Fig. 2.—Detailed View of a Portion of Quarry showing Weathered Rock.
How shall the facts before us be explained? It has been shown that the dense rock and the loose material are the same mineralogically, and grade from one into the other, and it is certainly rational to suppose that the latter is merely a changed form of the first. Some force must have been at work on the solid rock, destroying its coherency and converting it into loose sand. If we inspect the powdered rock, it will become apparent that this change has been brought about mainly by the process of weathering: surface water, with its ever-present acid impurities, has brought about the partial decay of the pyroxene and mica and caused the disintegration of the upper part of the rock. Water has not only attacked the rock from the upper surface, but has penetrated to considerable depths along the joint planes, working inward toward the center of each block until the mass becomes completely disintegrated. This process explains the concentric shells about cores of unaltered rock, each representing original joint blocks, which are seen in the second photograph. All our excursions into the field will show that this is not an isolated case, for wherever a ledge is exposed to our view there will be found a zone of weathered rock, varying in thickness from mere films to many feet.
By this process the greatest part of the materials constituting soils is formed, and the flora and fauna of the earth are rendered possible. Upon such products of decay the food supply of running water manifestly depends in a large measure, as will be pointed out on our next excursion; and were the scope of this article somewhat larger, it would be easy to show that the rock decay seen in our photograph has taken place in a length of time measured by something like ten thousand years. If all rock decayed as easily, and if the rate of decomposition, as determined here, held good for great distances from the surface, mountains two miles in height would become a prey to the force of chemical action in six and a half million years. We can not, however, give a time equivalent for the destruction of a mountain range, since decay, and consequent disintegration, is only one of the many forces acting to sap the strength of solid rocks and to tear them asunder. The above figures are given merely to make plain that the time necessary to accomplish the leveling of a mountain chain is but a small part of the earth's existence as such, great as this period may seem from the standpoint of human history.
We shall, if possible, time the second excursion immediately after a heavy rain, and we shall select for our objective point a place where the rain water, in its efforts to reach a stream, is forced to run down some steep declivity. Under such circumstances, the carrying power of the water will be very great, and we shall hope to find evidence of its work in transporting the products of rock weathering and other material broken up by the action of frost. A little diligence will soon reward us with the evidence which we seek. A local inequality of the ground, perhaps only a few feet across, is found filled with water—a minute, temporary lake caused by the recent heavy rainfall. Such little water bodies are extremely common, but the accompanying geological phenomena are, notwithstanding, none the less interesting, and the conclusions to be drawn from the evidence thus presented are none the less valuable.
If we examine the pool critically, it will be noticed that its shore line is cut by a little channel along which the overflow makes its escape. Further investigation will show that at another point along the shore, especially if we are fortunate enough to visit the locality very soon after a rain, there is a small rivulet entering the pool; and also that the entering stream is discolored with mud and carries more or less sand, while the escaping stream is nearly clear, and is free from all traces of coarse, sandy material. It is therefore evident that the sediment brought in by the stream has been left behind in the pool, and of course will be found deposited at its bottom, and it will appear that the only explanation of the inability of the water further to transport its burden is to be found in the fact that water loses nearly all its motion, and therefore its transporting power, on entering a stagnant pool. These are elementary truths, but an amplification of such simple phenomena is often fully capable of accounting for the most stupendous results.
Fig. 3.—Temporary Wet-weather Delta.
Having made these observations, let us look at the form assumed by the sediment when it is forced to fall to the bottom. At the point where the stream enters the pool there is seen an accumulation of material having a nearly level upper surface, presenting a scalloped or lobe-shaped outer margin, upon which the stream may be seen flowing and entering the water at one of the lobes. Other channels, though unoccupied by water, also lead to similar lobes. If we watch closely, we may be able to witness the growth of this body of sand, called a delta, as the falling sediment rapidly increases the size of the lobe; and also to perceive that as soon as the lobe is built out considerably in advance of the main body of sand, it will be easier for the stream to enter the water on one side of the scallop, thus abandoning its old mouth. In this manner the stream moves from one place to another, successively building the little scallops and continually carving new channels for itself. Fig. 3 is a photograph of such a delta, some three feet across, taken after the water had been drained away, and reveals its form in a characteristic manner. As we watch its growth, it will become evident that only the coarsest material transported by the stream goes to make up the delta, and that the clay and finest sand are deposited farther away, where the water is more quiet, or else pass out in the stream draining the pool. Let us look about a little. Not far from our miniature lake there are several others. In some the size of the delta is much larger in proportion to the area of the pool than is the case with the one first studied. We find in some cases that the stream has progressively built its delta completely across the old water surface. Taking a thin piece of board or a large knife, we can easily cut vertically through this sand deposit, thus exposing what is called a geological section. The sand grains of which the deposit is largely composed are seen to be arranged in layers nearly horizontal, and these layers are found to be due to alternations of sediment varying in fineness. This phenomenon is called stratification, and is what we should expect of the action of gravity operating on material of different sizes and densities suspended in a body of water. It has been found inexpedient to attempt to show a photograph of this section, owing to the smallness of the subject, but the same phenomena may be observed on a much larger scale in Fig. 5, which will be described below.