Geology, Vol. 1 [of 3]

by Thomas C. Chamberlin; Rollin D. Salisbury

The Influence of Rock.

The physical constitution, the chemical composition, and the stratigraphy of a rock formation, influence the rate at which it may be broken up and carried away. Clastic or fragmental rocks are usually stratified and made up of cemented pebbles (conglomerate), sand grains (sandstone), or particles of mud (shale). Igneous rocks, such as granite, are massive instead of stratified, and are usually made up of great numbers of interlocking crystals which bind one another together. Some crystalline rocks, such as schists, though not stratified, possess cleavage, which has much the effect of stratification, so far as erosion is concerned. All rocks are affected by systems of more or less nearly vertical cracks called joints. All these structures have their influence upon the rate of degradation.

Physical constitution.—Clastic rocks may be firmly cemented, or their constituents may be loosely bound together. The less the coherence the more ready the disintegration, and the finer the particles the more easily are they carried away. When the particles in transportation are angular they effect more wear on the bed over which they move, and on one another, than when they are round. The difference is great where the particles are large, and little where they are very small. If the materials carried be harder than the bed over which they pass, corrasion of the latter is favored.

Chemical composition.—Something also depends on the chemical composition of the rock, since this affects its solubility, and therefore its rate of decomposition. The more soluble the rock the larger the proportion of it which will be taken away in solution; but it does not follow that the most soluble rock will be most rapidly eroded, since the rate of erosion depends on abrasion as well as solution, and a rock which is readily soluble, as rocks go, may be less easily abraded than a rock which is made of discrete and insoluble particles bound together by a soluble cement. In such rocks, for example a sandstone in which the grains are cemented together by lime carbonate, the solution of the cement sets free a considerable quantity of sand, so that a small amount of solution prepares a large amount of sediment for removal. A stream might cut its valley much more rapidly in such a sandstone than in a compact limestone, though the latter is, as a whole, the more soluble. The constituent minerals of crystalline rocks resist solution and decay unequally, and when any one is dissolved or decomposed the rock crumbles and the less soluble constituents are ready for removal by mechanical means. So long as the material loosened by disintegration is removed, chemical heterogeneity favors erosion; but if the loosened débris is not removed erosion is not favored by chemical heterogeneity. In such a case erosion would be most rapid where the rock was most soluble.

Structure.—The structure of the rock has much to do with the rate of its erosion. Other things being equal, stratified rock is more readily eroded than massive rock, since stratification-planes are planes of cleavage, and therefore of weakness. Taking advantage of these planes the water has less breaking to perform to reduce the material to a transportable condition. For the same reason a thin-bedded formation is more easily eroded than a thick-bedded one.

Fig. 103 and 104.—Diagrams to illustrate the fact that a stream crosses many more cleavage-planes when the beds of rock are inclined than when they are horizontal.

The beds of stratified rock may be horizontal, vertical, or inclined, and inclined strata may stand at any angle between horizontality and verticality. In indurated formations the rate of erosion is influenced both by the position of the strata and by the relation of the direction of the flowing water to their dip and strike. On the whole the strata which are horizontal, or but slightly inclined, are probably less favorable for rapid erosion than those which are vertical or inclined at considerable angles. This is at least true where the layers are of uniform hardness and the joints infrequent.

Horizontal strata expose fewer cleavage planes to the water flowing over them than strata in any other position. In [Fig. 103] the stream which has the profile ad crosses bedding-planes at b and c. In [Fig. 104], where the beds dip up-stream, many more division-planes are crossed in the same distance. Since bedding-planes are planes of weakness, it follows that horizontal and nearly horizontal strata are not, under ordinary conditions of erosion, in a position favorable for most rapid wear. When strata are horizontal, it makes no difference which way the stream runs, for the current sustains the same relation to the cleavage-planes whatever its course.

In the case of incoherent material the position of the beds, or even their existence, has little influence on the rate of erosion. Such formations are weak in all directions, not simply along bedding-planes.