Mesquite Flat Sand Dunes are at the northern end of the valley floor and are nearly surrounded by mountains on all sides. The primary source of the dune sands is probably the Cottonwood Mountains which lie to the north and northwest. The tiny grains of quartz and feldspar that form the sinuous sculptures that make up this dune field began as much larger pieces of solid rock. In between many of the dunes are stands of creosote bush on the sand and on dried mud, which used to cover this part of the valley before the dunes intruded.