Outlines of the Earth's History: A Popular Study in Physiography

by Nathaniel Southgate Shaler

In any limestone district where the beds of the material are thick and tolerably pure—as, for instance, in the cavern district of southern Kentucky—the traveller who enters the region notes at once that the usual small streams which in every region of considerable rainfall he is accustomed to see intersecting the surface of the country are entirely absent. In their place he notes everywhere pitlike depressions of bowl-shaped form, the sink holes to which we have already adverted. Through the openings in the bottom of these the rain waters descend into the depths of the earth. Although the most of these depressions have but small openings in their bottom, now and then one occurs with a vertical shaft sufficiently large to permit the explorer to descend into it, though he needs to be lowered down in the manner of a miner who is entering a shaft. In fact, the journey is nearly always one of some hazard; it should not be undertaken save with many precautions to insure safety.

When one is lowered away through an open sink hole, though the descent may at first be somewhat tortuous, the explorer soon finds himself swinging freely in the air, it may be at a point some hundred feet above the base of the bottle-shaped shaft or dome into which he has entered. Commonly the neck of the bottle is formed where the water has worked its way through a rather sandy limestone, a rock which was not readily dissolved by the water. In the pure and therefore easily cut limestone layers the cavity rapidly expands until the light of the lantern may not disclose its walls. Farther down there is apt to be a shelf composed of another impure limestone, which extends off near the middle of the shaft. If the explorer can land upon this shelf, he is sure to find that from this imperfect floor the cavern extends off in one or more horizontal galleries, which he may follow for a great distance until he comes to the point where there is again a well-like opening through the hard layer, with another dome-shaped base beneath. Returning to the main shaft, the explorer may continue his descent until he attains the base of this vertical section of the cave, where he is likely to find himself delivered in a pool of water of no great depth, the bottom of which is occupied by a quantity of small, hard stones of a flinty nature, which have evidently come from the upper parts of the cavern. The close observer will have noted that here and there in the limestone there are flinty bits, such as those which he finds in the pool. From the bottom of the dome a determined inquirer can often make his way along the galleries which lead from that level, though it may be after a journey of miles to the point where he emerges from the cavern on the banks of an open-air river.

Although a journey by way of the sink holes through a cavern system is to be commended for the reason that it is the course of the caverning waters, it is, on the whole, best to approach the cave through their exits along the banks of a stream or through the chance openings which are here and there made by the falling in of their roofs. One advantage of this cavity of entrance is that we can thus approach the cavern in times of heavy rain when the processes which lead to their construction are in full activity. Coming in this way to one of the domes formed beneath a sink hole, we may observe in rainy weather that the water falling down the deep shaft strikes the bottom with great force; in many of the Kentucky caves it falls from a greater height than Niagara. At such times the stones in the basin at the bottom of the shaft are vigorously whirled about, and in their motion they cut the rocks in the bottom of the basin—in fact, this cavity is a great pot hole, like those at the base of open-air cascades. It is now easy to interpret the general principles which determine the architecture of the cavern realm.

When it first enters the earth all the work which the water does in the initial steps of cavern formation is effected by solution. As the crevice enlarges and deepens, the stream acquires velocity, and begins to use the bits of hard rock in boring. It works downward in this way by the mixed mechanical and chemical action until it encounters a hard layer. Then the water creeps horizontally through the soft stratum, doing most of its work by solution, until it finds a crevice in the floor through which it can excavate farther in the downward direction; so it goes on in the manner of steps until it burrows channels to the open stream. In time the vertical fall under the sink hole will cut through the hard layer, when the water, abandoning the first line of exit, will develop another at a lower level, and so in time it comes about that there may be several stories of the cave, the lowest being the last to be excavated. Of the total work thus done, only a small part is accomplished by the falling of the water, acting through the boring action of its tools, the bits of stone before mentioned; the principal part of the task is done by the solvent action of the carbonated waters on the limestone. In the system of caverns known as the Mammoth Cave, in Kentucky, the writer has estimated that at least nine tenths of the stone was removed in the state of solution.

When first excavated, the chambers of a limestone cavern have little beauty to attract the eye. The curves of the walls are sometimes graceful, but the aspect of the chambers, though in a measure grand, is never charming. When, however, the waters have ceased to carve the openings, when they have been drained away by the formation of channels on a lower level, there commonly sets in a process known as stalactitization, which transforms the scene into one of singular beauty. We have already noted the fact that everywhere in ordinary rocks there are crevices through which water, moving under the pressure of the fluid which is above, may find its way slowly downward. In the limestone roofs of caverns, particularly in those of the upper story, this ooze of water passes through myriads of unseen fissures at a rate so slow that it often evaporates in the dry air without dropping to the floor. When it comes out of the rocks the water is charged with various salts of lime; when it evaporates it leaves the material behind on the roof. Where the outflow is so slight that the fluid does not gather into drops, it forms an incrustation of limy matter, which often gathers in beautiful flowerlike forms, or perhaps in the shape of a sheet of alabaster. Where drops are formed, a small, pendent cone grows downward from the ceiling, over which the water flows, and on which it evaporates. This cone grows slowly downward until it may attain the floor of the chamber, which has a height of thirty feet or more. If all the water does not evaporate, that which trickles off the apex of the cone, striking on the floor, is splashed out into a thin sheet, so that it evaporates in a speedy manner, lays down its limestone, and thus builds another and ruder cone, which grows upward toward that which is pendent above it. Finally, they grow together, enlarged by the process which constructed them, until a mighty column may be formed, sculptured as if by the hands of a fantastic architect.

Fig. 13.—Stalactites and stalagmites on roof and floor of a cavern. The arrows show the direction of the moving water.

All the while that subterranean streams are cutting the caverns downward the open-air rivers into which they discharge are deepening their beds, and thereby preparing for the construction of yet lower stories of caves. These open-air streams commonly flow in steep-sided, narrow valleys, which themselves were caves until the galleries became so wide that they could no longer support the roof. Thus we often find that for a certain distance the roof over a large stream has fallen in, so that the water flows in the open air. Then it will plunge under an arch and course, it may be, for some miles, before it again arrives at a place where the roof has disappeared, or perhaps attains a field occupied by rocks of another character, in which caverns were not formed. At places these old river caverns are abandoned by the streams, which find other courses. They form natural tunnels, which are not infrequently of considerable length. One such in southwestern Virginia has been made useful for a railway passing from one valley to another, thus sparing the expense of a costly excavation. Where the remnant of the arch is small, it is commonly known as a natural bridge, of which that in Rockbridge County, in Virginia, is a very noble example. Arches of this sort are not uncommon in many cavern countries; five such exist in Carter County, Kentucky, a district in the eastern part of that State which abounds in caverns, though none of them are of conspicuous height or beauty.[7]

At this stage of his studies on cavern work the student will readily conceive that, as the surface of the country overlying the cave is incessantly wearing down, the upper stories of the system are continually disappearing, while new ones are forming at the present drainage level of the country. In fact, the attentive eye can in such a district find here and there evidences of this progressive destruction. Not only do the caves wear out from above, but their roofs are constantly falling to their floors, a process which is greatly aided by the growth of stalactites. Forming in the crevices or joints between the stones, these rock growths sometimes prize off great blocks. In other cases the weight of the pendent stalactite drags the ill-supported masses of the roof to the floor. In this way a gallery originally a hundred feet below the surface may work its way upward to the light of day. The entrance by which the Mammoth Cave is approached appears to have been formed in this manner, and at several points in that system of caverns the effect of this action may be distinctly observed.