Geoscience Reference
In-Depth Information
found that slump blocks ranged from minor forms that had
little effect on the overall yardang form to much larger
blocks that incorporated up to 20 % of the yardang's en-
tire volume. South of Dakhla, Egypt, talus masks up to
the lower one-third to one-half of 10-m high yardangs
(Brookes, 2003b).
liminary studies. Climate change influences the supply of
abrasive sediment, wind strength and direction, and the
infilling of yardang systems by sand and lacustrine de-
posits. In Namibia, subtle changes in Holocene sea level
affect the supply of beach sand that is driven inland to
feed an aeolian corridor (Corbett, 1993; Compton, 2006,
2007). In deflation basins associated with lakebeds, cy-
cles of erosion and sedimentation alternately lower and
fill basins (Washington
et al.
, 2006). Geologic and strati-
graphic investigation of the Qaidam Basin, China, has re-
vealed palaeoyardangs that were submerged during wetter
periods by lacustrine sediments, then revealed by mod-
ern erosion as the sediments are cut into new yardang
fields (Heermance, 2009, personal communication). Wind
strength and frequency varies not only seasonally and spa-
tially, but also over time frames of thousands of years. In
northern Chad, for example, modern sand-laden winds
blow 8 out of 12 months from the northeast, at daytime
velocities of 6 -8 m s
−
1
or greater. The Bodele low-level jet
21.2.3.6
Yardang age, rate of formation and role as
directional indicators
A wide range in yardang ages is reported, resulting from
differences in material erodibility, changing climate and
time available for formation. As seen in Table 21.3, the
majority of yardangs formed in softer sediments probably
developed in less than 2500 years. Small bedrock yardangs
may develop in less than 10 000 years, but megayardangs
probably take millions of years to develop fully.
The development of yardang fields over geologic time
is probably more complex than suggested by many pre-
Table 21.3
Yardang ages and rates of formation.
Erosion rate
(if known)
Location
Age
Material
Yardang scale
Source
Northwestern
Arabia
At least 400 000
years
Cambro-Ordovician
sandstone
Vincent and Kattan
(2006)
Um Al-Riman
Depression,
Kuwait
44 to 1500 years
'Muddy yardangs'
formed in
Quaternary
playa sediments
Small (0.2-5.4 m
in height)
0.5 cm/yr on
flanks; 1 cm/yr
at headward end
Al-Dousari
et al.
(2009)
Rogers Lake,
Mojave Desert,
California
Playa shoreline
deposits
Small
Headward
erosion: 2 cm/yr
Lateral: 0. 5 cm/yr
Ward and Greeley
(1984)
Qaidam basin,
China
Formed in less than
1500-2000 years
Silty sediment
Halimov and Fezer
(1989)
Lop-Nor region,
China
Formed in less than
1500 years
Soft sediments
0.2 cm/yr
Horner (1932),
McCauley, Breed and
Grolier (1977)
Western Desert,
Egypt
Formed in less than
2000 years
Unconsolidated
sediments
∼
4 m
0.2 cm/yr
Brookes (2003a)
Eastern Sahara
Several 1000 years
Soft playa muds
Small (several m
in height)
Haynes (1980)
Bodele
Depression,
Chad
Formed within 1200
to 2400 years
Diatomites
Small (4 m high)
Washington
et al.
(2006); Bristow,
Drake and Armitage,
(2009)
Payun Matru
volcanic field,
Argentina
Formed in less than
10 000 years
Basalt
2-3 m in height
Inbar and Risso (2001)
La Pacana Caldera
1-2 Ma
Ignimbrite sheets
Bailey
et al.
(2007)
Namibia and
Sahara: bedrock
yardangs
Estimate of
hundreds of
thousands to
Bedrock
Hagedorn (1968);
Mainguet (1970);
Goudie (2007)
