A dune begins with an obstacle—a stone, a shrub, a log,
沙丘形成於障礙——石頭，灌木叢，原木，
anything heavy enough to resist being moved by the wind.
一切重得足以抵抗被風吹動的東西。
This obstacle forms a
這種障礙在其背風面形成一個風幕
wind shadow
在氣流中，在忽快忽慢的風中制造旋渦，
on its leeward side
making eddies in the currents, now fast, now slow, of the air,
正如溪流裏的岩石在水中引起旋渦。
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.
Within the eddy the wind moves with less force and less velocity
力量和速度運動，生成地質學家稱之的
突變面。正是在這裏，風
than the airstreams on either side, creating what geologists call
the surface of discontinuity
要甩落它所負載的沙塵的一部分。沙子微粒，
. And it is here that the wind
順風沿著地球跳躍或顛跑，
tends to drop part of its load of sand. The sand particles,
開始積聚，
which hop or bounce along the earth before the wind,
在氣流中生成更大的旋渦
並捕捉更多的沙子。
begin to accumulate,
從而一個沙丘形成。
creating a greater eddy in the air currents
and capturing still more sand.
在橫截面觀看，沙丘呈現一個典型的輪廓。
It’s thus a dune is formed.
在迎風面螺旋角低而平緩——
Viewed in cross section, sand dunes display a characteristic profile.
與地平線成二十到二十五度角。在背風面
On the windward side the angle of ascent is low and gradual—
斜坡非常陡峭，通常大約三十四度角——
twenty to twenty-five degrees from the horizontal. On the leeward side
是沙和大多數其他松軟物質的休止角。
the slope is much steeper, usually about thirty-four degrees—
沙丘的陡沿被稱之為背風面
由於
the angle of repose of sand and most other loose materials.
當沙子被抬上迎風面並堆積於丘頂
The steep side of the dune is called the
或稍過丘頂時發生的滑動。
slip face
丘頂的重量
because of the slides
最終變得比下方的沙能夠支撐的還大，
that occur as sand is driven up the windward side
那麼額外的沙重重地落下背風面
and deposited on or just over the crest.
而整個沙丘
The weight of the crest
在主風的方向裏演進，直到遇到某些障礙
eventually becomes greater than can be supported by the sand beneath,
比如說山脈橫亙其間。
so the extra sand slumps down the slip face
這種運動，這種穿過地球表面
and the whole dune
宏偉緩慢的行進，與冰川的洗擦運動
advances in the direction of the prevailing wind, until some obstacle
外在相應，又與暗湧的悲傷
like a mountain intervenes.
－－其中有沙漠的裸露與遙遠，
This movement, this grand slow march
內在相應。
across the earth’s surface, has an external counterpart in the scouring
movement of glaciers,
愛德華·艾比（Edward Abbey, 1927—1989），美國著名生態文學作家。
and an internal one in the movement of grief
which has something in it of the desert’s bareness
and of its distances.