Mineral Guide > How the Earth was Formed > Caverns

Caverns

There are many hollows in the crust of the earth, and various names have been used to describe them. A "is a cavity held by the robbers or wild beasts. A "is a hiding place for hermits or for prisoners ; while a" cellar " is for storing provisions, or wines, and occasionally for raising mushrooms. A subterranean chapel, or burial-place, is called " a crept; " and a series of crypts is a" catacomb." The word " grotto " has the same etymology as " crypt," but carries the idea of natural or artificial decoration; hence " grotesque " means what is like the objects found in grottoes. Men open quarries, mines, wells and tunnels for practical uses. The Latin words from which we get " cave " and `' cavern " simply mean holes in the ground. But the Greek word for cave, " antron," was quite fanciful, meaning literally, " a breathing-place; " as if caverns were the nostrils whereby the Earth fills and empties her mighty lungs. One sees the force of this latter name on encountering the gale that almost always blows out from any large cavern.

Many who are familiar with surface geography fail to realize that there is, below the surface, a vast sunless realm, with hills and valleys, pits and plains, lakes, rivers, and cataracts: a subterranean world, whose contents, inhabitants and history teach some very curious and interesting lessons, and whose scenery is pretty, magnificent, or frightful, as the case may be. The science of caverns is now recognized by a vigorous "Society of Speleology," at Paris; and the magazine of that society, " Spelunca," is the only periodical exclusively devoted to the latest news from the underground world.

The question naturally arises as to the origin of caverns. The causes of their existence are various. Some of them are made by the agency of volcanoes, earthquakes, or other convulsions of nature. But most of them are made by the action of water. Marine caves are found wherever the billows have long pounded rocky cliffs with their liquid hammers, aided by the pebbles and boulders they have tossed against them. Thus the granite crags along the sea-coast have been gashed by arches, bored by natural tunnels, and honey-combed by grottoes. Few marine caves go deeper than daylight; the reason being that the stormy sea so soon destroys its own work. Hence those who seek vast caverns, amid whose winding avenues and vaulted rooms they can roam for miles, must visit regions remote from the ocean.

The largest of known caverns are found in limestone, and are due to the mechanical and chemical action of rain-water. This may be gentle and gradual, as in the case of the raindrops washing the softer material from cracks and seams in exposed ledges; or the falling water may be tumultuous, as at Niagara, and other cataracts, where ponderous rocks are rent asunder by its irresistible blows. Then, again, all rocks are porous, filtering the water through to form springs, large reservoirs, under-ground rivers, and finally caverns. The water is aided by the carbonic acid gas gathered from the atmosphere and soil, and which attacks limestone with energy, disintegrating or dissolving it, so that it may be carried away. When the subterranean water sinks to a lower level, or finds a new outlet, the stream-swept chasm becomes a cave, with rooms and galleries. Sometimes these are extremely dry; but usually the water continues to filter down through the rocks, hanging on the roof in countless drops, or spattering on the floor. The hanging drops, on evaporation, deposit tiny rings of carbonate of lime, which thicken into elongated cones called " stalactites." Those that fall on the floor deposit, not rings, but films, as thin as tissue-paper, laid film on film till thus the blunt "stalagmite" grows. This two-fold process often goes on till the stalactite and stalagmite meet to make a symmetrical pillar. Frequently, on a sloping roof, ridges take the place of rings, the result being alabaster curtains, some of them thick and brown, others thin and white, and others again delicately shaded, or striated, like onyx -a term commercially applied to the variety of stalagmite known as " Mexican onyx." The term "dripstone" is conveniently used for all these formations.

There has been much speculation as to the age of our largest American caverns. Guesses are almost useless in regions broken up by earthquakes, or even less violent vibrations. But in the Ohio valley there is a vast cavernous area, covering fully eight thousand square miles, and that has been comparatively free from disturbance. The sandstones and limestones of this region are none of them more recent than the Carboniferous Period, and they are very homogeneous in structure, resting in beds nearly horizontal. Hence the geological lesson is easy. This was once an immense plain slowly and gently uplifted from the primeval ocean, amid whose waters it stood as an island. Meanwhile it was worn by the water into hills called " knobs," and valleys known as " sink-holes." Some knobs are five hundred feet high, but all their strata are level, proving that they were made by erosion, instead of by mountain-making methods. The " sink-holes " are of every imaginable shape, having no outlet except by pits or crevices leading down to caverns, which are the gathering-beds for all the water of the region, whence it flows out again to feed a few large open rivers. So vast is this system of undermining that the united length of its passage-ways would make a tunnel going four times around the globe, were they placed end to end. Millions on millions of cubic yards of limestone have been first dissolved, then carried away in excavating these avenues. Hence the time required for making the great caverns of Kentucky, Indiana and Tennessee can only be measured by geological terms. They probably date back to the Tertiary Period.

Then there is the filling-up of caves, as well as their excavation, that figures in estimating their antiquity. The rate at which dripstone grows is by no means uniform. According to a long series of experiments and observations made by the writer it varies from an inch in a year to an inch in a century; the usual rate being about an inch in ten years. Geologists now agree that we cannot accurately tell the age of stalagmitic strata unless we know the conditions of formation. Ancient and modern growths strangely blend in the caverns of Luray, Vixginia ; where the first set were broken down and partly dissolved by an inundation of hot acidulated water, after which a new set grew to great size on the ruins.

In another Virginian grotto we saw a barricade of five huge fallen stalactites, each three feet thick, while a sixth grew on the spot where they had successively hung. Such results could be attained only by a process going on for ages.

In most caverns are signs of occupancy by men and animals. Some of these are in plain sight, but the most interesting remains are imbedded in clay-banks, or buried under rubbish and drip-stone. Visitors to the museums of Great Britain and continental Europe are impressed by the rich displays of skulls, tools, weapons and ornaments exhumed from caverns. The writer visited a grotto in Lozere whence three hundred human skeletons had been taken. Elsewhere he saw bones and arrow-heads under stalagmite four feet thick. From the cave of Gailenreuth were taken the remains of eight hundred bears, and one thousand were found in a single cave of Ojcow, in Poland. It is astonishing how much has been learned from such localities concerning prehistoric man and the associated animals.

Similar cave-hunting in America is more recent, but is now being pushed with vigor by our scientific associations and institutions. Mr. J. W. Corwith, of Chicago, generously fitted out an expedition for exploring the hill-caves of Yucatan, directed by the department of Archaeology in the University of Pennsylvania. The report of their adventures and discoveries is as thrilling as any romance. The exhibit of prehistoric relics from the cave-dwellings and rock-shelters of Arizona and Colorado delighted thousands of visitors to the World's Fair. Among fossil remains from the caves of Virginia, Pennsylvania, Kentucky, Indiana, Missouri and Tennessee, are extinct species, such as the mastodon, megalonyx, megatherium, and other creatures not now found in North America. But thus far no bones of palaeolithic man have been drawn from their depths to match the Calaveras skull of California, and the "glacial men" of New Jersey.
The living animals in our caves are peculiar. Some of them, as for instance the myriads of bats that flit in and out, are casual visitors. But the true subterranean species represent the infusorians, worms, crustaceans, spiders, vertebrates and mollusks. Many of them might seem insignificant to the careless eye; yet few objects have received more attention from men of science, who have observed their habits, dissected them, and microscopically examined their minutest particles, hoping to solve the question of their origin, and of their lessons as to evolution and natural selection. Cave fish, crawfish, crickets, spiders, myriapods, flies, fleas, and other forms of life, are as a rule white or translucent. Most of them are scavengers, living on decayed animal and vegetable matter swept in by underground streams. Most of them are blind, and some are literally " eyeless," having lost every vestige of organs of vision. By compensation some of them have long hairs projecting in all directions ; others have ludicrously lengthy legs; while others have attenuated feelers ten times as long as their bodies. The blind crawfish darts away from food that would be eagerly taken by one having eves, and then waves its long feelers as if for more information, and finally takes it with the utmost caution. The form of blind fish best known is the "Amblyopsis," varying in size from two to six inches ; colorless, with cartilage instead of bones, with no scales, and bringing forth its young alive, instead of laying eggs as most fishes do. Naturalists generally regard cave animals as survivors of the Quaternary Period, though some claim them as related to forms of deep-sea life, or even as modifications of the surface life of the region.

The majority of persons who visit caverns do so on account of the novel experiences or the unique scenery. The Mammoth Cave of Kentucky stands first among caverns remarkable for size and grandeur. The Colossal Cave, and others in its vicinity, are also of very great extent, though less widely known. The Wyandot Cave, of Indiana, rivals them in the magnitude of its chambers. Although comparatively barren of stalactites, these great caves boast of what might be called gardens of cave-flowers, that mimic every blossom of the conservatory, but really are curved crystals of gypsum. The Nicojack Cave, entered by an imposing gateway at the foot of the Raccoon Mountain, in Tennessee, is of vast dimensions, but its decorations are few and its winding passages perilous. The Marenga Cave of Indiana, and the Marble Cave of Missouri, as well as the Luray Cave and Grottoes of Shendun, in Virginia, are famous for splendid stalactites. The display at the World's Fair, from the Mammoth Crystal Cave, of the Black Hills, was much admired.

The writer himself has visited nearly three hundred caves in this and other lands. His latest trip was in 1897, with Messrs. Martel, Vire, and others, amid the " Causses " of southern France. The region was reached by a wild voyage in canoes down the gorges of the Tarn. Our wagon, with an outfit of provisions, tools, tents, rope-ladders, folding canvas boat, etc., made the peasants think we ran a traveling circus. To determine the channel of underground rivers we tinged them with aniline dyes and waited for the stained water to bubble up at the foot of tall cliffs. Inhabited cliffdwellings and rock-shelters for flocks were examined. Several caves were seen that have been described by the Societe de Speleologie. Finally, we found a new one of enormous depth, named " Aven-Armand," in honor of the first man who went down our swaying rope-ladder to the bottom. Armand was followed by others, who communicated with their helpers on the surface by means of a telephone. The cave has two shafts, one below the other, with a sloping way between, the total depth exceeding six hundred feet; and the array of stalactites is wonderful.

The most famous of all underground streams is the " Echo River," of Mammoth Cave, whose archway has a span of thirty feet or more, and whose width varies from twenty to two hundred feet, while its length exceeds half a mile. What is termed an " echo " is really a musical prolongation of sound for many minutes after the original impulse is given. River hall has a peculiar keynote that is known to the guides, who thus create remarkable chords. The best effects are had by simply rocking the boat and striking the water with the paddle. The concert that follows lasts for possibly half an hour, and is a medley of sweet bell-like tones, as if a myriad silver, iron, and golden bells had been rung by invisible gnomes, mingled with occasional shouts and shrieks.

Thus there is ample room for those who are ambitious for fame as cave-hunters, whether in the Old World or the New. Hundreds of known caverns are but imperfectly explored, and others nameless or unknown await their discoverers.