Dunes are likely to occur along stream valleys ([Fig. 2, Pl. II]), if their bottoms or slopes are of sand, and not covered by vegetation. Dunes along valleys are usually on the side toward which the prevailing winds blow. Thus they are more common on the east side of the Mississippi than on the west. Dunes may be formed in the valley bottoms, but the sand is often blown up out of the valley and lodged on the bluffs above.
Apart from these special classes of situations, any sandy region the surface of which is dry is likely to have its surface material shifted by the wind and piled up into dune ridges or hillocks ([Fig. 4, Pl. II]). Dunes probably reach their greatest development in the Sahara, where some of them take the form of hillocks, and some the form of ridges. Travelers in that region report that dune ridges are sometimes encountered the faces of which are so high and steep as to be difficult of ascent, and that parties have been obliged to travel miles along their bases before finding a break where crossing was practicable.
Fig. 20.—Wind-ripples. (Cross, U. S. Geol. Surv.)
Wind-ripples.—The surface of the dry sand over which the wind has blown for a few hours is likely to be marked with ripples ([Fig. 20]) similar to those made on a sandy bottom beneath shallow water, under the influence of waves. Like ripple marks made by the water, wind-ripples have one side (the lee) steeper than the other. While the ripples are, as a rule, but a fraction of an inch high, they throw much light on the origin of the great dune ridges. If the ripples be watched closely during the progress of a wind-storm, they are found gradually to shift their position. Sand is blown up the gentler windward slope to the crest of the ridge and falls down on the other side. The moment it falls below the crest of the ridge to leeward, it is protected against the wind, and is likely to lodge. Wear on the windward side is about equal to deposition on the leeward, and the result is the orderly progression of the ripples in the direction in which the wind is blowing, just as in the case of dune ridges.
Abrasion by the wind.—While the effect of the wind on sandy and dusty surfaces may be considerable, its effect on solid rock is relatively slight and accomplished, not by its own impact, but by that of the material it carries. The effect of blown sand on rock surfaces over and against which it is driven is perhaps best understood by recalling the effects of artificial sand-blasts, by means of which glass is etched. In a region where sand is blowing, exposed surfaces of rock suffer from a multitude of blows struck by the sand grains in transit. The result is that such rock surfaces are worn, and worn in a way peculiar to the agency accomplishing the work. If the rock be made up of laminæ which are of unequal hardness, the blown sand digs out the softer ones, leaving the harder projecting as ridges between them. Adjacent masses of harder and softer rock of whatever thickness are similarly affected. The sculpturing thus effected on projecting masses of rock is often picturesque and striking (Figs. [21] and [22]), and is most common in arid regions. Details of wind-carving are shown in [Fig. 23].
Fig. 21.—Wind-carved rock. (Green.)
PLATE II.
