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Climatic Geomorphology
blow essentially from a single direction (Sharp, 1949). A well-accepted theory is based in
the rotation of pebbles around a vertical axis, with rare overturning caused by excavation
beneath due to deflation. The capsizing can be also caused by expansion and drying of
clays, and by animal activity (Livingstone and Warren, 1996). This explains that the
smaller the ventifacts, the larger the number of facets they present, and vice versa.
Moreover, overtuming justifies the existence of ventifacts with facets in the upper part and
in the bottom. Some authors have pointed out that these micro-morphologies (polished
surfaces, pits, flutes, grooves, and spiral forms) are probably originated by a vortex of
aeolian dust (wind rotating around an axis), more than by sand impact (Maxson, 1940;
Higgins, 1956). These micro-morphologies can be reproduced in wind-tunnel experiments
(Dietrich, 1977; Whitney, 1979). In any case, most of the shapes related to ventifacts are
explained by sand grain impact, and aeolian dust erosion (Breed et al., 1989, 1997).
Wind direction can be determined from ventifacts and their associated forms, and, as a
consequence, fossil ventifacts provide good records of wind circulation during past
geological periods. If, in addition, they are dated using desert-varnish techniques (Dora,
1995), it is possible to know the paleoenvironmental conditions and the intensity of
aeolian activity.
6.2. Yardangs
The word yardang has its origin from the eastern limit of the Taklimakan desert in China.
The shape of the yardang looks like a capsized ship hull (Figure 17.10) even though in
many cases yardangs are flat topped and lack a keel. The windward face is wider, whereas
the leeside becomes narrower and looses height. Yardangs show varied forms
(Figure 17.11). Halimov and Fezer (1989) described eight types of yardangs in the
depression of Quaidam in China, according to its morphology, and they suggest a temporal
succession. The ratio between longitude and width varies from 3:1 up to 10:1 (McCauley
et al., 1977). Yardangs appear usually grouped, and their long axes are parallel to the
direction of the more intense winds. They occur in every large desert all over the world,
except in the Australian desert. They have been also identified on Mars (Greeley and
Iversen, 1985). Some authors (Cooke et al., 1993) distinguish yardangs according to their
size: microyardangs (1 m) (Figure 17.12), mesoyardangs (10 to 100 m) and megayardangs
(1 km). The more useful classification by Livingstone and Warren (1996) of landforms up
to 100 m long, called yardangs, and those of longer size, that form ridges (Mabbutt, 1977,
Laity, 1994) or megayardangs (Figure 17.13). Cross sections of megayardangs present a U
shape, with the bottom as the trough corridor. They are commonly covered by sand,
pavements, and sometimes, by barchan dunes, parallel to the ridges. Megayardangs are
well developed in the southern part of the Tibesti Massif (Central-Eastern Sahara), where
the ridges can reach up to 200 m in height, up to 4 km in length and a separation between
0.5 and 2 km; they cover a surface area greater than 600,000 km 2 (Hagedom, 1968;
Mainguet, 1972). Very large groups appear also in the hyperarid deserts of Egypt, Iran, and
Perf. Yardangs are carved generally in soft materials, such as aeolian sands and fluvial
sediments and Cenozoic lacustrine deposits, although many of the Saharan ridges are
excavated on Cambrian sandstones. They can be developed in a wide variety of
lithologies, from semi-consolidated sediments to granites or even harder rocks
Climatic Geomorphology
blow essentially from a single direction (Sharp, 1949). A well-accepted theory is based in
the rotation of pebbles around a vertical axis, with rare overturning caused by excavation
beneath due to deflation. The capsizing can be also caused by expansion and drying of
clays, and by animal activity (Livingstone and Warren, 1996). This explains that the
smaller the ventifacts, the larger the number of facets they present, and vice versa.
Moreover, overtuming justifies the existence of ventifacts with facets in the upper part and
in the bottom. Some authors have pointed out that these micro-morphologies (polished
surfaces, pits, flutes, grooves, and spiral forms) are probably originated by a vortex of
aeolian dust (wind rotating around an axis), more than by sand impact (Maxson, 1940;
Higgins, 1956). These micro-morphologies can be reproduced in wind-tunnel experiments
(Dietrich, 1977; Whitney, 1979). In any case, most of the shapes related to ventifacts are
explained by sand grain impact, and aeolian dust erosion (Breed et al., 1989, 1997).
Wind direction can be determined from ventifacts and their associated forms, and, as a
consequence, fossil ventifacts provide good records of wind circulation during past
geological periods. If, in addition, they are dated using desert-varnish techniques (Dora,
1995), it is possible to know the paleoenvironmental conditions and the intensity of
aeolian activity.
6.2. Yardangs
The word yardang has its origin from the eastern limit of the Taklimakan desert in China.
The shape of the yardang looks like a capsized ship hull (Figure 17.10) even though in
many cases yardangs are flat topped and lack a keel. The windward face is wider, whereas
the leeside becomes narrower and looses height. Yardangs show varied forms
(Figure 17.11). Halimov and Fezer (1989) described eight types of yardangs in the
depression of Quaidam in China, according to its morphology, and they suggest a temporal
succession. The ratio between longitude and width varies from 3:1 up to 10:1 (McCauley
et al., 1977). Yardangs appear usually grouped, and their long axes are parallel to the
direction of the more intense winds. They occur in every large desert all over the world,
except in the Australian desert. They have been also identified on Mars (Greeley and
Iversen, 1985). Some authors (Cooke et al., 1993) distinguish yardangs according to their
size: microyardangs (1 m) (Figure 17.12), mesoyardangs (10 to 100 m) and megayardangs
(1 km). The more useful classification by Livingstone and Warren (1996) of landforms up
to 100 m long, called yardangs, and those of longer size, that form ridges (Mabbutt, 1977,
Laity, 1994) or megayardangs (Figure 17.13). Cross sections of megayardangs present a U
shape, with the bottom as the trough corridor. They are commonly covered by sand,
pavements, and sometimes, by barchan dunes, parallel to the ridges. Megayardangs are
well developed in the southern part of the Tibesti Massif (Central-Eastern Sahara), where
the ridges can reach up to 200 m in height, up to 4 km in length and a separation between
0.5 and 2 km; they cover a surface area greater than 600,000 km 2 (Hagedom, 1968;
Mainguet, 1972). Very large groups appear also in the hyperarid deserts of Egypt, Iran, and
Perf. Yardangs are carved generally in soft materials, such as aeolian sands and fluvial
sediments and Cenozoic lacustrine deposits, although many of the Saharan ridges are
excavated on Cambrian sandstones. They can be developed in a wide variety of
lithologies, from semi-consolidated sediments to granites or even harder rocks