Geoscience Reference
In-Depth Information
Figure 21.4
Cross-section of a yardang in the Qaidam basin illustrating nearly level lacustrine beds with a thick, strong salt
capping that drapes across the form. The capping protects the underlying sediments from gullying (photo courtesy of P. Kapp).
a few thousand years. Although the development of a clas-
sic aerodynamic yardang shape appears easier to attain in
weakly consolidated rock, these materials are subject to
other erosional processes, which modify the form, includ-
ing solution (where salts are present), sheet flow, gullying
and mass movement (McCauley, Breed and Grolier, 1977;
Al-Dousari
et al.
, 2009). Moreover, the structure of the un-
derlying rocks often has a significant impact on yardang
form. Erosion tends to accentuate differences in the degree
of cementation, jointing or bedding (Grolier
et al.
, 1980).
Some yardangs have irregular or flat tops, the conse-
quence of more resistant cappings (Grolier
et al.
, 1980). In
Kuwait, for example, calcrete caps many of the yardangs
(Al-Dousari
et al.
, 2009), creating a flat top. The yardangs
of the Qaidam basin commonly show a thick (40-50 cm),
strong, salt capping that drapes across the form and ap-
pears to protect the underlying horizontally bedded la-
custrine sediments (Figure 21.4). There is no evidence
of fluvial erosion on this capping (R. Heermance, 2009,
personal communication).
Given sufficient time, yardangs develop in hard rocks,
including ignimbrite, basalt, dolomite, gneiss, schist and
sandstone (Goudie, 1989). In such materials, the yardangs
may be of great antiquity and indicate prolonged erosion
(Smith, 1963). In form, harder rocks often show aeolian
modification at the base and an irregular weathered top.
'Kharafish' topography (small-scale, sharp-ridged, in-
tensely fluted yardangs separated by sandy corridors) ex-
tends spatially to merge with swarms of larger yardangs
(hundreds of metres long and tens of metres high) in a
field that is estimated to be about 150 km in length and
tens of kilometres wide (Grolier
et al.
, 1980).
Yardangs of the Tibesti region and the Bodele Depres-
sion are formed both from hard, indurated sandstones
that appear as vast ridge-and-swale systems between the
Tibesti and Ennedi Mountains (Grove, 1960; Capot-Rey,
1961; Grove and Warren, 1968; Mainguet, 1968, 1970,
1972; Laity, 2009) and from much softer, easily erodible
lacustrine sediments derived from Lake Megachad (Bris-
tow, Drake and Armitage, 2009). The sandstone ridges
are up to 4 km in length (Mainguet, 1972) and 1 km in
width, whereas the yardangs formed in diatomite on the
floor of Lake Megachad have a scale of development ten
times less.
The development of megayardangs requires a unique
combination of features that includes hyper-aridity (rain-
fall
50 mm/yr), a unidirectional or narrowly bimodal
wind and relatively homogeneous rocks without complex
structures. They are absent in sites of active dune accu-
mulation, in areas with large alluvial fans, in mountainous
regions or where an integrated drainage system is present
(Goudie, 2008). It is possible that megayardangs may take
millions of years to form.
<
21.2.2.3
Megayardangs and mesoyardangs: scale
variations over distance
21.2.2.4
Interyardang corridors
Within a given wind regime, yardangs may vary in scale
by an order of magnitude. Yardangs of the Western Desert
The yardangs are separated by long, shallow depres-
sions, variously referred to as swales, couloirs, boule-
