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


The discussion of cave-forming processes and surface features of the Mammoth Cave area presented above has been given in order to provide a background for the description of the specific features seen on guided trips into the cave. Features seen and routes followed on these trips will vary from time to time, but those described here are the outstanding ones now being shown.

The Echo River Trip—For those who cannot stay long enough to take an all-day trip, shorter trips showing smaller portions of the cave are offered. One of these shorter trips is the Echo River Trip which starts through the Historic Entrance and takes about three hours. On the path leading to the cave entrance, the observant visitor will note that he walks down into a steep valley which has cut through the Big Clifty Sandstone into the cave-forming limestone below. In one side of this valley is the Historic Entrance, formed by solution eating into the limestone layers along the side of the valley and aided by collapse of thin, unsupported layers of limestone.

Fig. 9. Route map of Echo River Trip. (click on image for an enlargement in a new window)

Fig. 1. The Ruins of Karnak furnish spectacular evidence of the sculpturing action of water trickling down cave walls, enlarging cracks, leaving these tremendous limestone columns. Photo by W. Ray Scott, National Park Concessions, Inc.

Fig. 2. Bottomless Pit, 105 feet deep, a fine example demonstrating the power of water to dissolve limestone along vertical crocks or joints leading downward from one cave level to another. Photo by J. Wellington Young, National Park Concessions. Inc.


On this trip essentially the same features are seen as on the All Day Trip, at least down as far as Echo River. One of these features is the Bottomless Pit, situated just beyond the rockfall known as Giant's Coffin. This pit has a depth of about 105 feet. Directly above the pit is a dome-shaped circular "well" some 63 feet high. These pits and domes are developed at many points throughout the cave and are formed along vertical joints which tap some large water source lying above them. In many cases water drips or flows down through the pits and domes from higher levels or from the ground surface above. Sometimes waterfalls of this sort produce deep gouges in the walls through both solution and abrasion. Occasionally there are surface sinkholes directly above the domes.

Fig. 1. Stalactities, stalagmites, and rimstone dams in Mammoth Cave. The small rimstone dams or terraces in the lower part of the picture have developed where flowing water is slightly agitated or riffled by an irregular bottom surface. Photo by W. Roy Scott, National Park Concessions, Inc.

Fig. 2. Boone Avenue furnishes a good example of the narrow, steep walled type of cave passage formed by solution along a vertical joint in the solable limestone. Photo by W. Ray Scott, National Park Concessions, Inc.


After reaching Echo River and taking a boat ride on this underground stream, the visitor on this short trip is conducted out of the cave by way of Mammoth Dome and the Ruins of Karnak, a remarkable example of huge columns formed by continued sculpturing of limestone by water seeping down the walls, slowly carving out reentrants in the soluble rock.

The return route takes the party up a flight of stairs in the Mammoth Dome itself to a higher level at Little Bat Avenue and thence through Audubon Avenue, back to the Rotunda and the Historic Entrance. Mammoth Dome rises to a height of 192 feet within the cave.

The Frozen Niagara Trip—The relatively short, one and one-half hour trip, called the Frozen Niagara Trip, has retained its long-standing popularity through the years. An amazing variety of salactites, stalagmites, columns, and flowstone formations are observed during the course of this trip. A bus brings the visitor down along the road directly into the sinkhole which is responsible for the New Entrance to the cave. If one pauses here at the entrance and looks up he gets a fine view of the Big Clifty Sandstone capping the soluble limestone in which this sinkhole is formed.

Fig. 10. Route map of Frozen Niagara Trip. (click on image for an enlargement in a new window)

Entering the cave, the visitor first sees a number of pits and domes formed by water running down from the surface along joints in the limestone. After a heavy rain, water drips or flows through most of them, passing on down to lower levels. Some, like Roosevelt's Dome and Wilson's Dome, are, on an average, 130 feet high, and the visitor can look down into Silo Pit and other deep holes, averaging 160 feet deep. These are fine examples of the solutional power of running water in limestone.

The party proceeds along one of the five passageways leading into Grand Central Station. Nearby is the "Big Break," where collapse of a tremendous quantity of rock from the cave ceiling has littered the floor with rocks of all sizes. The broad, unsupported section of ceiling formed along the base of a single limestone layer seen farther on has been variously called the "Plastered Ceiling," "Smooth Ceiling," or "Fairy Ceiling." Blocks from this ceiling will come down sooner or later.1

1The collapse of rocks from the ceilings of cave passages is a very slow process, and no visitor to Mammoth Cave has ever been injured by such a rockfall.

The visitor then views Frozen Niagara, the largest single travertine deposit of the cave. This impressive flowstone feature, a large mass 75 feet high and 45 feet wide, was formed through centuries of slow deposition of limestone from water flowing and dripping down into this portion of the cave over blocks of fallen wall and ceiling rock. A stairway directly in front of Frozen Niagara leads down into the Drapery Room, so named because of the curtains or draperies formed by rows of stalactites growing together and by flowstone deposits of limestone over the surfaces of fallen rocks. Attractive dripstone formations may be seen in all directions. One person's attention may be caught by a stalagmite resembling a cactus; someone else may see an owl, a turtle, or a cat among the dripstone oddities. Interruptions and variations in the rates of dripping have contributed to the individuality of these formations. The guide may show the group a "strip of bacon" by placing a light behind a thin limestone deposit containing brownish streaks of iron, formed by oxidation during periods of nongrowth. One beautiful flowstone deposit in the Drapery Room is the Golden Fleece. Diffused by a lovely golden hue, it derives its color from the oxidation of iron minerals exposed to the air. Someone acquainted with Greek mythology no doubt named the formation for its resemblance to the famous golden ram's fleece in the story of Jason.

Fig. 11. The Cat, an odd shaped stalagmite in Colossal Cave.

Fig, 1. Along Onyx Colonade are seen examples of stalactites and stalagmites growing together to form columns, Stalactites are also present here in rows, the solution forming them having come down along cracks in the ceiling. Photo by W. Ray Scott, National Park Concessions, Inc.

Fig. 2. Crystal Lake probably owes its existence to name impervious rock layer lying beneath its surface, which prevented water dripping down constantly from above to continue on down to a lower level. From the lake one looks upward into Moonlight Dome, formed by the enlargement of a joint in soluble limestone. Photo by W. Ray Scott, National Park Concessions, Inc.


Further inside the cave and 60 feet below the observation point of the visitor lies the lovely green water of Crystal Lake, a small pool formed by the damming up of a stream behind a stalagmite that grew across the narrow passage here. On the water's edge is a stalagmite that resembles a bathing beauty when a light is turned on behind it. By varying the lighting effects, the bather, who is poetically called September Morn, can be made to change her costume.

Next the group enters a small passage called the Onyx Colonnade where there is a bewildering variety of columns, stalactites, and stalagmites. Many of these deposits have been given imaginary names, such as Wedding Cake, a stalagmite that has the layered appearance of a cake, and Lion's Cage, a section composed of rows of columns that look like the bars of a lion's cage. Each column was formed by a stalactite from above and a stalagmite from below joining. A few helectites are also present here.

In the Onyx Chamber again there is a wide variety of dripstone and flowstone deposits, many of them tinted in a variety of reds, purples, and yellows. The colors are due to the oxidation of minerals such as iron and manganese during interruptions in the growth of the formations. Located on one side of the Onyx Chamber is Rainbow Dome, a small dome 45 feet high, displaying many of the color effects noted above. One side of this dome is covered by a deposit of flowstone, Stage Curtain, showing the delicate folds of a thin theater curtain. Onyx Chamber itself contains one section of innumerable thin, fragile stalactites known appropriately as the Macaroni Factory. These slender hollow tubes are good examples of the early stages of stalactite growth. Their wet, shining surfaces show that they are still growing and their small size indicates rapid dripping. Rather than forming large stalagmites below, most of the water here flows off depositing calcium carbonate to form dams along the cave floor. Examples of terraces and dams built up around the edges of standing pools of water are numerous here. One particularly interesting long travertine dam has been called the Great Wall of China.

The Historic Trip—For those particularly interested in history, there is a special trip of about one and one-half hours duration to inspect features of historical significance. In the Historic Trip, the cave is entered through the Historic Entrance, and the group passes first into the Rotunda. There is ample time for close inspection of the saltpeter mining that made Mammoth Cave famous during the War of 1812, for the mining operations inside the cave furnished some four hundred thousand pounds of nitrates critically needed in making gunpowder. Many visitors ask about the origin of the saltpeter. Some authorities believe that it was derived from bat guano; others, though, prefer an origin from bacteria or other source in the dry cave earth.

Fig. 12. Route map of Historic Trip. (click on image for an enlargement in a new window)

Fig. 13. Portion of the saltpeter mining operations which was carried on in Mammoth Cave during the War of 1812. Vats were used in the leaching of nitrates from the dry cave earth. These and other features of the operation may be seen on the Historic Trip. Photo by W. Ray Scott, National Park Concessions, Inc.

Going down Broadway the visitor soon comes to Booth's Amphitheater, a large room formed in the soluble limestone and named for Edwin C. Booth, the famous Shakespearean actor. Booth visited the cave in 1876 and in this chamber he climbed up on a large ledge of rock and recited famous lines from Shakespeare from the natural stage.

As a party of visitors proceeds along the winding passage, the guide tells them to stop and look back. Silhouetted against the wall in the background is a perfectly outlined figure, Martha Washington's Statue. It is really a section of cave wall hidden from view by the dark curving walls in between, except for one small opening which, when backlighted, gives the effect of a statue in outline. This is much like looking through a keyhole into a lighted room.

Star Chamber is another feature of interest. In the subdued light of this part of the cave, the name seems very appropriate. As the guide turns up the light, the visitor can see white gypsum crystals standing out as "stars" against the background of a ceiling stained dark black by the manganese and iron oxides deposited there by water. The new growths of gypsum, pushing outward into the cave, have broken off the dark coating in places and have revealed the white star like growths.

Of considerable anthropological interest is the famous mummy of an Indian, who has been named Lost John, found in the cave. Items associated with the mummy have been carbon-dated to 2300-2400 years ago. Evidence of human occupation of Mammoth Cave is limited to the upper passages. This particular Indian, whose remains were found in 1935, apparently met an accidental death while collecting gypsum along a nearby ledge. He loosened rocks along the ledge and was crushed to death when a huge block weighing about five tons gave way. The mummy has been well preserved in the dry atmosphere of this part of the cave, which is not conducive to bacterial decay. The mummy is that of a man about 45 years old and five feet three inches tall.

Fig. 1. The Rotunda is a huge chamber 139 feet wide and 40 feet high, developed by collapse of large amounts of thinly bedded limestone from the ceiling over a long period of years. Photo by W. Ray Scott, National Park Concessions, Inc.

Fig. 2. Audubon Avenue, one of the large flat-ceiling passageways that is so typical of Mammoth Cave. The cave is developed in an almost uninterrupted limestone sequence representing Ste. Genevieve and Lower Chester ages. Photo by W. Ray Scott, National Park Concessions, Inc.


Fig. 14. Martha Washington's Statue, a silhouette formed by cave walls.

Evidence may be seen along Broadway of an experiment that was attempted in Mammoth Cave in 1842 and 1843, when little was known about tuberculosis. It was believed that the constant temperature of 54 degrees and the humidity of the air might benefit those suffering from tuberculosis. Thus, 12 huts were constructed along Broadway to house the fifteen or so persons suffering from this disease. They were brought into the cave and remained for several months without coming out. During the course of the experiment, all patients became worse, and at least two of them died.

The All-Day Cave Trip—The most interesting journey through the cave, for those who have the time, is the one featured as the All-Day Trip. In seven hours the visitor sees the greatest possible variety of underground features. Starting at the Historic Entrance, this trip takes the visitor down slope past the wooden leaching vats and pipes used in the manufacture of saltpeter. The visitor then enters a large chamber known as the Rotunda. This tremendous rounded room, although referred to as a "dome" has not been developed by the solution processes commonly associated with dome-pit formation. The Rotunda was formed instead by the collapse of large amounts of thinly layered limestone over a long period of time, resulting finally in this huge chamber which is 139 feet wide and 40 feet high. It is the largest structure of this sort in the cave.

Fig. 1. Spongy growths of calcium carbonate like this along Coral Avenue in Mammoth Onyx Cave are due to the carbonate deposition from solution seeping through the porous limestone. Photo courtesy of E. R. Pohl.

Fig. 2. St. Peter's Dome, in Onyx Chamber of Mammoth Cave, shows a profusion of dripstone features. Rapid dripping results in small stalactites and large stalagmites below. Photo by W. Ray Scott, National Park Concessions, Inc.


Fig. 15. Giant's Coffin, a large block of limestone estimated to weigh 2,000 tons. Photo by W. Ray Scott, National Park Concessions, Inc.

Broadway is a long, high-level corridor, about three miles in length and on an average of 40 feet high and 60 feet wide. Only a short portion of this extensive passage is shown on the present cave trips. It is a remarkable example of a broad cave passage developed originally along a porous limestone layer. Here Martha Washington's Statue and Giant's Coffin are seen.

Giant's Coffin is a huge block of fallen limestone lying on the cave floor. It is 50 feet long, 20 feet wide, and 16 feet high with the edges rounded by solution. This block has been estimated to weigh 2,000 tons. Many cave passages, particularly those with broad, flat ceilings, have been enlarged partly by the fall of rocks such as this. Cave enlargement by collapse or sapping is a common phenomenon and a natural consequence of the enlargement of passages with broad unsupported ceilings. Blocks are loosened by solution along the bedding planes and joints. Earthquake tremors may serve to bring down these blocks earlier than otherwise, but the fall would normally occur, anyway. Therefore, fallen blocks of limestone within the cave do not in themselves constitute evidence of earthquake shock. As previously stated, no visitor in this cave has ever been injured by such a rockfall. Beyond Giant's Coffin visitors follow lower passages.

Fat Man's Misery is a narrow winding passage which derives its name from the stooped and cramped position one must assume while passing through it. This extremely narrow channel was formed by a cave stream cutting down through a narrow joint in the limestone. Someone was so glad to get through this passage that he named the large chamber beyond Great Relief Hall.

Lower in the cave signs of water still at work can be seen. First, one comes to Dead Sea, an isolated pool of water which has an underground connection with Echo River to keep the pool supplied with water. The River Styx, another part of the Echo River system, is sometimes isolated but at times connected with Echo River, when water level in Green River is high and backs up into the cave. Most famous of the underground water is Echo River itself, several miles long and forming at present the main underground stream of the cave. Hydrologic studies suggest that Green River at high stages usually does not flow into the cave at Echo River but rather prevents Echo River from flowing out, thus ponding water underground and preventing visitors from entering lower parts of the cave.

By means of a short boat ride across the river, famous for its blind fish, the visitor reaches another of the main solutional channels, Silliman's Avenue. Here for the first time, the group sees some of the famous gypsum "flowers" in this dry part of the cave. Farther on in this same passage is another of the large "rooms" formed by the solution of limestone along cross-cutting fissures. Since the great Norwegian violinist, Ole Bull, gave a concert here in 1851, this chamber has been dubbed Ole Bull's Concert Hall. The end of Silliman's Avenue is marked by Mary's Vineyard, an interesting display of grape-like formations in the limestone deposits on the cave ceiling. These clusters are formed when drops of water carrying calcium carbonate are suspended for considerable time on the ceiling, allowing most of the solution to harden there.

A sharp turn to the right brings the visitor to a most welcome sight, Snowball Room. Here the group stops for lunch in surroundings which are not duplicated in any other dining room. "Snowballs" which cover the ceilings are gypsum "blisters" formed as the mineral is pushed outward into the cave by more gypsum forming behind.

After lunch the party proceeds to the narrow, steep-walled Boone's Avenue, a good example of one of the cave passages formed by water working its way down along a joint. Through the winding channels, past most fantastic gypsum crystals and needles in Kentucky Avenue, the party reaches Grand Central Station where five passages converge. Such a spot is formed by solution along intersection joints in the limestone plus stream erosion to widen and deepen the original channels. From this point on, the group comes into the upper cave levels which contain a fascinating variety of dripstone and flowstone. These deposits always bring forth exclamation of delight from all. This is the part of the cave where replenishment and growth of formations is best developed. Frozen Niagara, Drapery Room, and Onyx Colonnade make a fitting climax for this longest trip in Mammoth Cave. (For description of the last mentioned features, see Frozen Niagara Trip.)

Fig. 16. Map showing routes of All-day Trip and Scenic Trip. (click on image for an enlargement in a new window)

The Scenic Trip—Visitors have expressed much interest in a trip which is shorter than the All-Day Trip but still provides an opportunity to lunch in Snowball Room. In order to answer this need, the park service has set up a four and one-half hour trip appropriately called the Scenic Trip. Entrance is made through one of the man made openings, Carmichael Entrance. Suitable points for these man made openings were located by study of the geological conditions both on the surface and in the cave, and by making corings and soundings in order to determine those spots nearest the surface.

A short way inside the entrance a rockfall known as Rocky Mountains is seen. This is again evidence that the cave passages have been partially enlarged through the process of collapse and sapping. Soon the group sees the first of the wide variety of gypsum "flowers" developed so well in the dry part of the cave along Cleveland Avenue and some of the shorter passages leading from it. In Specimen Avenue and Florist's Garden are gypsum formations resembling roses, sunflowers, and other plants. Snowball Room is observed shortly thereafter, with its famous gypsum blisters resembling snowballs. Here the group pauses for lunch before proceeding down Boone's Avenue. From Snowball Room on, this Scenic Trip route merges with that of the All-Day Trip and the same features are seen.

Fig. 17. Snowball Room in Mammoth Cave. The "snowballs" were formed by layers of gypsum forced outward by the crystallization of gypsum in a porous limestone beneath. As growth continues such blisters often break open to form "flowers" (see Plate 4, Fig. 2). Gypsum is restricted to the drier parts of the cave. Lunch is served in this room for visitors on the All-day Trip and the Scenic Trip. It is 267 feet below the surface of the ground. Photo by Ray Scott, National Park Concessions, Inc.

Brief mention might be made of certain features which will be seen and which have not been previously described in the All-Day Trip. Such features would include two more rockfalls, one of which is called Mt. McKinley. Another is Grand Canyon, where the floor of one passage dropped down through the ceiling of another, thus producing a deep canyon. Proceeding onward, the group comes to Aero Bridge Canyon, so named because there used to be a cable car operating here to carry visitors from one side of this passage to the other.

Beyond Grand Central Station, past another rockfall known as Big Break, the visitors see the long broad expanse of Smooth Ceiling. Here one also observes the rock called Compass Needle, so named because numerous tests with a compass show this rock, which is shaped like a finger, points directly north. Farther on, the group passes Lover's Leap Canyon. No one has ever leaped into the canyon formed here, but a large number of fallen blocks of rock are visible in the canyon. From here the visitor ascends to the upper levels of the cave where dripstone and flowstone formations are so well developed.

Emerging from the cave into the outer air and light, the visitor may be startled by the sudden change in landscape and scenery, but he will never forget entirely the impressions of the cave trips he may have chosen through these "caverns measureless to man."

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Last Updated: 18-Jan-2007