VARIATIONS ON A PASSAGE IN EDWARD ABBEY

A dune begins with an obstacle—a stone, a shrub, a log,

anything heavy enough to resist being moved by the 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.

Viewed in cross section, sand dunes display a characteristic profile.

On the windward side the angle of ascent is low and gradual—

twenty to twenty-five degrees from the horizontal. On the leeward side

the slope is much steeper, usually about thirty-four degrees—

the angle of repose of sand and most other loose materials.

The steep side of the dune is called the

slip face

because of the slides

that occur as sand is driven up the windward side

and deposited on or just over the crest.

The weight of the crest

eventually becomes greater than can be supported by the sand beneath,

so the extra sand slumps down the slip face

and the whole dune

advances in the direction of the prevailing wind, until some obstacle

like a mountain intervenes.

This movement, this grand slow march

across the earth’s surface, has an external counterpart in the scouring

movement of glaciers,

and an internal one in the movement of grief

which has something in it of the desert’s bareness

and of its distances.