Ripplemarks on Death Valley Dunes. Photo from NPS archives.
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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.
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