Barchan

The sand is usually very well sorted in barchans, for it is constantly re-worked as the dune 'marches'. The marching also causes cross-bedding inside the barchan, with a dip parallel to the sand-fall face.

But to follow the dunes around the foot of their slopes is also tedious and one can walk for half a kilometre east or west, finding one barchan linked to another and no easy way through, and the unverifiable suspicion grows that one has been driven by insensible curves actually southwards.

The structure out here was revealed immediately—desert punctuated by oases in a geography of cruelty, barkhans or traveling sand-dunes a hundred feet high, which might or might not possess consciousness, […]

Barchans and transverse dunes are essentially of the same type, forming and migrating under a unidirectional wind regime. The difference between the two is related to the amount of sand: barchans are isolated mounds, whereas transverse dunes are composed of many barchans coalesced into a single, longer dune form[…].

Perhaps the most distinctive is the barchan dune, an isolated crescentic form with arms that stretch downwind. Barchans are not huge, often with heights of only a few meters.