A dune begins with an obstacle—a stone, a shrub, a log,
anything heavy enough to resist being moved by wind.

This obstacle forms a wind shadow on its leeward side,
making eddies in the currents, now fast, now slow, of the air,

exactly as a rock in a stream causes an eddy in the water.
Within the eddy the wind moves with less force and less velocity

than the airstreams on either side, creating what geologists call
the surface of discontinuity. And it is here that the wind

tends to drop part of its load of sand. The sand particles,
which hop or bounce along the earth before the wind,

begin to accumulate,
                                            creating a greater eddy in the air currents
and capturing still more sand.
                                                               It’s thus a dune is formed.