Sand on the move
 One
jump at a time
 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). |
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. |
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. |
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