 Dead Vlei has been claimed to be surrounded by the highest sand dunes in the world, the highest reaching 300–400 meters (350m on average, named "Big Daddy" or "Crazy Dune"), which rest on a sandstone terrace.The clay pan was formed after rainfall, when the Tsauchab river flooded, creating temporary shallow pools where the abundance of water allowed camel thorn trees to grow.When the climate changed, drought hit the area, and sand dunes encroached on the pan, which blocked the river from the area.