Page:Popular Science Monthly Volume 23.djvu/487

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brown sandstone, very unequally hardened and mixed with clay. Surmounting the whole is a bed of soft clay of varying thickness, mostly a sort of remnant, persisting only in mounds and the conical heaps already referred to.

PSM V23 D487 Diagrammatic section of a single butte.jpg
Fig. 1.—Diagrammatic Section of a Single Butte.

Now, all these strata, except the uppermost sandstone, have this common characteristic: they are composed of particles excessively fine, so that, if from any of the beds a little bit be taken, it may, when dry, be reduced between the fingers to an impalpable powder as fine as ashes. Even the lignite is no exception. If, now, in connection with this fact, and remembering the arrangement of the strata, we take into account the arid climate which prevails in all these regions, we are in position to understand much of the peculiar conformation of the Bad Lands. In the winter the snows are light, and in summer the rain that falls comes in sudden, violent, but short-lived storms. For perhaps half an hour after one of these storms, torrents flood the valleys and low plains between the hills, the rushing waters heavily charged with particles of clay, but particles so fine that they do not readily leave the water or become precipitated, but are borne on to the river, thence to the Missouri proper, which latter stream parts with them only as it blends in the clearer waters of the Mississippi. If, after the storm, we examine again the face of the bluff, we find it striated with numberless tiny channels, down which have just poured little rivulets of water hardly so much as wetting the surface, while from top to bottom the erosion has been about the same, the slightly increased density of the upper layer enabling them to sustain the brunt of the storm, and yet suffer no more wear than the softer strata beneath.