Geoscience Reference
In-Depth Information
different layers in the yardangs suggests an evaporite origin for the materials forming the huge yardangs (Farpoor
and Krouse, 2008). In addition to wind erosion, cracking, piping, rilling, slumping and salt weathering modify
the surface, suggesting that even in this very arid environment both wind and water are essential to the overall
development of yardangs. Fluvial processes dominate during the rainy season, allowing gullies to grow, but aeolian
processes prevail in other seasons. The yardangs are more affected by flooding and water erosion in the northern
part of the field, with wind streamlining increasingly important in the southern section (Krinsley, 1970).
The movement of sand is episodic, and thus the corridors
may be temporarily free of sand even in a wind-corrasion
landscape.
The re-entrant form at the front of the yardang reflects
the kinetic energy-flux profile for saltating sand. The lower
part of the yardang profile mirrors that of ventifacts, indi-
cating that similar processes are operating.
The importance of deflation as a process is not well
documented, relying largely on anecdotal evidence of ma-
terial loss from yardang flanks (Bosworth, 1922). Defla-
tion removes loose surficial material, including unconsol-
idated sediments or grains that have weathered out. Its
impact on yardang formation is greatest in poorly consol-
idated materials. On the basis of tree root exposures in
Mongolia, Ritley and Oduntuya (2004) infer that defla-
tion has removed up to 67 cm of material on one yardang
and up to 70 cm in the interyardang corridors. Muddy
yardangs in Kuwait lose mass by deflation of thin flakes
of a surficial crust (Al-Dousari
et al.
, 2009). However, in
some areas, strong surficial salt crusts bind poorly consoli-
dated sediments together and limit deflation (Figure 21.4)
(Halimov and Fezer, 1989). Studies of yardangs in the
coastal desert of Peru by Bosworth (1922) and McCauley,
Breed and Grolier (1977) suggested that abrasion is most
important in the troughs and deflation on the flanks and
crests. For the small and relatively low field of yardangs
at Rogers Lake, California, opinion is divided: McCauley,
Breed and Grolier (1977) felt that the streamlined shape
and smooth ridge crests were attained largely by deflation,
whereas Ward and Greeley (1984) considered abrasion the
most important process, dominating trough formation and
initial sculpting, with deflation helping to maintain the fin-
ished form, and Blackwelder (1934) argued that abrasion
by saltating grains both over and around the yardangs
shaped the forms.
tures. The presence of salts in deserts, particularly in as-
sociation with lacustrine sediments, means that salt solu-
tion processes and salt weathering play an important role
in yardang formation and modification. The Lut Forma-
tion of the Lut Desert, for example, consists of horizon-
tally bedded lacustrine sediments, 135-200 m in thickness
(Krinsley, 1970), that include gypsum and other more sol-
uble sulfate salts (Farpoor and Krouse, 2008). Elongate
depressions, commonly filled with salt, are orientated at
333
◦
, parallel to the prevailing wind and yardang axes
(Krinsley, 1970).
Solution features on yardangs are not uncommon and
help to elucidate the climatic history of a region and the
ongoing seesaw of wetter and drier climates that affect
yardang formation. They include honeycomb weathering
(such as the 20-42 cm diameter alveoles in Kuwait) and
deeper solution cavities (Al-Dousari
et al.
, 2009). Such
features may be best developed on the leeward slopes,
away from the abrasive action of the windward face. Ex-
tensive honeycomb weathering features are considered
indicative of wetter conditions in the past, and their par-
tial obliteration by aeolian abrasion on the lower yardang
slopes points to a return to drier conditions (Vincent and
Kattan, 2006). The strong chemical weathering of Egyp-
tian limestone yardangs in more humid climates of the past
produced solution features with interconnecting cavities
(Grolier
et al.
, 1980). At the present time, relict yardangs
of Precambrian dolomite in the southern Namib deflation
basin are being modified by solution (Corbett, 1993).
Mass movement
Mass movement of various types modifies yardang slopes.
Where soils are saline and clays swell and shrink, creep
processes operate and material moves slowly downslope
(Krinsley, 1970). Slump processes remove greater masses
of material and may substantively change yardang form.
Block failure produces talus around the base of yardangs.
The blocks can be undermined by either wind or water
erosion and failure may be further facilitated where joints
are present (Horner, 1932). Sandblasting of the windward
slope is a common cause of undercutting and subsequent
21.2.3.5 Secondary processes
Chemical weathering, solution features and salt
weathering
