MAMMOTH CAVE, a cave in Edmonson county, central Kentucky, U.S.A., 14' N. lat. and 86° 12' W. longitude. It is a distinct part of an extensive system of caves in the soluble St. Louis limestone, overlain by the Chester (Mauch Chunk) sand stone both of the Mississippian or Lower Carboniferous series. The area of the cave-bearing formation is over 8,000 sq.m. in southern Indiana, through central Kentucky and into northern Tennessee. The cave is said to have been discovered in 1809, when a hunter named Hutchins is reported as having pursued a bear into its entrance; but it must have been known earlier, for its entrance was designated in the county records of 1797. Readily accessible to the majority of the Eastern States it is visited by many tourists.
The St. Louis limestone throughout the cave-bearing area is massive and homogeneous, lying almost horizontal and showing few traces of tectonic or structural deformation. Owing to its relative purity and consequent solubility, it has been carved, chiefly since the Miocene period, by underground waters perco lating and flowing along its joint and fracture planes, into a great series of caves. Where its cap of Chester sandstone has given way it has been perforated by hundreds of "sink-holes," more or less funnel-shaped depressions distinctive of the landscape of the region, and interrupted by scarp-rimmed valleys with little or no relation to surface erosion. The depth to which the cavern has been cut has been determined by the level of Green river, to which the Mammoth cave system is tributary by subterranean passages opening along its banks. As Green river graved itself deeper into its bed, the dissolving and eroding waters passing through the limestone were enabled to proceed with their carving deeper and deeper. The thickness of the St. Louis limestone approaches and in places exceeds a thickness of 30o ft. The sec tions of the cave ordinarily traversed have been surveyed by civil engineers and geologists; but owing to insurmountable difficulties, many avenues are still unexplored. The temperature of most of the cavern and its passages is fairly uniform at 54° F. Just within the entrance a noticeable draught sweeps, outward for most of the year when the exterior air is the warmer, inward occasionally in winter when the exterior air is the colder. The upper galleries are dry; the lower damp owing to streams and pools, and the air is pure and wholesome.
The entrance is 118 ft. below the summit of a limestone bluff and 194 ft. above the level of Green river, but a half mile dis tant. The arch at the entrance has a span of 7o feet. The vesti
bule within the entrance rapidly contracts 30o ft. within to a passage called the Narrows where a gateway has been built. A short distance beyond the Narrows the passage opens upon the Rotunda, the first notable chamber of the main cave. It was in the Rotunda where during the War of 1812 and at other times nitre was prepared for powder, from the guano, chiefly the excre ment of myriad bats. By crude processes calcium nitrate worth $20,000 was obtained in 1914, when the industry reached its peak. The main cave is from 4o to 30o ft. wide and from 35 to 125 ft. high. It extends through the Rotunda, thence by Star Chamber, Chief City and minor chambers connected by long passages or narrow defiles to its rather abrupt termination 4 m. from the en trance. Though the entire cave extends under an area but 1 o m. in diameter, the main cave and the accessible tributary passages with their domes and chambers on its five different levels aggregate a length of at least 150 miles. The extent charted includes 225 avenues, 47 domes, 23 pits, eight cataracts, three rivers, two lakes and one sea. Streams and pools contribute to the majesty of the cave. They are navigable from May to October when Green river, with which they are connected, subsides. The Dead sea is a pool walled by cliffs 6o ft. high and in length ioo ft., along which a pathway runs to a stairway leading downward to the River Styx, a body of water 4o ft. long, crossed by a natural bridge. Lake Lethe, in a broad basin with mural cliffs 90 ft. high, becomes shallow and turbid at times. Many blind fish have been taken from it. A narrow path along Lake Lethe leads to a pon toon at the neck of the lake, and beyond it, a beach of fine yellow sand to Echo river, a stream or pool* m. long, 20 to 200 ft. wide and io to 4o ft. deep, with a symmetrical arched roof, varying in height from 19 to 35 feet. It is famous for the resonance of the tones given out by its vibrant stone which reverberate for from 10 to 3o sec. along its vaulted gallery. Other streams and pools, some of them even miles in length, occupy some chambers and galleries. They are fed by surface waters, which in the rainy season percolate and cascade into the cave in great volume, and collect in River Hall. For about seven months of the year these streams are unnavigable. When Green river is in freshet the waters in the cave become connected, sometimes rising 6o ft. above low-water mark.