Fig. 3. INDIANA.
U. S. Geol. Surv.
Scale, 2+ mile per inch.
Fig. 4. NEBRASKA.
Sand drifted over loose stones lying on the surface often develops flat or flattish faces or facets on them. These facets are likely to be three in number, and the exposed portion of the stone is likely to develop a sort of pyramidal shape, the three flattish surfaces being mutually limited by tolerably well-defined lines ([Fig. 24]). Thus arise the three-faceted stones (Dreikanter of the Germans) commonly seen where sands have been long in movement.
Fig. 22.—Wind-carved hillock of cross-bedded sandstone. Missouri River, Montana. (Calhoun, U. S. G. S.)
Not only does the drifting sand wear the surface over which it passes and against which it strikes, but the grains themselves are worn in the process. They are liable to be broken as they strike rock surfaces, and they are likely to strike one another in the atmosphere. In both cases they are subject to wear, and so to reduction to a finer and finer state.
The erosion accomplished by the wind is therefore of various sorts. The impact of the wind itself picks up the fine materials which are already loosened, thus wearing down the surface from which they are removed; the materials picked up wear the rock surfaces against which they are blown, and the transported materials themselves suffer reduction in transit.