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Unit 49

WIND AS A GEOMORPHOLOGICAL AGENT

Unit Overview

This unit examines the potential for wind to shape the physical landscape. The main sections are as follows:

  • Wind erosion
  • Degradational landforms
  • Aggradational landforms
  • Sand dunes
  • Loess

Compared to fluvial, glacial, and coastal erosion, eolian erosion is a more subtle process. Wind plays a larger role in less-vegetated landscapes. Only when wind picks up particles does its power to erode become important. Wind transports a significant amount of matter. This process occurs through suspension, saltation, and surface creep. Aggradational landforms include sand dunes and loess.

Unit Objectives

  • To examine the mechanisms of wind erosion and the landforms produced by this process
  • To relate various types of sand dunes to environmental controls
  • To note the importance and environmental significance of loess


Glossary of Key Terms

Backslope The windward slope of a sand dune.
Barchan A crescent-shaped sand dune with its points lying downwind; convex side of this dune is the windward side.
Deflation The process whereby wind sweeps along a surface and carries away the finest particles.
Deflation hollow Shallow desert basin created by the wind erosion process of deflation.
Desert pavement A smoothly weathered, varnish-like surface of closely packed pebbles that has developed on the upper part of an alluvial fan or bajada; no longer subject to stream braiding, such a surface is stable and may support desert vegetation.
Dune An accumulation of sand that is shaped by wind action.
Eolian Pertaining to the action of the wind.
Erg A sand sea; a large expanse of sandy desert.
Loess A deposit of very fine silt or dust laid down after having been blown some distance (perhaps hundreds of kilometers) by the wind; characterized by its fertility and ability to stand in steep vertical walls.
Longitudinal dune A long ridge-like sand dune that lies parallel to the prevailing wind.
Parabolic dune A crescent-shaped sand dune with its points lying upwind; concave side of this dune is the windward side.
Slip face The leeward slope of a sand dune.
Surface creep The movement of fairly large rock fragments by the wind, actually pushing them along the ground, especially during windstorms.
Transverse dune A ridge-like sand dune that is positioned at a right angle to the prevailing wind; usually straight or slightly curved.
Wind abrasion The erosion of rock surfaces by windborne sand particles.
Yardang A desert landform shaped by wind abrasion in the form of a low ridge lying parallel to the prevailing wind direction; most common in dry sandy areas underlain by soft bedrock.


Unit Outline

  • Wind erosion
    • Eolian processes are wind-related and shape the Earth's surface
    • Wind is both an aggradational and degradational force
    • Wind is only an erosive agent when it carries sand particles and performs wind abrasion
  • Degradational landforms
    • Deflation is the process of wind sweeping away fine particles of sand
      • deflation hollows are produced by this process
      • desert pavement is the remaining closely-packed pebbles on the surface
      • yardangs are low ridges that form parallel to wind direction, due to abrasion
    • Wind transportation (Fig. 49.4)
      • finest particles carried high in the air in suspension
      • larger particles bounce between the air and the ground, in saltation
      • largest rock fragments pushed along at the surface by surface creep
  • Aggradational landforms
    • Erg, or sand sea, is the main feature in arid environments
    • Ripples may be formed by saltation or surface creep
  • Sand dunes
    • An accumulation of sand that is shaped by the wind
    • Dune is active when the wind is constantly moving it, and it has no vegetation
    • Dunes become stable (fixed) when they enter a moister area, and support vegetation
    • Dune features (Fig. 49.6)
      • backslope (or windward slope)
      • crest (or top)
      • slip face (or leeward slope)
    • Dune forms (Fig. 49.7)
      • barchans are crescent-shaped dunes, the windward side is convex
      • parabolic dunes are crescent-shaped dunes, the windward side is concave
      • transverse dunes are straight or slightly curved, at right angles to wind direction, the windward side is concave
      • longitudinal dunes are long ridges, parallel to wind direction
    • Dune landscape research
      • dune morphology can provide information about past climates
  • Loess
    • Fine-grained sedimentary deposits originally left by glaciers, then picked up by wind
    • Distribution of loess deposits
      • extensive in Eurasia, some in North America, southern South America
    • Properties of loess
      • loess produces very fertile soil for agriculture
      • consists of quartz, feldspars, carbonates, clays, and other minerals
      • fertile through its entire profile, not just its upper layer
      • a homogeneous, porous substance that has vertical strength once water has developed passages within it
        • China's Loess Plateau


Review Questions

  1. Name the four major forms of sand dunes, using Fig. 49.7.
  2. Describe the processes of suspension, saltation, and soil creep.
  3. Explain the global distribution of loess, using Fig. 49.9 as a guide.