The dunes that surround the area are rich red and owe their red color to age, as over the thousands of years, the sand has literally rusted. 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. However, the Wikipedia article on dunes contradicts this claim; firstly, there is room for debate on the criteria, such as whether there are higher individual dunes or whether there are higher "average highest area" dunes elsewhere