Fig. 218.—Diagrams to illustrate the effects of obstructions of different types in arresting wind-driven sand. a, An unyielding obstruction which permits the wind to pass through it; b, a flexible and perforated obstruction; c, an unyielding closed barrier (after Schulze).
The wandering dunes.—Over the broad expanse of the desert, sand and dust, and occasionally gypsum from the saline deposits, are ever migrating with the wind; on quiet days in the eddying “sand devils”, but especially during the terrifying sand storms such as in the windy season darken the air of northern China and southern Manchuria. This drift of the sand is halted only when an obstruction is encountered—a projecting rock, a bush, or a bunch of grass, or again the buildings of a city or a town. The manner in which the sand is arrested by obstacles of different kinds is of great interest and importance, and is utilized in raising defenses against its encroachments. If the obstacle is unyielding but allows some of the wind to pass through it, no eddies are produced and the sand is deposited both to windward and to leeward of the obstruction to form a fairly symmetrical mound ([Fig. 218 a]). An obstruction which yields to the wind causes the sand to deposit in a mound which is largely to leeward of the obstruction ([Fig. 218 b]). A solid wall, on the other hand, by inducing eddies, is at first protected from the sand and mounds deposit both to windward and to leeward ([Fig. 218 c] and [Fig. 219]).
Except when held up by an obstruction, the drifting sand travels to leeward in slowly migrating mounds or ridges which are known as dunes. Their motion is due to the wind lifting the sand from the windward side and carrying it over the crest, from where it slides down the leeward slope and assumes a surface which is the angle of repose of the material. In contrast with this the windward slope is notably gradual, being shaped in conformity to the wind currents.
Fig. 219.—Sand accumulating both to windward and to leeward of a firm and impenetrable obstruction. The wind comes from the left (after a photograph by Bastin).
The dunes which are raised upon seashores, like those of the desert, are constantly migrating, those upon the shores of the North Sea at the average rate of about twenty feet per year. Relentlessly they advance, and despite all attempts to halt them, have many times overwhelmed the villages along the coast. Upon the great barrier beach known as the Kurische Nehrung, on the southeastern shore of the Baltic Sea, such a burial of villages has more than once occurred, but as in the course of time further migration of the dune has proceeded, the ruins of the buried villages have been exhumed by this natural excavating process ([Fig. 220]).
Fig. 220.—Successive diagrams to show how the town of Kunzen was buried, and subsequently exhumed in the continued migration of a great dune upon the Kurische Nehrung (after Behrendt).
Plate 7.