 Sand, varying in color and composition, covers 40 percent of the surface of the country, forming dunes that appear in all zones except the Senegal River Valley.Fixed sand dunes are composed of coarse, fawn-colored sand, while shifting ("mobile") dunes consist of fine, dustlike, reddish-colored sands that can be carried by the wind.Plateaus generally are covered with heavier blue, gray, and black sands that form a crusty surface over layers of soft, loose sand.