Sand in the Gran Desierto de Altar

 This is reportedly the largest continuous sand dune area in North America.   Apparently, the dunes are continually moving over the highway.  We saw various instances of this, but the road was always passable.

 http://en.wikipedia.org/wiki/Gran_Desierto_de_Altar

The Gran Desierto is best known for its magnificent Star Dunes, many in excess of 100 metres (330 ft) high. More than two-thirds of the Gran Desierto is covered by sand sheets and sand streaks. The remaining area is split equally between a western population of star dunes and an eastern set of transverse or crescentic dunes. Some of the larger crescentic dunes in the northeastern sand sea exhibit reversing crests, a transitional morphologic feature associated with star dunes.