=Shapes of dunes.=[10]—Dunes may assume the form of ridges or of hillocks. The ridges may be transverse to the direction of the prevailing wind or parallel with it. Where dunes assume the form of hillocks rather than ridges, a group of them may be elongate in a direction parallel to the dominant wind, or at right angles thereto. The shape assumed by a dune or a group of dunes depends on the abundance of the sand, the strength and direction of the wind, and the shape of the obstacle which occasions the lodgment. [Illustration: +Fig.+ 8.—Dune ridges parallel to the direction of the wind. Southwest part of India. Scale about 3 miles to the inch. (Cornish.)] [Illustration: +Fig.+ 9.—Dune ridges transverse to the direction of the wind. Scale about 3 miles to the inch. (Cornish.)] The incipient stages of dune formation are readily seen in many dry, sandy regions. The dune is likely to start in the lee of some obstacle, and to be elongate in the direction of the wind, especially if the wind be strong relative to the supply of sand. This shape is permanently preserved if the proper relations between the supply of sand and strength and direction of wind are preserved. In the dune region of the Indian desert[11] the prevailing winds are alternately the southwest and northeast monsoons, the former being the stronger. The supply of sand comes from the southwest. Near the southwest coast the dune ridges are parallel to the direction of the wind (Fig. 8); in the interior, where the winds are less strong, the dunes are transverse to it (Fig. 9); while between the districts where these two types prevail intermediate forms occur. The transverse dune ridges (Fig. 9) are said to be the result of the lateral growth and erosion of longitudinal dunes.[12] In regions of changeable winds the shape of the dunes is subject to great variation. Dunes are sometimes crescentic, the convexity facing the wind (Fig. 10). [Illustration: +Fig.+ 10.—Crescentic dunes in ground-plan, the convexities facing the wind. (Bokhara.) (Walther.)] Along coasts, dune ridges are often transverse to the wind, and groups of dune hillocks are frequently elongate in the same direction. Here the source of supply of the sand is itself an elongate belt, often transverse to the dominant wind, and the resulting dunes often have great length transverse to the wind. Where the wind has strong mastery over the sand, the longitudinal tendency is seen, even along coasts.[13] [Illustration: +Fig.+ 11.—Section of a dune showing, by the dotted line, the steep leeward (_bc_) and gentler windward (_ab_) slope. By reversal of the wind the cross-section may be altered to the form shown by the line _adc_. (Cornish.)] [Illustration: +Fig.+ 12.—Cross-section of a dune showing the profiles developed by scour of the wind on both flanks. (Cornish.)]