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. [Illustration: 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). [Illustration: 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. │ │ │ │ [Illustration: _A._ Ranges of dunes upon the margin of the Colorado │ │ Desert (after Mendenhall).] │ │ │ │ [Illustration: _B._ Sand dunes encroaching upon the oasis of Wed │ │ Souf. Algeria (after T. H. Kearney).] │ └─────────────────────────────────────────────────────────────────────┘