Barchan sand dunes are common on Mars and often form vast dune fields within very large (tens to hundreds of kilometers) impact basins. The regions upwind of barchans are usually devoid of sandy bedforms, so if you were walking in a downwind direction, then the barchans would seem to appear out of nowhere.
As you walk downwind, you would notice the barchans link up (“joining arms”) and eventually slope into featureless sand sheets. We call this progression of dunes a “Herschel-type dune field” named after the first place this sequence was described: Herschel Crater.
But here is something interesting: a barchan dune filling the upwind portion of a small impact crater in a Pac-Man-like shape. This “dune-in-a-crater” s nearly at… [More at link]