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1986; Bridges et al. , 2010). The loss of material lost per
impact, A ,is
1970), heights of 100 m and lengths measured in kilo-
metres in the Tablazo de Ica of the Peru-Chile Desert
(McCauley, Breed and Grolier, 1977, Beresford-Jones,
Lewis and Boreham, 2009) and tens of metres in height
and hundreds of metres in length for the larger features in
the Western Desert of Egypt (Grolier et al. , 1980). Mega-
yardangs, particularly when developed in resistant rock,
indicate a wind regime that has remained strong and stable
over very long periods of time (Smith, 1963).
The long axes of yardangs parallel the prevailing wind
or, more likely, the wind of highest velocity (Horner,
1932; Krinsley, 1970). Yardangs seldom occur as individ-
ual landforms. They typically appear in small groupings
or 'swarms', but also occur in immense'fleets', 'fields'
or yardang complexes (Figure 21.1). The parallelism of
the individual ridges gives a distinctive grain to the to-
pography. Within the fields, yardang axes are largely sub-
parallel, but the whole field may describe a gentle curve,
following the direction of the prevailing winds. On the
south coast of Peru, dune fields and yardang complexes
curve gently through an arc of 180 to converge on the
city of Ica (McCauley, Breed and Grolier, 1977). In North
Africa, the yardang and ridge-and-swale systems follow,
in a great arc, the deflection of the Trade Winds around
the Tibesti Mountains (Laity, 2009).
The term yardang has been applied to a broad range
of wind-eroded features, some highly streamlined and
others much less so. Some authors limit the use of the
term yardang to distinctly aerodynamic forms. 'Ridge-
and-swale system' has been applied to parallel, unstream-
lined forms (most notably those bordering the Tibesti
Mountains of the Sahara). These are also referred to as
megayardangs (Goudie, 2007). Brookes (2001) prefers
the use of the term 'aeolian erosional lineation' (AEL),
which incorporates both yardang and ridge systems. In
this chapter, the term yardang will be used in a general
sense to describe any ridge-like form modified by the
wind, whether highly streamlined or not.
V 0 ) n ( D
D 0 ) m
A
=
S a ρ p ( V sin
α
where V 0 and D 0 are the threshold particle speed and
diameter that will initiate erosion (Scattergood and Rout-
bort, 1983). Anderson (1986) calculated values of n
=
2 and m
3 on the basis of abrasion experiments. The
loss of material upon impact is roughly proportional to
the kinetic energy of the impact. The rate of abrasion then
becomes
=
Rate
=
s a qf
where q is particle flux and f the wind frequency (Greeley
et al. , 1984).
21.2.1.2
Deflation
Deflation plays a role in eroding softer rocks and poorly
consolidated sediments and therefore influences yardang,
pan and depression formation, but not the development
of ventifacts. There have been few field or laboratory
studies of deflation. Often, its role is suggested by field
relationships, where mass loss has occurred, but features
typical of sand abrasion (such as grooving) are absent.
Unlike abrasion, the process of deflation is not limited
by elevation. Therefore, it can work across the entire sur-
face of a feature, such as a yardang, particularly if other
processes operate to weather and loosen material. Salt
weathering and wetting and drying processes are impor-
tant precursors to deflation. Even sandstone can be made
friable by salt efflorescence, preparing it for deflation by
the wind (Haynes, Mehringer and Zaghloul, 1979).
21.2.2
Yardangs
A yardang is an elongate ridge that shows clear signs
of having been eroded by the wind (Hedin, 1903). The
yardang may be formed in either hard (basalts, sand-
stone, granite, dolomite, etc.) or soft (lacustrine) ma-
terials, with rock type influencing form, scale and rate
of formation. Yardangs vary greatly in scale, from mi-
croyardangs (centimetre-scale ridges) (Worrall, 1974), to
mesoyardangs (metres in height and length), to mega-
yardangs (tens of metres high and often more than 1000 m
in length) (Cooke, Warren and Goudie, 1993; Laity, 2009).
Most impressive in scale are the megayardangs (Cooke,
Warren and Goudie, 1993; Goudie, 2007), whose dimen-
sions attain heights of 60 m in the Lut Desert (Krinsley,
21.2.2.1
Yardang form and scale relationships
In their most classic form, yardangs have been described
as resembling the 'hull of a racing yacht turned upside
down' (Bosworth, 1922, p. 295). The windward face
is typically blunt-ended, steep and high, with the lee-
ward end declining in elevation and tapering to a point
(Bosworth, 1922; McCauley, Breed and Grolier, 1977;
Whitney, 1985) (Figure 21.2). The face is often undercut
and sometimes has a moat.
In reality, the form of yardangs varies considerably,
from 'classic' forms (Figure 21.2) to those that at first
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