Mammoth Cave lies in an area of central Kentucky between the Barren and Green Rivers, and Beaver Creek. In this area of several hundred square miles are numerous caves, of which Mammoth Cave is the most famous. Mammoth Cave is at least 150 miles long, and is thought to be the longest in the world. It is impossible to be sure, since there may be caves we don't know about.
The cave area of central Kentucky has, on the surface, an abundance of rich farm land. Underlying the entire area is a 330 foot (100 meter) thick formation of limestone and dolostone. Dolomite is very similar to limestone, except that it is rich in magnesium. Most of the caves in the world are formed in limestone. This is not surprising as limestone is fairly abundant at the surface of the earth. No other type of rock occurring so often at the surface dissolves so readily as limestone. The best recipe for caves is a mixture of limestone and water.
In this part of central Kentucky it rains about 48 inches a year. A scientist would say it has a mean annual rainfall of 48 inches. The runoff from this large amount of water should be about 20 inches per year. This is not an abnormal amount of water for most areas; but since there are almost no streams in the area, the water, which usually runs off into creeks and small streams must go somewhere, runs into large sinkholes. Some of these sinkholes are larger than a tractor, or as large as a house, sometimes even larger.
The water in these sinkholes then flows into the thick layer of limestone and dolostone. It then begins to move through the system of conduits (somewhat like pipes), larger channels and the caves themselves. As it moves this water, which contains carbonic acid, eats away part of the calcite in the limestone and dolostone. This calcite then mixes with the carbonic acid to form a solution of calcium bicarbonate (which we know better as baking soda). When this calcium bicarbonate loses carbon dioxide it can then, under proper conditions (found in the Mammoth Cave area), create stalactites and stalagmites. These rock formations can be formed with many other materials included, such as gypsum, iron pyrites, limonite, and so forth. It is this variety of materials that accounts for the variegated colours found in these rock formations. The most common material is calcium carbonate. After the water that started on the surface as rain, moves through the cave system, it is discharged from springs into the Green River.
The Green River has played an important part in the development of Mammoth Cave, because underground water must actively circulate for solution and cave-forming processes to take place. Unless there is an outlet for this water at the surface there can be no underground movement of water. The constant movement of water in the area means that the Mammoth Cave system is constantly growing. Most of the water from the cave area is eventually carried away by the Green River.
Dr. Stephen Austin is a scientist who has studied the geology of the Mammoth Cave area. His studies have led him to the conclusion that the caves cannot be as old as many geologists have previously thought. His findings are based on an analysis of another scientist, J. V. Thrailkill, who studied the rate groundwater movement in caves in other areas of the world. By using Dr. Thrailkill's figures Dr. Austin discovered that "if the dissolving power of the acid in one square kilometer of central Kentucky is carried in one conduit, a cave one meter square and fifty-nine meters long could form in a year!"
Professor Swinnerton, another geologist, gives a similar estimate. He calculates that a cave three feet high and over 120 feet long could be formed for every square mile of the entire area each year. He considers this a conservative estimate, which means that caves actually form much more rapidly than even these figures would indicate. Both Swinnerton and Austin note that the full dissolving power of the acid present in Kentucky's soil is not used in the formation of these caves, because the climactic conditions are not exactly right. In a cooler and more humid climate such caves could form even more rapidly. Many scientists believe there was a cooler and more humid climate was normal during what they call the Pleistocene epoch. If this is true it would have increased both the rate of groundwater flow, and the rate of cave solution, so that caves would form much more rapidly than they do at present.
It is this which leads Dr. Austin to the conclusion that Mammoth Cave must not be very old. It is certainly much older than you or me, or even the oldest person we know, but it is young in the manner in which many geologists describe the earth. Many scientists think that the Pleistocene epoch lasted about two million years, and that this is when many caves were formed. Dr. Austin points out that, assuming present rates and conditions (which are less than they would actually have been under the conditions supposed to be existing in the so-called Pleistocene), in two million years all the limestone in Kentucky would be completely dissolved! What this really means to us is that if the 330 foot thick layer of limestone and dolostone which underlies the cave area was really two million years old, you would now be reading about Mammoth Canyon instead of Mammoth Cave.
Dr. Austin says: "Any reasonable estimate of the volume of limestone actually removed by solution of Kentucky caves and karst would be insignificant compared to that predicted by an evolutionary model." In other words, as large as the cave system is at present, it is very small compared to what we would expect if the caves were several million years old. This means that instead of being many millions of years old, Mammoth Cave and the entire cave system of the area is only some thousands of years old.
Stephen A. Austin, "Origin of Limestone Caves," Impact Institute for Creation Research. No. 79 (Jan 1980): 1-8.
Ann Livesay, Geology of the Mammoth Cave Natural Park Area. Rev. ed. by Preston McGrain. Lexington: University of Kentucky, 1962.
George W. Moore, "Limestone Caves," in The Encyclopedia of Geomorphology ed. R. W. Fairbridge. New York: Reinhold Book Co., 1968.
A. C. Swinnerton, "Origin of Limestone Caverns," Bulletin. Geological Society of America. 43 (Sept 1932): 663-693.
Groundwater containing dissolved carbon dioxide (CO2) dissolves calcite CaCO3) which is the chief constituent of limestone. The dissolved carbon dioxide combines with water to form carbonic acid and this solution converts part of the calcite to soluble calcium bicarbonate. (Note that in rich soil the water becomes even more acidic because it takes up more carbon dioxide.) Here is the chemical formula for what happens:
CO2 + H2O H2CO3 + CaCO3 Ca+2 + 2HCO3-1
The formula for dolomite is: