To those who love the tang of adventure in strange and untrodden places there is no experience, nowadays, that compares with opening up and exploring caverns. To find a mountain that has never been ascended, or a region on the earth's surface that has never been mapped, one must make long journeys and spend a fortune. Caves may be found wherever there are thick beds of permeable limestone with sink-holes on the surface, or other evidence of subterranean water-courses. The descent calls for no costly equipment. It may be made at any season of the year. The trip will take only a day or two. And cave exploration is a sport that yields quick results: the moment you get underground you are face to face with the unknown.
Yes, there may be nothing new under the sun, but under the earth nearly everything is new. It is safe to say that not one per cent of the subterranean passages in our limestone regions have been explored. In Kentucky alone, according to Professor Shaler, there are at least 100,000 miles of caverns that have not filled up. A similar honeycombed formation extends over large parts of Indiana, Illinois, Missouri, Arkansas, and Tennessee. Superb caverns have been found in the Shenandoah Valley of Virginia, in the Black Hills of South Dakota, and in other parts of our country. Very few, even of our best-known caves, have yet beer? completely explored. There are hundreds, and per haps thousands, that no man ever has entered. The} are sealed or masked from our observation, and yet have left marks by which their existence can be proved.
The surface indications of a cavernous region are easy to read. Take the Ozark Plateau, for example. Anyone traveling crosscountry from the Missouri River toward Arkansas will notice that the surface rock mostly is limestone and that it is commonly porous or fissured, being easily "eaten" by the elements. Often he will observe what geologists call vermicular limestone, full of little holes like those that earthworms bore in the soil, or like what "sawyers" bore in pine timber. He will cross some fine rivers, generally very clear, but will marvel at the almost total absence of brooks and spring branches; this even in a country that is distinctly mountainous.
In summer one may travel sometimes for a day in the Ozarks without finding running water. He may come to the perfectly dry bed of a water-course that evidently drains a considerable territory, and his driver will tell him that this "dry fork" carries surface water only for a short time after a heavy rainfall. The real drainage stream flows underground.
When a spring is met in this region it is likely to be a large one. A typical "big spring" boils out of a hillside and fills a crater-like basin sixty to a hundred feet in diameter. Its surface is blue as indigo. The water is so clear that, by immersing your face, you can see the white bottom forty or fifty feet below. The outlet is strong enough to turn a mill, and forms at once a creek navigable by canoes. Such are the St. James Spring on the Meramec, the Round or Blue Spring on the Current River, Brvce's Spring on the Niangua, and Mammoth Spring near the Missouri-Arkansas line.
On wide plateaus, where the drainage is not abrupt, our traveler will see numerous funnel-shaped depressions in the fields, into which surface water either disappears quickly after a rain, or collects in ponds, according to whether the vent of the "sink-hole" is open or has been closed. Often one comes to a place where the fields are fairly pock-marked with such holes.
All this tells a plain story. There are few small springs and brooks because the surface rock is so porous or fissured that rain almost immediately seeps through it to underground channels. The sink-holes are simply old cavern chambers with the roofs fallen in. Generally there are deeper chambers or galleries below them, into which the drainage flows if the sieve or tube in the funnel'? neck has not been closed by accident or design. The great springs are outlets of subterranean rivers.
Whenever the underground waters have eroded a channel at a lower level than that which drained the original gallery the latter is left dry and forms an extensive cavern that can be opened for exploration. This is provided that the old passages have not filled up again by a process that will be described hereafter.
Some of the caverns already known in the Ozarks are of noble dimensions. The Marble Cave, forty miles from Marionville, Mo., has been traversed for many miles, and to a depth of 400 feet below the surface. One of its vaulted chambers is 350 feet long, 125 feet wide, and 195 feet high, by actual measurement. Three miles away is the exquisitely beautiful Fairy Cave, which is entered through a sink-hole 100 feet deep. There are said to be over a hundred known caves in Stone County alone.
Crossing the Mississippi into southern Illinois, we find a cavernous limestone belt in comparatively level country. Near Burksville is a cavern that is said to have been explored fourteen miles one way and six miles in the opposite direction, without finding either end. It has a lake, and a river in which there are blind fish.
Southern Indiana has scores of caves that contain eyeless fish and crustaceans, beds of niter, epsom salts, great deposits of alabaster, and Indian relics. In the Wyandot Cave is a domed chamber 1,000 feet in circumference and 185 feet high, from the floor of which rises a pyramid to within 50 feet of the roof. In another vast hall is a symmetrical pillar 40 feet high and 75 feet in periphery, rising from a base that is 300 feet around, the whole mass being solid, homogeneous alabaster as white as snow.
The finest cavern district in the world is about the head waters of the Green River, in Kentucky. Here the limestones have a depth of several hundred feet, and hence are peculiarly favorable for the formation of stupendous caverns. Edmonston County by itself has some five hundred caves, one of which, the Mammoth Cave, is certainly the largest that has yet been discovered on the globe. Within a section of about ten square miles, and a thickness of 300 feet, where this gigantic cavern is centered, there are probably more than 200 miles of galleries large enough to permit, the passage of a man. The "Long Route" for visitors in the Mammoth Cave, which is mostly quite smooth and easy, takes eight or nine hours of steady walking at an average pace of two miles an hour. One of the domes is 300 feet high. The Mammoth Dome is about 400 feet in length, 150 feet in width, and from 80 to 250 feet high, according to position.