Sossusvlei is about 66 km past the Sesriem gate. The last 6 km can only be traversed with 4WD vehicles as the concrete road ends and sand begins (the place where the concrete road ends is known as "2x4 parking" as any non-4WD vehicle must stop there). Sossusvlei is a clay pan, of roughly elliptical shape, covered in a crust of salt-rich sand. While the pan has been shaped over time by the Tsauchab river, the actual flooding of the pan is a relatively rare event, and sometimes several years pass between one flood and the next one. The river is dry most of the year, and even when it is not, it carries relatively little water to the vlei. The vlei is surrounded by high orange-reddish dunes, partially covered by a vegetation comprising grass, bushes, and some tree (mostly of species Acacia erioloba).