Geoscience Reference
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Figure 21.1 Yardangs of the Qaidam basin, China, are formed from lacustrine sediments. The field has developed through
complex cycles of sedimentation and aeolian erosion, in a deflation basin that has alternately filled and lowered (photo courtesy
of R. Heermance).
glance are difficult to recognise as yardangs (Figure 21.3)
(Halimov and Fezer, 1989). Within any given material,
the form of the yardang is influenced by differences in
resistance to weathering and erosion, determined by such
factors as the degree of cementation, bedding and joint-
ing patterns (Grolier et al. , 1980) and by the length of
time the yardang has been eroded (Vincent and Kat-
tan, 2006). These factors lead to spatial zonations in
yardang form.
Grolier et al. (1980) note that one of the critical ele-
ments distinguishing yardangs from other hills or insel-
bergs is that they must appear to be streamlined by the
wind and have a length that exceeds the width, usually by
a ratio of 3 : 1 or greater. Wind tunnel experiments sug-
gest an ideal length-to-width (L/W) ratio of 4 : 1 (Ward
and Greeley, 1984), independent of scale. However, these
ideal proportions are probably achieved only after very
long periods of time, and actual ratios vary considerably
both within and between yardang fields. Length-to-width
ratios of yardangs in coastal Peru range from 3 : 1 in
the northern part to 10 : 1 in the southern part of the
field (McCauley, Breed and Grolier, 1977). In the Sahara,
streamlining varies according to yardang size, with larger
forms rather irregular in contour and smaller ones dis-
playing a more streamlined and smoothed surface (Smith,
1963). Bosworth (1922) also observed that smaller Peru-
vian yardangs (
6 m) attain greater perfection of form,
with corners and points, which initially offer resistance to
the wind, worn off by erosion to create a 'windform'
shape. Individual freestanding yardangs may be more
streamlined than those in groupings (McCauley, Breed
and Grolier, 1977).
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