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In-Depth Information
(LLJ) increases the frequency and magnitude of episodes
of wind erosion, and simulation models suggest it would
have been even stronger during the Last Glacial Maximum
(LGM) (Washington
et al.
, 2006).
Like ventifacts, yardangs are directional structures that
are important geological indicators of modern and palae-
owind directions. Donner and Embabi (2000) observed
abrasional features on ventifacts and yardangs at 10
sites in the Western Desert of Egypt and concluded that
east-west alignments reflected the southward displace-
ment of the mid-latitude westerlies in a pre-Holocene cold
arid phase (20 000 to 10 000 y BP). Brookes (2003a)
concluded that the cross-cutting feature alignment re-
flected Early Holocene west and northwest flows and Late
Holocene north and northeast flows.
formation are relatively rare. Although they occur in desert
regions on all continents, there is considerable variation
in their scale, development and spatial extent. Compre-
hensive discussions of the Earth's major yardang fields
are provided in McCauley, Breed and Grolier (1977) and
Goudie (2007). The following section provides some ad-
ditional details.
There are multiple groupings of yardangs in Asia, many
of which have not been described fully in the literature.
Chinese yardangs range from mesoyardangs to mega-
yardangs. In the northwest Junggar Desert and the north-
eastern Taklimakan they cover an area of less than 1 %
(Sun, 2002). Yardangs in the Lop Desert, or Lop Depres-
sion, located in the Tarim basin, were made famous by
Hedin (1903). They are developed in old lake and al-
luvial sediments (H orner, 1932; Goudie, 2007), where
rainfall is generally less than 20 mm/yr, vegetation is
sparse (
21.2.3.7
Yardangs and desert dust
<
10-30 % cover) and there is relatively little relief
(Songqiao and Xuncheng, 1984). Megayardangs, 20-25 m
in height, occur in the Qaidam basin (Figure 21.9). A
small (5 km
2
) field of yardangs has been described in
Mongolia, formed in weakly consolidated sandstones and
mudstones among a group of dome dunes (Ritley and
Oduntuya, 2004).
Several areas of documented yardangs are found in
South America, including classically streamlined forms
near the Ica Valley of Peru (up to 1 km in length)
(Bosworth, 1922; McCauley, Breed and Grolier, 1977;
Beresford-Jones, Lewis and Boreham, 2009), yardangs
formed on ignimbrite sheets of La Pacana Caldera, Chile
(Bailey
et al.
, 2007), and megayardangs up to 10 km in
length in Holocene basaltic flows in the Payun Matru vol-
canic field in the southern Andes Mountains, Argentina
(Inbar and Risso, 2001).
As discussed earlier, yardangs are extensive in the
Sahara Desert. In southern Africa, they are prominent
features in coastal Namibia (Corbett, 1993; Compton,
2007; Goudie, 2007). As early as 1887, Stapff described
yardangs formed from bedrock in the Kuiseb Valley as
'aerodynamic landforms'.
Yardang development in North America, Australia and
Europe is very limited in scale and extent. In North
America, a small but well-studied group of yardangs
occurs at Rogers Lake, California (Blackwelder, 1934;
Ward and Greeley, 1984). Australia's deserts have ex-
tensive dune fields, but appear to lack the aridity and
transport of sand required to form yardangs (Twidale,
1994; Goudie, 2007). Only minor examples have been
reported (West
et al.
, 2009). Europe has a small, relict
grouping of yardangs in the semi-arid Ebro Depression of
Spain (Gutierrez-Elorza, Desir and Gutierrez-Santolalla,
The abrasional and deflational processes associated with
the formation of yardangs and their corridors bring about
an attendant increase in atmospheric dust (Chapter 20),
particularly when the surface material is composed of
readily erodible lacustrine sediments. Studies of the main
source regions for global dust indicate that many of them
are large basins of internal drainage, with the Bodel´eDe-
pression of North Africa alone responsible for between 6
and 18 % of global dust emissions (Todd
et al.
, 2007).
Within this depression, the palaeolake diatomite sedi-
ments of Lake Chad are moulded into yardangs as a result
of atmospheric conditions conducive to wind erosion, in-
cluding a strong Bodele low-level jet (Washington and
Todd, 2005; Schwanghart and Schutt, 2008), topographic
channelling (Mainguet, 1996) and very gusty surface con-
ditions (Engelstaedter and Washington, 2007; Washington
et al.
, 2006). The northeast Harmattan winds are funnelled
and intensified as they pass between the Tibesti and Ennedi
Mountains in northern Chad.
The deflation of material during yardang formation pro-
duces dust that is subsequently deposited elsewhere. An
important geomorphic question is the relative contribution
of desert dust derived from yardang fields to the loess de-
posits that are widespread in central Asia. Sun (2002) in-
vestigated the origin, age and provenance of loess in high
mountain areas of China and concluded that the defla-
tion of old lacustrine deposits, including the yardang-rich
Junggar and Tarim Basin, provided only a minor source
of silt-sized particles.
21.2.3.8
Global yardang systems
