The topography of Namibia can be divided into four regions. At 2,000m, the highest land is the central plateau that runs roughly from north to south, from south of Keetmanshoop to north of Otjiwarongo. This is hilly, verdant country where most of Namibia's best farmland is concentrated.
To the west of this plateau, the land falls off in a dramatic escarpment down to the Namib Desert, one of the world's oldest deserts which stretches for 1,600km beside the Atlantic Ocean. The escarpment, and the incisions that have been cut through it by river action over the years, provides some of Namibia's most spectacular scenery. Below, the Namib is a flat coastal plain whose profile is broken only by shifting dunes and the odd towering inselberg.
East of the central plateau, the land slopes off much more gradually, merging into the great sand-sheet of the Kalahari Desert. A plateau standing at about 1,000m, stretching from Namibia into Botswana and even beyond, this is rolling country with vegetated sand-dunes.
Barchan, or crescentric, dunes arise wherever sand-laden wind deposits sand on the windward (up-wind) slopes of a random patch on the ground. The mound grows in height until a 'slip-face' is established by sand avalanching down on the sheltered leeward (downwind) side. The resulting dune is therefore in a state of constant (if slow) movement – sand is continuously being deposited and blown up the shallow windward slope and then falling down the steep leeward slope. This slow movement, or migration, is more rapid at the edges of the dune (where there is less wind resistance) than in the centre, which results in the characteristic 'tails' of a mature barchan.
Fairly constant winds from the same direction are essential for the growth and stability of barchan dunes, which can migrate from anything up to six metres a year for high dunes to 15 metres a year for smaller dunes. Probably the best examples of barchan dunes occur in Namibia's Skeleton Coast, where some of the dune crests are highlighted by a purple dusting of garnet sand. You'll see them 'marching' across the road near where the D2345 turns from the main C34 coastal road.
Where the prevailing wind is interrupted by crosswinds driving in sand from the sides, a long seif or longitudinal dune is formed, instead of a swarm of barchans. The shape of seif dunes is that of a long ridge with high crests, parallel to the direction of the prevailing wind. They commonly occur in long parallel ranges, such as those south of the Kuiseb River which show up so clearly on satellite photographs.
Sand-sheets occur when the land is vegetated with grass and scrub, or is covered with rocks and pebbles. Then the force of the wind is broken and it becomes less homogenous. In such situations poorly developed seif dunes or irregular barchans form, and may often join together to some extent, making an undulating sand sheet. From this platform of coarser sand, more erratic dunes often rise.
Sand-sheets, in one form or another, are the most common dune formation in southern Africa, since the 'text book' conditions needed to form perfect barchan or seif dunes are rare. However, the principles remain the same and 'imperfect' dunes of barchan or seif origin are widespread throughout the Kalahari and Namib deserts.