beaver Bend Member of the Girkin Limestone (lower Chesterian Series, lower Upper Mississippian). Reported lithologies in the Beaver Bend include lime mudstones (micrites; micritic limestones), some fossiliferous to oolitic wackestones, and lime mudstones having patches of coarsely-crystalline calcite spar.
It is not certain exactly when Mammoth Cave was discovered. A story about its 1797 discovery by a man named Houchins (or Hutchins) is fictional (the park service knows this, but still tells the story during cave tours). However, Mammoth Cave’s earliest acknowledgement in available historical records dates to 1797.
Entrance dale to Mammoth Cave’s Historic Entrance (looking NW) - the cave mouth shown above is downhill from the park’s visitor center, accessed via this paved path. A small creek (left side of photo) has carved this little valley. This creek valley intersected & breached level B of Mammoth Cave (= the 2nd highest level in the cave system), resulting in the Historic Entrance. The Green River is ~0.3-0.4 miles further downhill.
Big Clifty Sandstone (looking SW) - the rocks along the entrance dale (see above photo) are quartzose sandstones of the Big Clifty Sandstone (Chesterian Stage, lower Upper Mississippian). This is the “insoluble caprock” unit on Mammoth Cave Ridge. The sandstone has obvious, well-developed cross-bedding, indicating deposition by a one-directional current. Large talus blocks of Big Clifty Ss. line most of the paved path from the visitor center down to the Mammoth Cave Historic Entrance.
Mammoth Cave, Historic Entrance (looking NW) - the rocks here are Beaver Bend Member limestones (Girkin Ls.).
A small waterfall occurs at Mammoth Cave’s Historic Entrance. Water flow is minimal to nonexistant during dry spells. A mixture of limestone rubble and tree branches litter the floor of the cave mouth directly below this waterfall (see photo below). Water does not pond here, but trickles into the cave for a very short distance, then disappears by downward percolation, helping to enlarge one or more inaccessible vadose passages.
It is customary to use fanciful (& sometimes silly or nonsensical) place names in caves. Of course, none of these names have real meaning in a geologic sense. The names are still useful, however. The human mind processes information about objects or places more readily if a name is available. This phenomenon is referred to by some as “name magic”.
Kentucky Avenue (= Grand Avenue)
Echo River Spring
Dripping Springs Escarpment
Much of the info. presented here & in the following pages is synthesized from the following:
Lee, E.F. 1835. Map of the Mammoth Cave, Accompanied with Notes. 1 sheet.
Bishop, S. 1845. Map of the Explored Parts of the Mammoth Cave of KY. Louisville. Morton & Griswold. 1 sheet. [facsimile published 1985]
Bullitt, A.C. 1845. Rambles in the Mammoth Cave During the Year 1844, by a Visiter. Lousville. Morton & Griswold. 101 pp. 1 fold-out map. [facsimile published 1985]
Richards, C. 1868. The Mammoth Cave, pleasures and pains of western travel, uncertainties of railways and steamboats, a woman's experience under ground, the Mammoth Cave from an unromantic point of view. The New York Times September 7, 1868. [warning: article contains anti-male sexist commentary and anti-nature commentary]
Hovey, H.C. 1880. One hundred miles in Mammoth Cave. Scribner's Monthly Magazine 20: 914-924. [facsimile published 2000]
Kämper, M. 1908. Map of the Mammoth Cave, Kentucky. 1 sheet. [facsimile published 1981]
Hovey, H.C. & R.E. Call. 1912. Mammoth Cave of Kentucky, with an Account of Colossal Cavern, Revised Edition. Louisville. John P. Morton & Company. 131 pp.
Watson, P.J. (ed.) 1974. Archeology of the Mammoth Cave Area. New York. Academic Press. 255 pp.
Palmer, A.N. 1981. A Geological Guide to Mammoth Cave National Park. Teaneck, New Jersey, USA. Zephyrus Press. 196 pp.
George, A.I. 1990. Effects of the New Madrid Earthquake (1811-1812) damage to the Mammoth Cave saltpeter works, Kentucky. Journal of Spelean History 24: 10-12.
George, A.I. & G.A. O'Dell. 1992. The saltpeter works at Mammoth Cave and the New Madrid Earthquake. The Filson Club History Quaterly 66: 5-22.
Cave Research Foundation. 1993. Mammoth Cave System. Dayton. Cave Research Foundation. 1 sheet.
Kennedy, M.C. & P.J. Watson. 1997. The chronology of early agriculture and intensive mineral mining in the Salts Cave and Mammoth Cave region, Mammoth Cave National Park, Kentucky. Journal of Cave and Karst Studies 59: 5-9
Granger, D.E., D. Fabel & A.N. Palmer. 2001. Pliocene-Pleistocene incision of the Green River, Kentucky, determined from radioactive decay of cosmogenic 26Al and 10Be in Mammoth Cave sediments. Geological Society of America Bulletin 113: 825-836.
Santucci, V.L., J. Kenworthy & R. Kerbo. 2001. An Inventory of Paleontological Resources Associated with National Park Service Caves. National Park Service. 50 pp.
Thompson, B. & J. Thompson. 2003. Images of America, Mammoth Cave and the Kentucky Cave Region. Charleston. Arcadia Publishing. 128 pp.
White, W.B. & E.L. White. 2003. Gypsum wedging and cavern breakdown: studies in the Mammoth Cave System, Kentucky. Journal of Cave and Karst Studies 65: 43-52.
Palmer, A.N. 2007. Cave Geology. Dayton. Cave Research Foundation. 454 pp.
Arthur N. Palmer, pers. comm., 2009 & 2010.