Geoscience Reference
In-Depth Information
Figure 21.7
Yardangs of the Qaidam basin are formed from lacustrine sediments. This photograph illustrates flooding of part of
the yardang complex. The field has developed through complex cycles of sedimentation and erosion, in a deflation basin that has
alternately filled and lowered (photo courtesy of R. Heermance).
and the forms themselves are the result of the constant
interplay between wind and water. Indeed, during climat-
ically wetter periods, earth flows, solution and gullying
may be of sufficient magnitude that aeolian features are
masked (Krinsley, 1970).
Studies of contemporary yardang fields show that short-
term changes to the surface of yardangs can be extensive
during infrequent rains and floods. Yardangs may expe-
rience seasonal fluctuations, with gullies deepening and
extending in the rainy season and aeolian processes dom-
inating at other times of the year (Krinsley, 1970). In the
Lut Desert of Iran, the upper surfaces of many yardangs
are extensively gullied (see, for example, Figures 88, 89
and 90 in Krinsley, 1970, Part II) and may even be cut
into buttes with vertical cliffs (Figure 21.8). The degree
to which water or wind erosion affects the yardang form
varies spatially within the field. The northern yardangs are
more frequently inundated by floodwater and thus the hills
have near-vertical cliffs, which are the result of floodwater
erosion at the base, with salt stains marking the extent of
these events (Krinsley, 1970). The steep lower slopes are
further modified by wind abrasion. Moving southward in
the field, the yardangs are increasingly modified by the
wind, until they grade into sand ridges (Krinsley, 1970).
The yardings of the Lut Desert are discussed in more detail
in Box 21.1.
The long-term (1-2 Ma) interactions of fluvial and ae-
on an ignimbrite sheet in La Pacana Caldera, Atacama
Desert, are reflected in the morphology of the features.
Today's average rainfall is
10 mm, but the region has
experienced wetter periods in the recent geologic past.
Bailey
et al.
(2007) use a classification system that as-
sumes that relatively straight ridges are associated with
wind erosion and sinuous channels with fluvial erosion. A
range of features were found, marking the relative signifi-
cance of wind and water according to location: (1) features
dominantly eroded by fluvial processes, (2) fluvial chan-
nels with some aeolian modification, (3) complex forms,
resulting from fluvial and aeolian erosion, and (4) forms
that were dominantly formed by aeolian processes (Bailey
et al.
, 2007).
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Wind erosion: abrasion and deflation
Abrasion removes mass from the yardangs by a 'sand-
blasting' process. Evidence includes polish, intense flut-
ing and small-scale erosional lineaments on yardangs,
generally within the lower one to three metres where
saltation is most active, and the development of a steep,
often undercut, upwind face (a re-entrant form) (Hobbs,
1917; Bosworth, 1922; Hagedorn, 1971; Grolier
et al.
,
1980; Donner and Embabi, 2000). The corridors allow
the passage of sand as ripple or dune forms. Trailing
ridges or streams of sand can extend from the down-
