A gust of predawn wind flows down from the mountains. The tail vane of an unlocked windmill turns with the wind; steel grates on rusted steel. The sound is like that of a human groan. The Indian and Burns look toward the corral a hundred yards away and the tall tower—a skeleton of metal—standing within it. There they see, dangling on a rope, black in silhouette against the eastern sky, the first of the hanged men.

Desert Images

OF ALL NATURAL FORMS the sand dunes are the most elegant—so simple, severe, bare. Nature in the nude. Nothing can mar for long their physical integrity. Broken down by foot traffic or machines, the sand is re-formed by the winds into fresh new dunes, formally perfect, advancing with the winds across the desert, where nothing but a mountain range can halt their progress.

Sand—unlike dust—is not airborne at great altitudes. Students of the matter have learned that even the strongest winds can seldom lift sand more than a foot or two above the ground. Where the ground is hard and fairly flat, however, the wind steady, the supply of sand sufficient, this lifting power is enough to form dunes.

A dune begins with any obstacle on the surface—a stone, a shmb, a log, anything heavy enough to resist being moved by the wind. This obstacle forms a wind shadow on its leeward side, resulting in eddies in the current of the air, exactly as a rock in a stream causes an eddy in the water. Within the eddy the wind moves with less force and velocity than the airstreams on either side, creating what geologists call the surface of discontinuity. Here the wind tends to drop part of its load of sand. The sand particles, which should be visualized as hopping or bouncing along the surface before the wind (not flying through the air), begin to accumulate, the pile grows higher, becoming itself a barrier to the wind, creating a greater eddy in the air currents and capturing still more sand. The formation of a dune is under way.

From Beyond the Wall. Essays from the Outside (1984)

Viewed in cross section, sand dunes display a characteristic profile. On the windward side the angle of ascent is low and gradual—twenty to twenty-five degrees from the horizontal. On the leeward side the slope is much steeper, usually about thirty-four degrees—the angle of repose of sand and most other loose materials. The steep side of the dune is called the slip face because of the slides that take place as sand is driven up the windward side and deposited on and just over the crest. When the deposit on the crest becomes greater than can be supported by the sand beneath, the extra sand slumps down the slip face. As the process is repeated through the years, the whole dune advances with the direction of the prevailing wind, until some obstacle like a mountain intervenes. At this point the dunes, prevented from advancing, pile higher. At Death Valley and in Great Sand Dunes National Monument in Colorado the highest dunes reach five hundred feet. The only higher sand dunes are in Iran, where they attain a world’s record of seven hundred feet.