Dune
a low hill formed by drifting sand. A dune is not a dust formation. Where dry sand is exposed the wind cuts it away and whirls the grains or slides them along the surface until they settle by their own weight into a place of shelter. This drifting takes place usually on a coast where a wind from the water has an unobstructed sweep. A very ordinary wind carries dry sand up a long slope, piling it just over the ridge. As the operation continues the far edge of a dune grows higher and higher, and travels or wanders farther from the shore. The traveler by rail around the southern end of Lake Michigan passes the faces of dunes advancing southward. They tower up to the height of two or three tall trees. Dunes are not infrequent in sandy prairie countries, as in the extreme western parts of Nebraska and Kansas. Cape Cod, southeastern France, the shores extending along the Bay of Biscay to the Spanish border, the Netherlands, Denmark along the North Sea, and Northeastern Prussia along the Baltic, have serious problems to meet. Forests, fertile farms, villages, and mills are buried; harbors are filled; and the courses of rivers choked up. A dune near Haarlem in the Netherlands rises 197 feet above the sea level. Many of the old caravan cities of Asia have been buried completely, not by sandstorms, but by the insidious approach of sandwalls that came nearer and nearer, a few yards each year, until walls, temples, houses, and palaces were overwhelmed beneath a vast pile of sand. The wind also cuts great hills away. The transporting power of the wind varies as the sixth power of its velocity. A current of four miles an hour has sixty-four times the carrying power of a two-mile wind. The governments of the regions mentioned have made strenuous efforts to control the sands. Some account of the success of Massachusetts in binding sand areas by transplanting grass, may be found in an article on BEACH GRASS.
Another method of protection is the formation of an artificial dune near the coast. The French build a fence of brush or boards. As soon as that is covered, they build another on top of it, until a ridge is formed so high and so steep on the seaward side that sand ceases to ascend it. A height of thirty-three feet is said to be sufficient near the coast. Another method of arresting sand, or rather of keeping it in place, is that of covering it with a layer of fine beach or heather until a sward of grass or a growth of trees can be established.