Geoscience Reference
In-Depth Information
Figure 21.3
The form of yardangs may vary considerably within a single basin. These pyramidal forms are one of eight mor-
phologies recognised by Halimov and Fezer (1989) in the Qaidam basin (photo courtesy of P. Kapp).
The length-to-width ratio of the yardangs is related
to the time available for abrasion and the structure and
lithology of the material into which it is cut. Brookes
(2003b) illustrates yardangs cut into exhumed meander
scrolls in a region of unidirectional winds, where elongate
features develop parallel to the long axis of the scrolls and
short stubby forms occur where the wind is perpendicular
to the grain of the scrolls. Wind tunnel experiments by
Ward and Greeley (1984) indicate that if the original form
is broad, erosion will decrease the width, whereas for
a more elongate form, the dominant change will be a
decrease in length. Table 21.1 illustrates the large range in
length-to-width ratios that have been identified in different
yardang fields.
Lineation and streamlining are often associated with
scale-specificity (Evans, 2003). Yardangs within any
given field are largely similar in scale and show great
periodicity in wavelength (Mainguet, 1970, 1972), but be-
tween different areas, the scale varies according to wind
activity and intensity, the time span over which erosion
has acted and rock type. The crest spacing is similar within
any given area: 20-40 m in yardangs formed in soft di-
atomites and 1.6 km in sandstone megayardangs in the
Sahara Desert (Mainguet, 1970; Evans, 2003).
It has been suggested that the spacing of small meso-
yardangs may be related to the regular spacing of vege-
tation (Cooke, Warren and Goudie, 1993). Many bushes
accumulate aeolian sediments and develop into nebkhas
(vegetation-anchored dunes) that became stabilised over
time. They are later eroded by the wind to form yardangs,
which inherit the regular spacing of the plants (Capot-Rey,
1957; Mahmoudi, 1977).
Although most yardangs are elongate or tear-dropped
in form, reflecting a largely unidirectional wind regime,
more unusual shapes can develop when winds come
from two different directions. At a site in Farafra, Egypt,
small chalk yardangs are abraded by both westerly and
north-northwesterly winds to become L- or S-shaped
(Donner and Embabi, 2000).
Table 21.1
Representative length-to-width ratios of
yardangs.
Average L :
W ratio
Location
Source
1.5 : 1
Kuwait
Al-Dousari
et al.
(2009)
1.8 : 1
Mongolia
Ritley and Odontuya
(2004)
3 : 1
Western Desert,
Egypt
Grolier
et al.
(1980)
4 : 1
Rogers Lake, CA,
USA
Ward and Greeley
(1984)
21.2.2.2 Material properties: influence on yardang form
and development
Yardangs form in many different lithologies. They are
perhaps most common and widespread in softer materi-
als, most notably lacustrine sediments. On dry lakebeds,
5 : 1
Central Asia
Halimov and Fezer
(1989)
10 : 1
Borkou and Lut
Deserts
Cooke, Warren and
Goudie (1993)
