270
Climatic Geomorphology
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Figure 12.14. In the upper part, cross section of a gilgai (after Harris, 1968). In the middle-lower part,
classification of gilgai morphology (Verger, 1964).
after a year. Another way of producing gilgai is given by Hallsworth et al. (1955) and
expressed graphically by Verger (1964) (Figure 12.16). During a dry period, particles
fall into the interior of the soil cracks and this added material is believed to cause the
microrelief during humid periods, because it provokes significant pressure deep within the
soil. It is also important to distinguish between the various mechanisms due to the action of
distinct forces in diverse situations, in relation to cracking and the expansion-contraction
cycles (Knight, 1980), which helps in understanding gilgai genesis more adequately.
4. Desert varnish
A fine, dark film developed on exposed material commonly occurs in arid regions,
and this is called desert varnish or rock varnish. In some deserts, such as those in SW
USA, this covering occurs on 75% of the rocky surfaces (Engle and Sharp, 1958).