Page 555 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 16 Oceans, Coastal Systems, and Wind Processes 519
▲Figure 16.28 A ventifact. One of the wind-eroded rocks in the Dry Valleys area of antarctica, a snow-free polar desert with winds reaching speeds of 320 km · h−1. [Scott Darsney/Lonely Planet images/getty images.]
On Earth, some yardangs are large enough to be detected on satel- lite imagery. The Ica Valley of south- ern Peru contains yardangs reaching 100 m in height and several kilome- tres in length. The Sphinx in Egypt was perhaps partially formed as a yardang, whose natural shape sug- gested a head and body. Some scien- tists think this shape led the ancients to complete the bulk of the sculpture artificially with masonry.
Desert Pavement
The work of wind deflation is im- portant for the formation of desert pavement, a hard, stony surface— as opposed to the usual sand—that commonly occurs in arid regions (Figure 16.30a). Scientists have put forth several explanations for the formation of desert pavement. One explanation is that deflation literally
is abrasion. Like the sandblasting of streets and build- ings for maintenance, this process is accomplished by a stream of compressed air filled with sand grains that quickly abrade the surface. Since sand grains are not lifted to great heights above the ground surface, abrasive action in nature usually is restricted to a dis- tance of no more than a metre or two above the ground. Variables that affect natural abrasion rates include the hardness of surface rocks, wind velocity, and wind constancy.
blows away loose or noncohesive sediment, eroding fine dust, clay, and sand and leaving behind a compacted con- centration of pebbles and gravel (Figure 16.30b).
Another hypothesis that better explains some des- ert pavement surfaces states that deposition of wind- blown sediments, not removal, is the formative process. Windblown particles settle between and below coarse rocks and pebbles that are gradually displaced upward. Rainwater plays a part, as wetting and drying episodes
Rocks that are pitted, fluted (grooved), or polished from eolian erosion are called ventifacts (liter- ally, “artifacts of the wind,” shown in Figure 16.28). They usually be- come aerodynamically shaped in a direction determined by the con- sistent flow of airborne particles in prevailing winds. On a larger scale, deflation and abrasion together are capable of streamlining multiple rock structures in a landscape in alignments parallel to the most effective wind direction, thus pro- ducing distinctive, elongated for- mations called yardangs. Abrasion is concentrated on the windward end of each yardang, with deflation operating on the leeward portions. These wind-sculpted features can range from metres to kilometres in length and up to many metres in height (Figure 16.29).
▲Figure 16.29 A field of yardangs. abrasion from consistent, unidirectional winds shaped these yardangs in the Qaidam Basin, northwest China. [Xinhua/Photoshot.]