Nature in the nude. From Desert images, by Edward Abbey.
Sand dunes are conspicuous and spectacular features of many desert landscapes.
They bear testimony to the timeless power of wind and the alternating states of rock through geological time. Broken down by weathering and erosion, rocks are eventually turned into loose fragments, particle by particle, over vast stretches of time. Thus only particles of a certain size are capable of building dunes. Unlike dust and silt, sand particles are too heavy to remain airborne by strong winds and are bounced along the ground for great distances and at high velocities. A dune begins to form against some kind of obstruction on the ground such as a piece of wood, a rooted plant, or a rock - any object that is resistant to being moved by the wind.