Earth Features and Their Meaning / An Introduction to Geology for the Student and the General Reader

by William Herbert Hobbs

Fig. 189.—Diagram to show how an impervious layer conducts the descending water in a lateral direction to issue in surface springs.

The trunk channels of descending water.—While within the unconsolidated rock materials near the surface of the earth, it is clear that water can circulate in proportion as the materials are porous and so relatively pervious. As the pore spaces become minute and capillary, the difficulty of permeation through the materials becomes very great. Thus in the noncoherent rocks it is the coarse gravel and the layers of sand which serve as the underground channels, while the fine clays have the effect of an impervious wall upon the circulating waters. In coarse sand as much as a third of the volume of the material is pore space for the absorption and transmission of water. Even under these favorable conditions the movement of the water is exceedingly slow and usually less than a fifth of a mile a year.

Fig. 190.—Sketch map of the Oucane de Chabrières near Chorges in the High Alps, to illustrate the corrosion of limestone along two series of vertical joints (after Martel).

Within the hard rocks it is the sandstones which have the largest pore spaces, but in nearly all consolidated rocks there are additional spaces along certain of the bedding planes, the joint openings ([Fig. 190]), and the crushed zones of displacement, so that these parting planes become the trunk channels, so to speak, of the circulating water. It is along such crevices that in the course of time the mineral matter carried in solution by the water is deposited to produce the ore veins and the associated crystallized minerals.

The caverns of limestones.—Where limestone formations have a nearly flat upper surface, a large part of the surface water enters the rock by way of the joint spaces, which it soon widens by solution into broad crevices with well-rounded shoulders. At joint intersections solution of the limestone is so favored that the water may here descend in a sort of vertical shaft until it meets a bedding plane extending laterally and offering more favorable conditions for corrosion. Its journey now begins in a lateral direction, and solution of the rock continuing, a tunnel may be etched out and extended until another joint is encountered which is favorable to its further descent into the formation. By this process on alternating shafts and galleries the water descends to near the surface of the water table by a series of steps, and is eventually discharged into the river system of the district ([Fig. 191]). Within the larger caverns the water at the lowest level usually flows as a subterranean river to emerge later into the light from beneath a rock arch.

Fig. 191.—Diagram to show the relation of caverns in limestone to the river system of the district and to the “swallow holes” upon the surface.

From the plan of a system of connecting caverns it may often be observed that the galleries of the several levels are alike directed along two rectangular directions which indicate the master joint directions within the limestone formation. This is especially clear from the map of the galleries in the explored portions of the Mammoth Cave ([Fig. 192]).

Swallow holes and limestone sinks.—Above the caverns of limestone formations there are selected points where the water has descended in the largest volume, and here funnel-shaped depressions have been dissolved out from the surface of the rock. In different districts such depressions have become known as “sinks”, “swallow holes”, entonnoirs, and Orgeln. Wherever the depressions have a characteristic circular outline, there can be little doubt that they are the product of solution by the descending water, and have relatively small connections only with the subterranean caverns. They have thus naturally collected upon their bottoms the insoluble clay which was contained in the impure limestone as well as a certain amount of slope wash from the surface. Inasmuch as the clays are impervious to water, the bottoms of these swallow holes are better supplied with moisture than the surrounding rock surfaces, and by nourishing a more vigorous plant growth are strongly impressed upon the landscape ([Fig. 193]).