Page:Popular Science Monthly Volume 33.djvu/554

538 the name of drift or flying sand. While single grains have but little cohesive force, sand behaves toward the wind like water; and the method of the formation of dunes is undoubtedly very similar to that of the formation of waves. If the sand was quite even and horizontal and the wind blew regularly in the same direction, it would not get at the sand. But the surface of a sand bed is not even; it consists of the roundish heads of the sand grains that form the upper layer. The wind blowing over them moves them out of their place, and, the individual grains being roundish, they roll. The continuous pressure of the wind extends their movement, and these grains striking upon the projecting grains that are still at rest, disturb them, and the movement spreads more and more. The smaller grains at last no longer touch the ground, and only the heavy ones retain the springing character of their motion, till the wind is restrained and weakened by some fixed objects—plants or buildings—and is compelled to let part or all of its load fall. These objects are thus exposed to be submerged in sand; and hence it is that we so often see fields of low plants and even villages overwhelmed. It is interesting to observe the different ways in which different objects receive the wind. A tight wall does not catch the sand immediately in front of itself. A furrow is formed just before it, through the generation of side-currents, which receive the sand from actual contact with the wall. There is then formed a sand-ridge parallel to the wall, but at first separated from it by a hollow; but afterward, when the ridge has become high enough to shield the wall from the wind, the side-currents are extinguished, and the sand advances to it. The sand driven over the wall by the wind falls at a considerable distance behind it. A striking illustration of this process was formerly to be seen at the church of Altpillau, on the Baltic. The village, which had previously surrounded the church, was removed farther to the east on account of the presence of the sand, but the church had to be left where it was. A sand-ridge some twelve or twenty feet high was formed around it, but nowhere reached the walls of the building; and while the congregation were obliged to climb over the ridge, they never found the church-doors buried. A broken wall, an open fence, or a quick-set hedge behaves quite differently toward the advancing sand. No furrow is formed in front of it. The air-current forces a large part of its load through the openings, but is so weakened by the obstruction that it drops it before and behind the fence. A little wall is formed around a tree-trunk, which is not, however, of great extent behind it. In isolated bushes and tufts of herbage, the intervals between the single stalks are filled up with sand, and a little mound is gradually formed.

Like the inland drift-sands, the dunes of the coast also migrate.