FORMATION OF SEDIMENTARY ROCKS AND CLAYS.
Erosin and Transportation.—§ 21. As stated above, rain which falls upon the surface is divided into three portions ; one part evaporates ; one sinks into the ground; and one slides off the surface into the nearest stream or drainage channel. Water, like any other body sliding down an inclined plane, develops energy, and this energy enables it to pick up and carry obstructions, not too heavy, which it finds in its path. The law which governs the carrying power of water is that its carrying power varies as the sixth power of the velocity, i. e., if a current moving at a given rate is able to carry particles weighing one ounce, another current moving with double that velocity will carry stones weighing 64 ounces. This law makes it clear that water flowing over loose material will pick up portions which lie on its bed and carry them away. Rain falling on the surface of crystalline rocks would attack the more easily decompos able and convert them into earthy material, thus disintegrating the rock and covering it with a loose layer made up of clay, hydroxide of iron and other non-aluminous earthy matters, and grains of undecomposed minerals such as quartz, the less readily decomposable silicates, and the more resistant crystals of feldspar and other aluminous minerals. A portion of the next shower that falls will run off the surface and will carry with it more or less of this disintegrated material; the amount depending on the velocity. If the ground slopes sufficiently and there is no obstruction to the run-off, the loose material will be carried away as fast as it is formed, but if the flow is in any way obstructed, the granular matter will accumulate and the surface will soon be covered with vegeta tion whose roots help to bind the particles. Where fallen leaves and stems protect the surface they interfere with the action of running water and so prevent removal. In mountain regions on steep forested slopes the residual clays have often accumulated to a depth of 30 to 100 ft. In such places the grains at the surface are continually moving down the slope but so long as the forest covering remains, this action is so slow that new material is manufactured at the bottom faster than that on the surface is removed. So soon, however, as the forest covering is removed, rapid erosion sets i n and the accumulated material is speedily carried away.
When these surface-waters with their load of debris are gathered into a stream each hard grain becomes a tool with which the stream tears up its bed. Each undecomposed fragment as it is carried down the slope
acquires energy with which it strikes effective blows upon those portions of the stream-bed or banks which resist its progress, and in this way it loosens fresh fragments which are soon added to the load carried by the stream. It should be remembered that the fragments so added are not decomposed, but may be called silt or rock flour.
Transported Clays.—§ 22. Whenever the slope of the stream-bed is lessened the water loses velocity and consequently carrying power. It is thus compelled to deposit the larger particles in its load, and these accumulate to form beds of gravel or sand according to the velocity which the stream still retains, but the finer portions are carried on until some further reduction of velocity compels the stream to drop them also. In this way running water gathers the residual clays and other products of decomposition from the often widely separated deposits in which they were formed, mixes their ingredients, carries them to points more or less distant, assorts them and deposits the coarser or heavier grains in beds of sand or gravel while with the finer portions it builds up beds of transported clay.
If in the places where these clays are deposited, the current remains practically constant for long periods, thick beds of clay nearly or quite uniform in composition and texture will result; but as every heavy rain which falls on any part of the area drained by the stream increases its volume and consequently its velocity, the places where it deposits its clay may change frequently, and consequently the deposits at a given point may form alternating layers of coarser or finer material, fine clay, coarse clay, sands or gravels, the thickness of the individual layers being governed by the amount of sediment the stream was carrying and the length of time during which the velocity remained constant. Heavy deposits of transported clays, except as noted below, usually represent old lake beds or ponds, into which the clays are brought by streams. The clay brought by one of these streams will usually be uniform in com position, with varying texture, but that brought by different streams may be different if their drainage basins do not lie on the same kinds of rock. Consequently the deposits in such lake beds are likely to he more or less in pockets.