Note 438 — Chapter V

Notwithstanding the general tendency of isolated coast dunes and of the peaks of the sand ridges to assume a conical form, Andresen states that the hills of the inner or landward rows are sometimes bow-shaped, and sometimes undulating in outline.—Om Klitformationen, p. 84. He says further that: "Before an obstruction, two or three feet high and considerably longer, lying perpendicularly to the direction of the wind, the sand is deposited with a windward angle of from 6° to 12°, and the bank presents a concave face to the wind, while, behind the obstruction, the outline is convex;" and he lays it down as a general rule, that a slope, from which sand is blown, is left with a concavity of about one inch of depth to four feet of distance; a slope, upon which sand is dropped by the wind, is convex. It appears from Andresen's figures, however, that the concavity and convexity referred to, apply, not to the horizontal longitudinal section of the sand bank, as his language unexplained by the drawings might be supposed to mean, but to the vertical cross-section, and hence the dunes he describes, with the exception above noted, do not correspond to those of the American deserts.—Om Klitformationen, p. 86.

The dunes of Gascony, which sometimes exceed three hundred feet in height, present the same concavity and convexity of vertical cross-section. The slopes of these dunes are much steeper than those of the Netherlands and the Danish coast; for while all observers agree in assigning to the seaward and landward faces of those latter, respectively, angles of from 5° to 12°, and 30° with the horizon, the corresponding faces of the dunes of Gascony present angles of from 10° to 25°, and 50° to 60°.—Laval, Mémoire sur les Dunes de Gascogne, Annales des Ponts et Chaussées, 1847, 2me sémestre.