dune

Wind and a generous supply of granular material are needed to make a dune, which can arise in arid places or along the shore of a lake or ocean, or even along the banks of a river such as the Arkansas River in western Kansas. Dunes are classified by their shape, which is a function of sand and topography interacting with the wind. Sand particles begin to move when wind velocity reaches about eleven miles per hour. The particles move by skipping over the ground, a motion called saltation. The grains are kicked briefly into the air and then fall with a forward glide, hitting other grains on the ground and putting them into motion as well. Relatively fine sand collects this way to produce dunes with a clearly marked crest that drops off to a steep slip face on the sheltered, leeward side. Coarse sand produces rounded dunes without these features. One of the largest dune fields in the United States is the Algodones Dunes, extending southeasterly more than forty miles from Glamis in Imperial County, California, to the southwestern corner of Arizona and into Mexico. The highest dune on America’s East Coast is the 140-foot high Jockeys Ridge on the Outer Banks of North Carolina; it’s an example of a coastal dune. Sometimes, continuing wind causes dunes to move—as at White Sands, New Mexico, where the dunes of loose gypsum sand move twenty-four feet a year. If the winds and sand supply diminish, an advancing or migrating dune might acquire a covering of vegetation and settle down as a fixed or stabilized dune. The dune environment often has unfamiliar acoustic properties and dune-created wind patterns that sculpt the features of the dune field. Wanderers among dunes may feel a thrill in their alien presence, be enchanted, or, like early saints, see visions in mirages and hear voices in the falling sand.

D. J. Waldie