Barchan dunes are defined as arc-shaped, and contain well-sorted, mature sand grains. In the geological record, finding these dunes can lead to all sorts of interesting information being gathered about the ancient system, as the two "horns" found either side of the dune will always face down wind.
Barchans can "join" with other Barchan dunes, growing and growing until they form ridges that can extend for hundreds of kilometres.
Aside from the Afar Depression, other spectacular examples include La Joya in Peru, and Parangkusumo Beach in Indonesia; the dunes of Parangkusumo are comprised of volcanic sands that have their origin in Mt Merapi, and have taken thousands of years to form and migrate.
Barchan sand dunes have also been found on Mars, giving clues to the systems operating on the Red Planet.
Image: George Steinmetz