The 200 metre high sand dunes at Sossusvlei in the Namib Desert in Africa? After the raging floodwaters of Noah's Flood (violent enough to create the Grand Canyon so presumably also destructive of any pre-existing dunes made of desert sand) had receded. Correct?
http://commons.wikimedia.org/wiki/File: ... usvlei.JPG
I hope I am not talking barchans.