418
Climatic Geomorphology
In conclusion, aeolian ripples are closely related to sand saltation, although new
models suggest that reptation provides the initial conditions for tipple formation, and
they quickly evolve by saltation impacts and, as a consequence, by grains transported
by reptation (Lancaster, 1995).
6. Aeolian erosion and resulting landforms
When particles are lifted by the wind from the surface and impact on the rocks at
significant speeds, they produce aeolian erosion landforms. They are more widely spread
in areas of scarce or absent vegetation, especially in arid zones, but are also present in
periglacial and marine beaches environments.
Wind erodes by deflation, that is, the result of entrainment and transport of loose
material of the ground. Average deflation rate is about 1 mm/year (Cooke et al., 1993).
Abrasion is the mechanical wearing out of cohesive material. Ballistic impacts of grains
cause surface rock fracture, until the surface becomes stabilized. This suggests that
abrasion speed reaches a maximum and later decays. Most authors estimate that abrasion
is produced only by sand grains, however others think also that the aeolian dust originates
abrasion, although secondarily (Greeley and Iversen, 1985). Abrasion speed rates are very
variable, about several millimeters per year, and they depend on the type of rock, wind
speed and amount of grains provided by the wind. The height where maximum abrasion
occurs fluctuates between 0.1 and 0.4 m. These figures have been obtained from field
experiments during intense storms (Sakamoto-Arnold, 1981) and from theoretical models
(Anderson, 1986).
6.1. Ventifacts
Ventifacts are pebbles with smooth surfaces, polished and faceted by the impact of
particles carried by the wind (Figure 17.9). Pits, flutes, grooves, and helicoid patterns are
developed both in boulders and in rock outcrops. These micro-morphologies appear only
in weathering-resistant rocks.
Facets are oriented perpendicularly to wind direction. The number of facets is very
variable, and up to 20 have been described (Higgins, 1956). The most common, however,
is the presence of two or three facets that intersect forming edges. The German words
einkanter, zweikanter, and dreikanter are commonly used to indicate that they have one,
two, or three facets, respectively. Pits are small closed depressions that can be originated
by the action of wind over a nonhomogeneous rock or on pre-existent holes. They are
inclined 55 to 90 ~ from wind direction and, in consequence, they indicate the windward
side. Where the angle decreases considerably we find the transition toflutes (Sharp, 1949).
These have a spoon-shaped form, with dead ends and U-shaped cross sections. Commonly
they are open to the windward. Their length, width, and height vary greatly, from
millimetric sizes up to centimetric ones (Cooke et al., 1993). They appear in horizontal
surfaces or in gentle slopes, in very different kinds of rocks, and they are not related to the
structure (Breed et al., 1989; Laity, 1994). Grooves are opened in both extremes and are
longer than flutes. They indicate, as those described before, the predominant wind
Climatic Geomorphology
In conclusion, aeolian ripples are closely related to sand saltation, although new
models suggest that reptation provides the initial conditions for tipple formation, and
they quickly evolve by saltation impacts and, as a consequence, by grains transported
by reptation (Lancaster, 1995).
6. Aeolian erosion and resulting landforms
When particles are lifted by the wind from the surface and impact on the rocks at
significant speeds, they produce aeolian erosion landforms. They are more widely spread
in areas of scarce or absent vegetation, especially in arid zones, but are also present in
periglacial and marine beaches environments.
Wind erodes by deflation, that is, the result of entrainment and transport of loose
material of the ground. Average deflation rate is about 1 mm/year (Cooke et al., 1993).
Abrasion is the mechanical wearing out of cohesive material. Ballistic impacts of grains
cause surface rock fracture, until the surface becomes stabilized. This suggests that
abrasion speed reaches a maximum and later decays. Most authors estimate that abrasion
is produced only by sand grains, however others think also that the aeolian dust originates
abrasion, although secondarily (Greeley and Iversen, 1985). Abrasion speed rates are very
variable, about several millimeters per year, and they depend on the type of rock, wind
speed and amount of grains provided by the wind. The height where maximum abrasion
occurs fluctuates between 0.1 and 0.4 m. These figures have been obtained from field
experiments during intense storms (Sakamoto-Arnold, 1981) and from theoretical models
(Anderson, 1986).
6.1. Ventifacts
Ventifacts are pebbles with smooth surfaces, polished and faceted by the impact of
particles carried by the wind (Figure 17.9). Pits, flutes, grooves, and helicoid patterns are
developed both in boulders and in rock outcrops. These micro-morphologies appear only
in weathering-resistant rocks.
Facets are oriented perpendicularly to wind direction. The number of facets is very
variable, and up to 20 have been described (Higgins, 1956). The most common, however,
is the presence of two or three facets that intersect forming edges. The German words
einkanter, zweikanter, and dreikanter are commonly used to indicate that they have one,
two, or three facets, respectively. Pits are small closed depressions that can be originated
by the action of wind over a nonhomogeneous rock or on pre-existent holes. They are
inclined 55 to 90 ~ from wind direction and, in consequence, they indicate the windward
side. Where the angle decreases considerably we find the transition toflutes (Sharp, 1949).
These have a spoon-shaped form, with dead ends and U-shaped cross sections. Commonly
they are open to the windward. Their length, width, and height vary greatly, from
millimetric sizes up to centimetric ones (Cooke et al., 1993). They appear in horizontal
surfaces or in gentle slopes, in very different kinds of rocks, and they are not related to the
structure (Breed et al., 1989; Laity, 1994). Grooves are opened in both extremes and are
longer than flutes. They indicate, as those described before, the predominant wind