Page:Popular Science Monthly Volume 25.djvu/616

600 At the deepest point, fresh erosion or corrosion is taking place, while the steep bank adjacent is being rapidly worn away (see Diagram No. III).

The features to be described can only be satisfactorily observed in time of low water. The bank above the river on the deep side is then generally very high, often rising perpendicularly twenty feet or more above the surface of the water. This high bank, thus exposed to the view of the navigator in the river, affords a most excellent opportunity of studying the manner in which the material composing the general valley has been deposited, the various agencies that combined to form the deposit, and the approximate time required for the accumulation of a given thickness of this alluvium.

These walls of loose earth are always very conspicuously stratified, the layers having various thicknesses and different colors. As many as a dozen distinct strata can usually be seen, often very definitely marked off from one another. The color of these layers enables the observer to determine, with considerable certainty in any case, whether it was due to a wash from the neighboring hills, whose color can be directly compared, or to a deposit from the river itself, brought in time of flood from points higher up, or, as is often the case, from vegetable, mold which long immunity from disturbance has allowed to accumulate. Some idea of the time occupied in the total deposit may be formed from the presence of forests of cottonwood (Populus monilifera, Ait.) which line the river. These trees are sometimes of great size, measuring three or four feet in diameter, and, although the cottonwood is a rapidly growing tree, there can be no doubt that many of the trees are two or three hundred years old. But the mere presence of these forests standing upon the surface of the latest stratum of the general valley is by no means the only time-measure we have. A careful observer, though merely walking among them, might perceive that some of them have their bases buried to some little depth with alluvial earth or vegetable mold. This fact, which would escape any one who was not specially looking for evidences of it, becomes striking when the edges of the strata are viewed from the river.

As the river wears away the previously formed deposits of its valley, it at length approaches the portion that has bad time to become covered with these forests. Undaunted, it attacks this portion also, and begins the work of felling the trees. Their roots are laid bare, the solid earth on which they have stood for ages is swept away, and one after another these ancient giants succumb to the rapacity of the waters, and fall powerless into the raging current. Every step in the process by which this result is accomplished may be seen by watching these eroded banks while floating down the stream. The river, as it passes one of these doomed forests, is choked with snags, through which the surging waters roar, and among which it is extremely difficult and often dangerous to guide a boat. These snags are of all