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Geology of Mojave National Preserve: Kelso Dunes

Sand on the Move - One Jump at a Time

Dune formation Once bedrock is broken down into blocks, water begins to transport them downstream. Eventually large blocks may be jostled around enough to be broken into sand-sized grains. Sand and other sediment usually ends up deposited along the sides of streams, in lakes, or in the ocean.

You probably already know that sand dunes are sculpted by wind. As long as streams flow and lakes stay wet, the sediment in them is protected. When lakes or stream beds dry out the sediment is exposed to the wind and the particles are ready to move!

All it takes is a bit of breeze (16 kilometers/hour or 10 miles/hour) to put fine sand in motion. The finest grains may be suspended in the air and carried along (suspension). Heavier grains tend to bounce along as they are lifted into the air, fall back to the ground, then bounce back up again (saltation). The heaviest grains the wind can move are usually nudged along by impact from bouncing, saltating grains ( impact creep).

Dune formation diagram

Pile up!

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.

close up of Aztec Sandstone crossbedding
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 Aztec Sandstone (Jurassic). The sloping lines or laminations you see are the preserved slip faces of a migrating sand dune. The formation, when seen in crossection, is called crossbedding.

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