• Abstract and Figures
• Public Full-text

mammoth Cave National Park eneath the surface of southcentral weak carbonic acid (the same acid as in soda Kentucky lies a world that is virtually pop) dissolved a network of tiny microcaverns unparalled. It is a labyrinth character- along the cracks. As the land continued slowly ized by mile upon mile of dark, seemingly rising, the Green River eroded its channel endless passageways. The geological process deeper. The water in the network of Bresulting in this world that we refer to as Mam- microcaverns drained through the limestone moth Cave began hundreds of millions of years under the plateau toward the river. Just as ago and continues today. Indeed, the complex- rivulets converged into streams above ground, ity of the system adds to its uniqueness and the water flow paths through the limestone also story of its formation is noteworthy. converged into incrementally larger flow paths. The Ancient World As rainwater continued to enter the system and 350 million years ago was a very different time more limestone was dissolved, the microcaverns than today. The North American continent was enlarged. Because the major drains carried the located much further south; at that time Ken- most water, they enlarged the most. Caves were tucky was about 10 degrees south of the equa- forming. As the Green River cut deeper, the tor, and a shallow sea covered most of the water table continued dropping to the same southeastern United States. The warm waters base level as the Green River. New under- supported a dense population of tiny organisms ground drains formed at levels lower than the whose shells were made of calcium carbonate older ones, and the older channels emptied. (CaCO3). As these creatures died, their shells Thus the oldest cave passages are the closest to accumulated by the billions on the floor of the the surface, and the youngest horizontal pas- ancient sea. In addition, calcium carbonate sages are the deepest underground. At the precipitated from the water itself. The build-up present water table, cave passages are still of material continued during the next 70 million being formed. years until some seven hundred feet of lime- stone and shale was deposited. Late in the deposition of the limestone, about fifty to sixty Surface Clues feet of sandstone was deposited over much of As you approach the vincinity of Mammoth the area by a large river system that emptied Cave, several clues suggest the existence of into the sea from the north. caves. Roadcuts along highways have vertical exposures of layered grayish rock, often broken About 280 million years ago, the sea level into irregular blocks at the top where erosion started to drop and the continent began to rise, has widened vertical cracks across layers. exposing layers of limestone and sandstone. Between the layers you may see the tiny open- The stage was set for the formation of the ings in the limestone that are the first stage in Mammoth Cave. Forces at work beneath the the formation of a cave. earth's crust caused it to slowly rise, buckle and twist, causing tiny cracks between and across The landscape along the highway also has layers of limestone and sandstone. At the same special characteristics. You will not see surface time river systems as we know them today slowly streams. Instead, you will see myriads of crater- developed. By about three million years ago a like depressions called "sinkholes." These sandstone-capped plateau stood above the sinkholes are places where run-off may quickly Green River, and a low, almost flat limestone enter the limestone aquifer. Cave drains carry plain extended southeast of what is now Inter- the dissolved limestone away, and the surface state 65. soil settles, creating the bowl-shaped depres- sion. If the sinkholes drains become plugged Acid Rain with soil, then the water cannot drain under- Rain water, acidified by carbon dioxide in the ground and a pond forms. Occasionally the soil, seeped downward through millions of tiny drain becomes unplugged and a pond as large cracks and crevices in the limestone layers. This as several acres will disappear overnight.This karst geology kind of landscape is called karst topography. It path of flow. The result The result is numerous is found along and to the southeast of Interstate small to moderate-sized interconnecting pas- 65 near Mammoth Cave National Park and sages and only a few large ones. referred to as the Sinkhole plain. At its south- east edge surface streams sink underground Fourth, vertical shafts are formed where water joining the drainage of thousands of sinkholes. flows off the edge of the sandstone caprock and Continuing northwest they become the under- seeps down into the limestone below. These ground rivers of Mammoth Cave. shafts are geologically much younger than the horizontal passages, and they intersect these The Uplands older passages only by chance. The drains of Driving northwest from Cave City or Park City, the shafts, however, eventually join the actively you start to climb a line of bluffs rising some forming passages at the water table, thus three hundred feet above the sinkhole plain. adding to the cave's interconnections and These bluffs are the Chester Escarpment -- the complexity. border between the unprotected limestone of the Sinkhole Plain and the Mammoth Cave Plateau. Finally, the caprock on the plateau protects older upper level passages from destruction. Beyond the top of the escarpment the plateau is This is in contrast to the situation found on the divided into broad, flat sandstone-capped uncapped Sinkhole Plain. There the surface of ridges separated by steep, limestone-floored the land continues to drop, because upper level valleys with many sinkholes. Very little water is passages of caves collapse and are eroded able to penetrate the sandstone caprock, so the away as fast as newer and lower passages are limestone below is protected from erosion. Most formed at the level of the water table. of the early discoveries in Mammoth Cave were beneath these ridges and valleys, and all the Creation ... and Destruction entrances are in the valleys. Cave passages also collapse in Mammoth Cave. As the valleys between the flat-topped Putting It All Together ridges widen and deepen they intersect the A unique combination of circumstances has oldest upper level passages. Usually this made Mammoth Cave the longest cave in the collapse results in a "terminal breakdown"; but, world, with more than 345 miles of mapped sometimes we can enter the cave at the break- passages. First, the karst setting has a large down of jumbled blocks of limestone and area for potential cave formation. The upstream sandstone. The Historic Entrance to the cave is headwaters of Mammoth Cave are out under the easy to enter because water draining off the sinkhole plain. Most of the passages large sandstone caprock has dissolved much of the enough for people to enter are under the escarp- breakdown, creating a huge opening to one of ment, the plateau, and the flat-topped ridges the largest passages in the Mammoth Cave with their intervening valleys. Springs along the system. Because the rapodly flowing water here Green River are the downstream outlets of is not saturated with limestone minerals, it ground rivers such as Echo and Roaring Rivers. cannot deposit the stalacitite and stalagmite formations we think of as decorating caves. Second, the Green River valley has deepened slowly due to many interruptions during the ice Artistry in Stone ages (Pleistocene). As a result, major passages As water and time enables the removal of were formed and Mammoth Cave contains limestone and the formation of cave passages, multiple levels. so too, they enable the deposition of "cave decorations" called speleothems. These decora- Third, the limestone is made up of many differ- tions include both the familiar gypsum flowers ent layers with different characteristics; there- and needles. Although these speleothems seem fore as the underground water sought lower and to grow magically from the walls, ceiling, and lower levels, each layer provided a different floors, they are actually formed by the processes Karst geology of dissolutoin and precipitation. The two most The Dry Formations common types are composed of the major Sulfate speleothems, like gypsum flowers, are mineral in limestone, calcium carbonate deposited in dry passages beneath the sand- (CaCo3) and by salts of a minor component, stone caprock. Calcium sulfate (gypsum) is sulfates (SO4). much more soluble than calcium carbonate and can be carried toward cave passages by Carbonate speleothems, such as stalactites, are the slight amount of water that seeps through deposited in passages where there is no sand- the sandstone caprock. The water in the damp stone caprock above. Here, vertically seeping limestone is slowly drawn by capillary action water dissolves calcium carbonate and can into dry passages (85%-95% relative humidity) redeposit it if the water drips into an air-filled from all directions. As the water evaporates passage. The water loses carbon dioxide (CO2) gypsum is deposited. At its most spectaclar, this to the cave air, much like a soda pop loses CO2 mineral (CaSO4) *2(H2O) forms white to gold bubbles when opened. The loss makes the flower-like structures that seem to ooze and curl water less acidic, so it is unable to hold as much from the wall, ceiling, and floor much like icing calcium carbonate in solution. The calcium from a cake decorator's nozzle. In fact, gypsum carbonate is then precipitated as travertine speleothems grow from the base. This phe- speleothems. nomenon helps explain why they can form loose crusts or blisters and how gypsum growing in The shape of the speleothems depends on limestone cracks can force off bits of limestone where and how fast water enters a cave pas- and gypsum from the ceiling and wall.

Load more

  3

Copy Link