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A B

FIGURE 20.14 (A) A stream-carved mountainside before glaciation. (B) The same area after glaciation, with some of the main features
of mountain glaciation labeled.

fragments, eventually producing a powdery, silt-sized sediment particles by carrying them in suspension. Both can move larger
called rock flour. Suspended rock flour in meltwater from a gla- and more massive particles with increased velocities. Water is
cier gives the water a distinctive gray to blue-gray color. denser and more viscous than air, so it is more efficient at trans-
Glaciation is continuously at work eroding the landscape in porting quantities of material than is the wind, but the processes
Alaska and many mountainous regions today. The glaciation that are quite similar.
formed the landscape features in the Rockies, the Sierras, and across Two major processes of wind erosion are called (1) abrasion
the northeastern United States took place thousands of years ago. and (2) deflation. Wind abrasion is a natural sandblasting process
that occurs when the particles carried along by the wind break off
small particles and polish what they strike. Generally, the harder
Myths, Mistakes, & Misunderstandings mineral grains such as quartz sand accomplish this best near the
ground where the wind is bouncing them along. Wind abrasion
Unchanging as the Hills? can strip paint from a car exposed to the moving particles of a
dust storm, eroding the paint just as it erodes rocks on the sur-
It is a mistake to say something is “unchanging as the hills.” The
hills may appear to be tranquil and unchanging, but they are face. Rocks and boulders exposed to repeated wind storms where
actually under constant attack, weathered and eroded bit by bit. the wind blows consistently from one or a few directions may be
As more and more of the hills are carried away over time, they planed off from the repeated action of this natural sandblasting.
are slowly changing, eventually to cease to exist as hills. Rocks sculptured by wind abrasion are called ventifacts, after the
Latin meaning “wind-made” (Figure 20.15).

WIND
Like running water and ice, wind also acts as an agent shap-
ing the surface of the land. It can erode, transport, and de-
posit materials. However, wind is considerably less efficient
than ice or water in modifying the surface. Wind is much less
dense and does not have the eroding or carrying power of
water or ice. In addition, a stream generally flows most of the If wind is predominantly from one direction,
time, but the wind blows only occasionally in most locations. rocks will be planed off or flattened
on the upwind side.
Thus, on a worldwide average, winds move only a few percent
as much material as do streams. Wind also lacks the ability
to attack rocks chemically as water does through carbonation
and other processes, and wind cannot carry dissolved sedi-
ments in solution. Even in many deserts, more sediment is
moved during the brief periods of intense surface runoff fol-
lowing the occasional rainstorms than is moved by wind dur-
ing the prolonged dry periods. With a persistent shift in wind direction,
additional facets are cut in the rock.
Flowing air and moving water do have much in common
as agents of erosion since both are fluids. Both can move larger FIGURE 20.15 Ventifact formation by abrasion from one or
particles by rolling them along the surface and can move finer several directions.

512 CHAPTER 20 Shaping Earth’s Surface 20-12