Geoscience Reference
In-Depth Information
Table 21.2
Climatological conditions for yardang development.
Location
Wind velocity
Rainfall
Source
Um Al-Rimam
Depression, Kuwait
Mean at 10 m: 4.5 m/s;
maximum recorded 38 m/s
Mean: 112 mm/yr
Al-Dousari
et al.
(2009)
Namibia
22-28 m/s (gusts at diurnal peak,
October to March)
5-20 mm/yr
Corbett (1993)
Rio de Ica rock
plateau, Peru
Unimodal from south: mean:
7.5-9 m/s; maximum gusts:
22.7-31.9 m/s
Mean: 0.3 mm/yr
Beresford-Jones, Lewis and
Boreham (2009)
Lut Desert, Iran
Maximum average velocity,
April: 9.35 m/s
Mean:
<
10 mm/yr
Ehsani and Quiel (2008)
Lop Depression
Mean:
<
20 mm/yr
Songqiao and Xuncheng
(1984)
La Pacana crater,
Atacama Desert
Mean:
<
10 mm/yr
Bailey
et al.
(2007)
Bodele Depression,
Chad
Mean from northeast: 6-8 m/s
17 mm at Faya Largeau
Goudie and Middleton
(2001)
3. Aridity and an absence of vegetation. Annual rainfall
is usually less than 100 mm and often less than 10 mm
(Table 21.2.)
abrades the bottom and sides of the trough, causing pro-
gressive steepening of the yardang slopes (Blackwelder,
1934). The wind (and occasionally water) erosion causes
the passages to erode more rapidly than the yardangs (Hal-
imov and Fezer, 1989). Over time, the troughs widen and
eventually breach the ridges at places of weakness. As
abrasion intensifies on the yardang prows, the ridges be-
come shorter and smaller (Halimov and Fezer, 1989).
Mass wasting, weathering and fluvial erosion reduce
the yardang summits as the troughs deepen by erosion
(Brookes, 2001).
The end product of aeolian erosion in yardang fields
may be a plain. Over a distance of 75 km, Vincent and
Kattan (2006) document a downwind progression from
sandstone mesas and eroded canyons to megayardangs,
yardangs, rock pavements and ultimately dunes. The
megayardangs are up to 40 m in height and hundreds
of metres in length. As sand supply and exposure in-
crease downwind, yardangs decline in height to less than
25 m. Further downwind, they become even smaller, until
the landscape transitions into a planar surface composed
of hundreds of square metres of abraded bedrock, upon
which can be seen the slightly raised remnants of an-
cient yardangs. The rock pavements are believed to be
extensions of the yardang corridor floors. The downwind
decline in yardang size may be related to abrasion rates,
which are affected by the downwind increase in the avail-
able sand load. Finally, the pavement ends in a dune field,
where the crests of barchanoid forms are oriented perpen-
dicular to the yardangs.
4. A pre-existing lineament of weakness that the wind can
exploit (gullies, joints, etc.).
5. A supply of abrasive sediment: relative to most dune
environments the region is 'sand poor' (McCauley,
Breed and Grolier, 1977).
6. For yardangs formed on lakebeds, a low water table.
21.2.3.2
Initial conditions and yardang development
Yardangs are probably initiated during arid climatic
phases when erosive winds, charged with sand, exploit
the axial trends of pre-existing lineaments, most com-
monly relict fluvial dissection systems (Horner, 1932;
Blackwelder, 1934; Krinsley, 1970; McCauley, Breed and
Grolier, 1977; Laity, 1994, 2009; Brookes, 2001; Bailey
et al.
, 2007; Al-Dousari
et al.
, 2009; Bristow, Drake and
Armitage, 2009), joint systems (Mainguet, 1970; Neev
and Hall, 1992; Vincent and Kattan, 2006), as for in-
stance in sandstones, or cooling cracks in volcanic de-
posits (Bailey
et al.
, 2007). For yardangs of the Arabian
Peninsula and the Lut Desert, Neev and Hall (1992) argue
for tectonic influences, suggesting that the linear trends
of yardangs, as well as their locations and patterns, are
strongly influenced by pre-existing swarms of fractures.
