Geoscience Reference
In-Depth Information
Figure 21.5
The corridors in yardang fields are cut by migrating sand, ripple trains and barchans. Location: Qaidam basin (photo
courtesy of R. Heermance).
occurs in the corridors, with the yardang form adjusting to
this new 'base level'. The yardang troughs act as conduits
for the transport of sand (and sometimes water) from one
region to another. In the Lut Desert, for example, sand is
moved from the north to the south through the yardang
field, trailing out into sand ridges at the southern margin
of the field (Krinsley, 1970). Over time, troughs widen to
become corridors (Brookes, 2001).
The troughs and corridors range in cross-sectional
form from U-shaped to flat-floored (McCauley, Breed
and Grolier, 1977), becoming increasingly level as the
yardangs become more widely spaced (Blackwelder,
1934; Krinsley, 1970). Flattening may also be the result of
erosion that has met more resistant strata. The troughs may
be partly or completely buried with sand, which is present
in the form of ripples, megaripples or dunes (Krinsley,
1970; Mainguet, 1970; McCauley, Breed and Grolier,
1977; Grolier
et al.
, 1980; Al-Dousari
et al.
, 2009).
The ripple trains sometimes diverge at the head of the
yardang and converge in the downwind direction around
the flanks (McCauley, Breed and Grolier, 1977). Migrat-
ing barchans, with axes paralleling the ridges and resul-
tant winds, also signify the movement of sand through
the yardang field (Figure 21.5). The corridor floors fre-
quently show marks of aeolian erosion, including longi-
tudinal striations and shallow erosional basins (Mainguet,
Mountains, whereas the intervening corridors are light in
tone owing to abrasion by moving sand and barchan dunes
(Mainguet, 1972).
In extremely arid regions, erasure by wind action re-
moves any morphological evidence of fluvial erosion from
the corridors (Mainguet, 1970; Grolier
et al.
, 1980). In
other areas, hybrid corridor forms develop that incorporate
both water and wind erosion characteristics (McCauley,
Breed and Grolier, 1977; Bailey
et al.
, 2007); e.g. the
troughs may retain a degree of sinuosity as a relict of
their former status as a watercourse (McCauley, Breed
and Grolier, 1977).
21.2.3
Yardang formative processes
21.2.3.1
Environmental requirements for yardang
formation
The following environmental requirements for yardang
formation have been identified:
1. Strong, unidirectional or seasonally opposing winds
(with one direction stronger than the other). Mean
winds commonly equal or exceed the threshold ve-
locity for sand movement, with higher gusts common,
particularly in the windy season (Table 21.2).
