A Short Lesson In Geology and Chemistry
Many hurry to and through and away from Mammoth Cave; but let us go in a more leisurely manner. Suppose we begin by a stroll amid the rounded hills that environ Cincinnati. We find their flanks full of corals, shells, crinoids, and other marine objects by myriads. These are fossils, yet perfect as if freshly cast up from the sea. But we observe that the limestone lies in thin, level layers, with no signs of volcanic or earthquake action. They were gently cut down by an undermining process that left no caverns, because the strata are so thin that they can not hold together. This is the same Lower Silurian formation that elsewhere made the famous “bluegrass region,” causing Central Kentucky to be the fairest bit of the globe’s surface known.
Go by steamboat down the Ohio to Madison, Indiana, and the scenery changes with the geology. Near the river are still seen the thin blue limestone strata that we saw at Cincinnati, but capped by the marble heights of the Upper Silurian. Cascades from the cliffs wash out the thinner, softer material, making wide, shallow grottoes, each being, as a rule, at the head of a ravine, which is a cave in ruins.
At the charming city of Louisville we encounter another geological change, and meet a striking proof that the region was once flooded by the ocean, namely, the grand old coral reef over which tumble the Falls of the Ohio. It used to bristle with branching corals like stag-horns and was strewn with tens of thousands of more delicate varieties, car-loads of which have since been carried away; but enough remain to show that all this country was uplifted by continental forces from a primeval sea. Probably its altitude was once above the present level, to which it has been reduced by causes some of which are still at work.
Rambling through the valleys and examining their rocky beds, we find fissures no doubt caused by that continental uplifting to which we have referred. These cracks, or “joints,” are visible over large areas, wherever the country rock is exposed. Usually they run at nearly right angles with one another, north and south lines crossing those from east to west. The joint-walls may closely fit, or have been parted to make channels by which falling rain might be drained.
You have noticed that soda-water roughens and eats away the marble slab on which the soda-fountain rests. On asking the reason you are told that it is due to the carbonic acid gas (carbon dioxide) with which the water is charged. In nature this same gas is formed by the decay of animal and vegetable matter. Rainwater absorbs it from the atmosphere and while sinking through the loam and soil. It also takes up humous acids, which aid in the work of corrasion effected on reaching the limestone. Mechanical energy assists chemical action in slowly dissolving and removing the limestone particles.
All limestone caves, great and small, were carved by this slow yet irresistible process. The downward flow follows the joints till a lateral “bedding-plane,” or something else, turns the stream horizontally, when there results a widening of the passageway. Should the roof collapse there would be “a tumble-down” within and perhaps a “sink-hole” without. Should the cave cut through from one bedding-plane to another, a series of galleries would result; the upper ones dry as tinder and the lower ones wet with water that finally reaches the drainage level, whence it emerges into some open valley.
Occasionally the whirling water bores straight down through all galleries, making what is termed a pit, or a dome, according to the point of view. Standing pools deposit nitrous earth and various other mineral substances. Water trickling through the roof evaporates, each drop laying down its load of the bicarbonate of lime to create a stalactite; or a stalagmite if it first falls on the floor. A general and convenient term is “dripstone,” masses of which are found at almost any crossing of the joint-planes. Should “fixed air” (carbon dioxide), which is fifteen times as heavy as the atmosphere, settle into the lower parts of any cave, it would make visiting dangerous or fatal. But air currents and other causes make every part of Mammoth Cave free from any except the sweetest, purest air ever inhaled.