The extensive sand dunes that cover areas, such the Saharah Desert and other sandy desert terrains, are called dune fields. Sand dunes vary in shape and size, and are formed by the velocity of the wind. Sand dunes tend to be longer on their side that is windward. The lee of the wind forms the sand dunes shorter side called the slip face. The valley or dipped gap between the sand dunes is called the slack. There are five forms of eolian, or air motion, formed sand dunes: Crescent, Linear, Star, Dome, and Parabolic.
For sand dunes to form there are a couple of prerequisites: a fine grained surface and relatively strong, consistent winds. The force of the wind, once it reaches a fluid threshold, moves the individual sand grains in two distinct ways, by saltation and surface creep. The primary form movement of the sand into forming sand dunes is saltation. Wind moves over and picks up grains of sand from the surface deposits giving them a forward momentum. In cases of a more coarse surface, such as pebbles or other forms of hard surface, the grains of sand will bounce directly off of the surface and back into the air with the wind once again providing it a forward momentum or push. This downwind movement on a course of rough surface is moved at a much faster rate than the form of saltation on a surface composed of finer sand grains, where rather than bouncing along the surface the sand grain will strike the softer sandy surface, burying itself and causing the expulsion of a secondary grain into the air that will then be blown downwind and follow the same pattern. The movement of sand grains along the surface by saltation is now about 75 percent of all sand moved by the wind. After saltation has started the grain of sand will cease to require direct wind pressure to be lifted into the air.
Surface creep is what happens with larger grains of sand that are too heavy to be lifted into the air by the wind and moved. When these heavier grains of sand are struck by saltating grains of sand, though the wind does not have the ability to force them into the air, it does aid them into a minor forward push along the surface. These grains of sand can be nearly two hundred times the bulk the saltating grains of sand. Surface creep accounts for approximately 25 percent of all grains of sand transported by wind.
As grains of sand start to heap up the dunes and ripples are able to form. The sand is moved and piled by the wind until it collapses under its own weight, resting when reaching its level of steepness that create stability in the base and dune. The average angle of a sand is 30-34°; this is the angle of repose. Dunes continue to form in the direction that the wind blows, migrating with the wind’s force as the sand repeats the cycle of ascending the windward dunes side to the crest and slips back down the slip face of the dune.
This is the striking process of sand dunes forming and continuing to restructure themselves over time. The same sandy landscape of these desert terrains is sculpted into beautiful dunes and ripples by the force of the wind as has happened for ages and will continue to beatify these areas, it seems, until the end of time itself.