沙丘形成於障礙——石頭，灌木叢，原木，

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

一切重得足以抵抗被風吹動的東西。

anything heavy enough to resist being moved by the wind.

這種障礙在其背風面形成一個風幕

This obstacle forms a

在氣流中，在忽快忽慢的風中制造旋渦，

wind shadow

on its leeward side

正如溪流裏的岩石在水中引起旋渦。

making eddies in the currents, now fast, now slow, of the air,

在旋渦之內，風在兩側以氣流較少的

exactly as a rock in a stream causes an eddy in the water.

力量和速度運動，生成地質學家稱之的

Within the eddy the wind moves with less force and less velocity

突變面。正是在這裏，風

than the airstreams on either side, creating what geologists call

要甩落它所負載的沙塵的一部分。沙子微粒，

the surface of discontinuity

順風沿著地球跳躍或顛跑，

. And it is here that the wind

開始積聚，

tends to drop part of its load of sand. The sand particles,

在氣流中生成更大的旋渦

which hop or bounce along the earth before the wind,

並捕捉更多的沙子。

從而一個沙丘形成。

begin to accumulate,

creating a greater eddy in the air currents

在橫截面觀看，沙丘呈現一個典型的輪廓。

and capturing still more sand.

在迎風面螺旋角低而平緩——

It’s thus a dune is formed.

與地平線成二十到二十五度角。在背風面

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

斜坡非常陡峭，通常大約三十四度角——

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

是沙和大多數其他松軟物質的休止角。

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

沙丘的陡沿被稱之為背風面

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

由於

當沙子被抬上迎風面並堆積於丘頂

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

或稍過丘頂時發生的滑動。

The steep side of the dune is called the

丘頂的重量

slip face

最終變得比下方的沙能夠支撐的還大，

because of the slides

那麼額外的沙重重地落下背風面

that occur as sand is driven up the windward side

而整個沙丘

and deposited on or just over the crest.

在主風的方向裏演進，直到遇到某些障礙

The weight of the crest

比如說山脈橫亙其間。

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

這種運動，這種穿過地球表面

so the extra sand slumps down the slip face

宏偉緩慢的行進，與冰川的洗擦運動

and the whole dune

外在相應，又與暗湧的悲傷

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

－－其中有沙漠的裸露與遙遠，

like a mountain intervenes.

內在相應。

This movement, this grand slow march

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

愛德華·艾比（Edward Abbey, 1927—1989），美國著名生態文學作家。

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.