Kentucky Geological Survey Special Publication 7 Geology of the Mammoth Cave National Park Area

Age of Rocks—The rocks of the area date back about 325 million years¹ to that division of geological time called the Mississippian Period. Vast regions of this state and many others were then covered by shallow seas in which layer upon layer of clay, silt, sand, and limestone were forming. The limestone was formed from mineral matter in the sea water and from the shells and other parts of animals and plants that lived there. Fossil remains can be seen in these rocks in many places.

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¹Estimates of various geological age dates are revised from time to time as further information and data are gathered. The time of formation of the limestone in which Mammoth Cave is formed is estimated to be between 310 and 325 million years ago.

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Sediments from nearby land sources were carried by rivers and streams to these Mississippian seas and were deposited there as mud, now hardened into shale, and sand and gravel, now hardened into sandstone. Similar processes now in operation are forming layers of sediments in ocean and lake basins. Thus, about 1,200 feet of Mississippian limestones, sandstones and shales came into existence. The caves were formed much later.

Earth Movements Affect Cave Area—At the beginning of the Pennsylvanian Period, crustal movements of the earth caused the seas to withdraw from this area as the whole region was slowly warped upward. During this slow upraising, rivers flowing over the newly formed land surface deposited layers of sand and gravel, some in delta-like form, which were later covered by other layers above to form rocks known to be Pennsylvanian in age. At the close of the Paleozoic Era, the earth's crust in this region was warped upward into a great dome-like structure called the Cincinnati Arch. This feature can be seen in the cross-sectional view of the state (frontispiece). Located on the western flank of the Arch, the rocks of the Mammoth Cave area dip about 30 feet to the mile in a northwest direction, toward the Western Coal Basin.

The later history of the area is mainly one of erosion, when great thicknesses of rock were removed from the Cincinnati Arch. Some of this material accumulated as sediments which helped to form the younger rocks of the Purchase area in western Kentucky. Later uplifts, occurring to a greater degree in the east than in the west, rejuvenated or reactivated all the streams in the region. Erosive forces thus produced the present landscape features.

Fig. 1. Geologic time chart showing the relation of the age of the rocks in the Mammoth Cave area to those of the rest of Kentucky. The scenic features of this interesting region have been forming during the last 25 or 30 million years in rocks which are 310 to 325 million years old.

Among the weathering forces at work in the Mammoth Cave area has been one called carbonation. This is a process whereby certain gases from the air, such as carbon dioxide, are dissolved in water to form carbonic acid. Pure water cannot dissolve limestone readily, but most water is not pure. Instead, it contains these dissolved gases and acids which greatly increase the chemical reactions that may occur. When such water comes into contact with limestone, calcium carbonate of the limestone reacts with the carbonic acid to form calcium bicarbonate, which is a very soluble compound. In this form much carbonate material is dissolved in water and carried away. The fact that limestone is soluble in water accounts for the development of many of the features, both on the surface and underground, which intrigue the visitor.

Surface Features—Coming into the park from the southeast, along Kentucky Highway 70, one crosses a rolling flat lowland surface underlain by the St. Louis Limestone. This limestone, being soluble in the water which seeps down through it, has been eaten away until now the surface is literally pitted by depressions which are called sinkholes. The large number of sinkholes has led some geologists to call this the Southern Sink Hole Plain, while others know it as a part of the Pennyroyal Plateau. It also has been referred to as the "Land of Ten Thousand Sinks." To the west a high plateau rises about two hundred feet above the lower sinkhole plain. This conspicuous landscape feature is known as the Dripping Springs Escarpment. It is capped by the Big Clifty Sandstone, beneath which are thick layers of limestone. In certain of these limestones Mammoth Cave itself has been formed.

Fig. 2. A north-south cross section through the formations in the vicinity of Mammoth Cave would pass through the rocks shown above. In the soft, soluble limestone lowlands, caves and sinkholes are developed. The highlands above the Pennyroyal Plateau and separated from it by the Dripping Springs Escarpment are protected from rapid erosion by the Big Clifty and Pottsville sandstones.

If one approaches the cave area by way of Park City on Kentucky Highway 255, there is also a striking display of sinkholes in the Pennyroyal Plateau, and about two and one-half miles from Park City on the way north toward Mammoth Cave, the road goes up onto the top of the Dripping Springs Escarpment. The visitor who is interested in the origin of place names can see the spring called Dripping Spring, which has given its name to this escarpment. Located on Route 31W about eight miles west of Park City, this spring issues from the face of the escarpment itself.

Fig. 3. Aerial view of Pennyroyal Plateau (sinkhole plain), Dripping Springs Escarpment, and sandstone-copped Mammoth Cave Plateau. The sinkholes in the foreground are in the Ste. Genevieve Limestone. The Big Clifty Sandstone outcrops at the top of the escarpment. Photo by W. Ray Scott, National Park Concessions, Inc.

Route 255 merges with Route 70 about five miles south of Mammoth Cave. Along both sides of the highway approaching the cave may be seen numerous large, irregularly circular sinkholes. One of the largest in the area is Monroe Sink on the north side of the road about 2-1/2 miles west of Cave City. Another fine example is the New Entrance to Mammoth Cave, a sinkhole which will be seen by all visitors who take the Frozen Niagara Trip.

There are very few surface streams in the cave area other than the Green and Nolin Rivers, despite the average rainfall of about 50 inches. Many streams bear such suitable descriptive names as Lost River and Sinking Creek. A surface stream may suddenly enter a sinkhole or joint in the limestone and disappear. The limestones are honeycombed with openings formed by solution processes, and the water may reach great underground depths by following these openings. Perhaps a mile or two downstream, the water which disappeared will suddenly reappear at the surface as a large spring. One of the most striking examples of such disappearing streams may be seen by taking a short trip along one of the side roads within the park boundaries going in a southwest direction. About four miles from Mammoth Cave, one approaches a large sinkhole shown on the map as Cedar Sink. This is a special type of sinkhole, probably produced originally by collapse of limestone layers over a cavernous opening, that reveals a short section of an underground stream. A stream of water emerges from beneath a steep wall at one end of the depression, flows slowly across the floor of the sink, and disappears at the base of a rocky bluff on the other side.

During a long period of time most of the streams of this area have become adjusted to an underground course and can no longer be seen at the surface. As an indication of this fact, the visitor may note the numerous streamless valleys in the park area. These valleys look exactly like any other steep-sided stream valleys except that none of them has a stream of permanently flowing water in it. When rain falls, the water passes rapidly underground through sinkholes and joints.

The observant visitor may notice, however, that there are several ponds formed on top of the Dripping Springs Escarpment, and he may wonder about their origin. Such ponds are formed locally where there is an impervious layer of shale capping the Big Clifty Sandstone, thus sealing off the underground seepage channels through which the rest of this area is drained.

Many underground streams are still at work today in this region, carving out innumerably more passages. Areas underlain by limestones having such features as sinkholes and disappearing streams are described as having karst¹ topography. This term was applied originally to a mountainous limestone belt along the eastern shore of the Adriatic Sea in Italy and Yugoslavia. Karst topography is extremely well developed in the area around Mammoth Cave.

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¹The term karst is derived from the Yugoslavian word kras meaning stone, which is the root for the Italian place name, Carso.

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Importance of Green River—One obvious exception to the underground drainage pattern of the local streams is Green River, which flows quietly along through the park in its winding course about 300 feet below the level of the hotel. Not far from the hotel, Echo River emerges from its underground course in the cave to join Green River. Echo River contributes to the water supply of the main stream, as do many other springs and underground streams in the limestone belt.

Fig. 1. The Glen Dean formation is one of the fossiliferous Mississippian limestones. It occurs above the limestone in which Mammoth Cave was formed, with other limestones and sandstones intervening. Remains of crinoids, brachiopods, and bryozoans indicate a marine environment while the limestone was forming. Fossils shown include: A. Archimedes, a bryozoan; B. fragment of crinoid; C. Spiriferina, a brachiopod. Fig. 2. The cliff-forming Big Clifty Sandstone of the Mammoth Cave area is composed of firmly cemented sand grains. Withstanding erosion more effectively than the soluble limestones, it stands above the low limestone plain to cap the Dripping Springs escarpment (see Figure 3). Photo by W. Ray Scott, National Park Concessions, Inc. PLATE 1

Green River certainly must have played a significant role in the development of the cave. Active circulation of underground water is essential to solution and cave-forming, and there must be some outlet again at the surface if there is to be active water movement. The valley of Green River and its tributaries supply this needed outlet, and it is in the vicinity of these valleys that the caves have been formed.

The cave-forming processes are fairly well limited (though not entirely) to the limestone above the level of the deepest valley. Stream valleys increase in size and as they become deeper the zone of cave development also extends deeper. The lower cave passages are the youngest, and as they are progressively formed the upper levels become relatively dry as the water passes on down to the lower levels.

Fig. 4. The successive stages in the erosion cycle of a limestone area such as Mammoth Cave are shown above. Surface streams are diverted underground by way of sink holes, and crevices are opened in the soluble limestone, thus giving rise to such streams as Sinking Creek and Lost River. (After Lobeck.)

Another factor to be considered is the dip of the limestone beds. Moving water follows porous strata, bedding planes, and joints. In the first two cases, the water will be passing primarily down dip. Thus, water following such beds on the south side of Green River will have an outlet in the valley of Green River, but water on the north side moving down dip will reach the water table level and go down no further. Active water circulation is therefore fairly well restricted to the area south of the river. Since water moves downward along joints on both sides, there will be some cave passages on the north side too, but these will be limited in size and number.

Fig. 5. An early stage in the cutting of the Green River Valley is shown above. Active water circulation in the dipping limestones is limited to the few upper layers on the up-dip side where the water can find an outlet into the Green River Valley lying below.

Fig. 1. Through this quiet spring the water of Echo River emerges to join Green River, thus furnishing the active water circulation necessary for the solution processes leading to the formation of the large cavern system. Photo by W. Ray Scott, National Park Concessions, Inc. Fig. 2. Echo River, at one of the lowest levels developed in Mammoth Cave, varies in depth with the seasons. Sometimes water from Green River backs up into Echo River filling this channel entirely with water, leaving behind silt and sand. Photo by W. Ray Scott, National Park Concessions, Inc. PLATE 2

As lower outlets are made available through valley deepening, additional porous zones and bedding planes become available for active water circulation. It is primarily along such zones that the different cave levels are determined. With any upwarping of the earth's crust, valleys are cut still deeper and the development of solutional features is accelerated.

Thus, it can be seen that in the Mammoth Cave area, where thick layers of well-jointed limestones occur, solutional processes are favored by the presence of the deep Green and Nolin River valleys, situated so that they provide ample outlet for underground water drainage. In addition, these valleys aid in localization of cavern development by furnishing steep gradients for all nearby streams, surface or subsurface. As the slopes become steeper, more tributary streams along valleys in the well-jointed limestone are diverted to underground courses since water will take the easiest and quickest path downward to seek its lowest level.

Locally there is also the hard sandstone caprock which protects the underlying limestone from active destruction by weathering processes. This sandstone lies directly under the soil at the cave hotel and may be seen in several places as one goes down into the valley in which the Historic Entrance is located.