The Work of Wind and Deserts - The Changing Earth: Exploring Geology and Evolution - page 385

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Figure 15.3 Ventifacts

Wind

1 2 3

b Large ventifacts lying on desert pavement in Death Valley
National Monument, California.

a A ventifact forms when wind-borne particles (1) abrade the
surface of a rock, (2) forming a fl at surface. If the rock is moved,
(3) additional fl at surfaces are formed.

variable dimensions result from
differential erosion of surface
materials. Ranging in size from
several kilometers in diameter
and tens of meters deep to small
depressions only a few meters
wide and less than a meter deep,
defl ation hollows are common in
the southern Great Plains region
of the United States.
In many dry regions, the
removal of sand-sized and smaller
particles by wind leaves a surface
of pebbles, cobbles, and boul-
ders. As the wind removes the
fine-grained material from the
surface, the effects of gravity and
occasional heavy rain, and even
the swelling of clay grains, rear-
range the remaining coarse parti-
cles into a mosaic of close-fi tting
rocks called desert pavement
(Figure 15.3b and

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Figure 15.4 Yardang A profi le view of a streamlined yardang in the Roman playa deposits
of the Kharga Depression, Egypt. Yardangs form by wind erosion.

Figure 15.6).
Once desert pavement forms, it protects the underlying ma-
terial from further defl ation.

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These ridges may then be further modifi ed by wind abrasion
into their characteristic shape. Although yardangs are fairly
common desert features, interest in them was renewed when
images radioed back from Mars showed that they are also
widespread features on the Martian surface.

WIND DEPOSITS
Although wind is of minor importance as an erosional
agent, it is responsible for impressive deposits, which are
primarily of two types. The fi rst, dunes, occur in several dis-
tinctive types, all of which consist of sand-sized particles
that are usually deposited near their source. The second is
loess, which consists of layers of wind-blown silt and clay
deposited over large areas downwind and commonly far
from their source.

Deflation
Another important mechanism of wind erosion is defla-
tion, which is the removal of loose surface sediment by
the wind. Among the characteristic features of deflation
in many arid and semiarid regions are deflation hollows
or blowouts ( ◗ Figure 15.5). These shallow depressions of