Death Valley geology field trip

Death Valley Dunes

Death Valley Dunes. Photo from NPS archives.

Sinuous sculptures of sand

Death Valley's most accessible sand dunes are just a few miles from Stovepipe Wells. Tucked into Mesquite Flat in the north end of the park, these dunes 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.

Breaking down bedrock

Even in this parched climate, the effects of weathering take their toll on rock. High in the mountains, which receive quite a bit more moisture than the valley floor, bedrock is broken down into blocks. Flash floods, speeding storm waters that rush down from bare mountain slopes during intense desert storms, grind the rock to smaller pieces as it hurtles toward the valley floor. Eventually large blocks may be jostled around enough to be broken into sand-sized grains.

Sand and other sediment usually ends up deposited on alluvial fans or on the valley floor. In Death Valley's climate, it doesn't take long for the sediment to dry out and become exposed to the prevailing northwest winds.

Migrating sand grains may be suspended in the air (suspension), bounced along (saltation), or nudged along by impact from bouncing, saltating grains(impact creep).

Sand on the move

All it takes is a bit of breeze (16 kilometers/hour or 10 miles/hour) to whisk fine sand into motion. The grains may be suspended in the air, bounce along, or nudged along by impacts from bouncing grains, depending upon the grain size and wind strength. Almost all blowing sand remains within a meter of the surface as it migrates.

Ripplemarks on Death Valley Dunes. Photo from NPS archives.

Ripples and dunes

Once sand begins to pile up, ripples and dunes can form. Wind continues to move sand up to the top of the pile until the pile is so steep that it collapses under its own weight. The collapsing sand comes to rest when it reaches just the right steepness to keep the dune stable. This angle, usually about 30-34°, is called the angle of repose. Every pile of loose particles has a unique angle of repose, depending upon the properties of the material it's made of.

The repeating cycle of sand inching up the windward side to the dune crest, then slipping down the dune's slip face allows the dune to inch forward, migrating in the direction the wind blows. As you might guess, all of this climbing then slipping leaves its mark on the internal structure of the dune. The image on the right shows sand dune structure preserved in the Jurassic Aztec Sandstone at Lake Mead National Recreation Area. The sloping lines or laminations you see are the preserved slip faces of a migrating sand dune.