Page:Popular Science Monthly Volume 25.djvu/613

Rh low country the Missouri here possesses are generally to be found on the north or left bank of that river (see Diagram No. I).

Without attempting a description of those strange and interesting mauvaises terres, which are the favorite theme of popular writers, I shall endeavor to give some idea of the process by which the valleys of these rivers have been formed and of the action of the rivers within their present bed. It is quite evident that the entire configuration of the land-surface of the region has been the result of erosion, and distinct breaks or even low cliffs sometimes occur, showing the edges of the horizontal strata. At intervals of from five to ten miles small streams or creeks fall into the river, often entirely dry in summer, sometimes containing a small quantity of perfectly transparent water, but so charged with alkali as to whiten the pebbles over which it flows, and to render its use by man or beast almost impossible. These creeks, locally denominated coulées—a name given them by the early French explorers—have excavated valleys of different lengths and widths, and between these occur narrow plains, or even mere ridges. Of the immense volume of solid earth and rock that has been brought down by the process of eroding these terraces, creek-valleys, etc., only a minute fraction has been retained, but this has been deposited near the river, forming an alluvial bottom of varying width. This alluvial deposit it is the function of the river perpetually to wear away, while at the same time laying down new matter, with which it is constantly charged, to take its place. The result is, that throughout the lower portions of these rivers, and also in the Missouri Yalley below their junction, the bed of the river is perpetually shifting its position in the general valley. When we contemplate the entire history of the river, the valley must be regarded as due to this process, and its great width relatively to that of the stream itself can only thus be accounted for. But, if we contemplate it only at a given time, as the present, the valley appears to consist of two quite distinct parts, viz., the riverbed and the valley proper, raised above it and gradually sloping back on one or both sides to the foot of the first terrace. If, in time of low water, we compare these two parts, the latter will appear to be stable, while the former will clearly show that it is unstable. There was probably never a time in the river's history when these two distinct features did not exist much as now, though no one can say how many times the river may have worn away the stable portion of its valley on one side while it was forming anew on the other, and afterward receded and carried off the last-formed valley, leaving its previous bed to be again filled up until it has regained all the aspect of permanence which it previously possessed. This crossing and recrossing by the river-bed of the general valley, proceeding simultaneously with the work of lateral erosion, have gradually lowered the valley to its present position and are still lowering it. In a certain sense this applies to all rivers and river-valleys, but nowhere perhaps