Chapter 18
Aeolian accumulations
1. Introduction
Aeolian accumulations are the result of sedimentation of particles carried by the wind,
producing deposits of sand and dust. We shall begin with the study of the aeolian sands,
analysing their distribution, processes and types of accumulations.
Dunes are, undoubtedly the most outstanding of desert forms (Figure 18.1). Research
on these landforms has experienced a boost during the last two decades with the use of
satellite images. Aeolian sands cover about 5% of the Earth's surface (Thomas, 1997d),
and 20% of the arid zones of the world are occupied by sand accumulations. The
proportion of this cover varies according different deserts, less than 1% in North and South
America (Lancaster, 1995) up to 50% in Australia (Mabbutt, 1977). The word erg or sand
sea means a large surface covered by sand, at least 125 km 2 (Fryberger and Ahlbrandt,
1979; Thomas, 1997d). The erg is called koum in Central Asia and nafud in Arabia. We
can see the global distribution of great ergs in Figure 18.2, where active ergs are
distinguished from marginal ergs, stabilized by vegetal colonization. Most ergs are active
with an average annual precipitation of 150 mm (Wilson, 1973). The largest number of
sand seas is concentrated in the Northern Hemisphere. Wilson (1973) identified 58 ergs
with a surface extent greater than 12,000 km 2 in the different deserts all over the world,
most of them in the Sahara desert, where the erg Chech in southern Argelia (312,000 km 2)
is the largest in this desert. The erg of Rub al Khali, which occupies one-third of the
Arabian Peninsula (Figure 18.3), has a surface area of 560,000 km 2, and the Great SandyGibson Desert, in western Australia, covers a surface area of 630,000 klTl 2.
In some areas it is difficult to distinguish between coastal and desertic dunes. Examples
dealing with this problem occur in the coastal zones of Namibia (Lancaster, 1982;
Lancaster and Ollier, 1983), Pert] (Finkel, 1959) and Oman (Goudie et al., 1987). In some
zones, continental dunes obtain their sand supply in the coastal regions, whereas in other
areas ergs have prograded towards the sea (Pye and Tsoar, 1990).
A hierarchic classification of ergs (Wilson, 1972; Lancaster, 1994) is based on
the identification of three classes with distinct spatial patterns: (1) aeolian ripples (0.1
to 1 m); (2) isolated individual dunes (50 to 500 m); and (3) compound and complex
dunes (> 500 m). Each element in this hierarchy responds to the dynamics of the wind
pattern of the region.
Other large bodies of aeolian sands do not have dune forms and are called sand sheets.
Their surface area ranges from a few square kilometers up to 100,000 K1TI 2 in the eastern
Sahara (Breed et al., 1987). Their development is controlled by vegetation, grain size,
shallow phreatic levels, seasonal floods, and the occurrence of superficial crusts (Kocurek
and Nielson, 1986).
Aeolian accumulations
1. Introduction
Aeolian accumulations are the result of sedimentation of particles carried by the wind,
producing deposits of sand and dust. We shall begin with the study of the aeolian sands,
analysing their distribution, processes and types of accumulations.
Dunes are, undoubtedly the most outstanding of desert forms (Figure 18.1). Research
on these landforms has experienced a boost during the last two decades with the use of
satellite images. Aeolian sands cover about 5% of the Earth's surface (Thomas, 1997d),
and 20% of the arid zones of the world are occupied by sand accumulations. The
proportion of this cover varies according different deserts, less than 1% in North and South
America (Lancaster, 1995) up to 50% in Australia (Mabbutt, 1977). The word erg or sand
sea means a large surface covered by sand, at least 125 km 2 (Fryberger and Ahlbrandt,
1979; Thomas, 1997d). The erg is called koum in Central Asia and nafud in Arabia. We
can see the global distribution of great ergs in Figure 18.2, where active ergs are
distinguished from marginal ergs, stabilized by vegetal colonization. Most ergs are active
with an average annual precipitation of 150 mm (Wilson, 1973). The largest number of
sand seas is concentrated in the Northern Hemisphere. Wilson (1973) identified 58 ergs
with a surface extent greater than 12,000 km 2 in the different deserts all over the world,
most of them in the Sahara desert, where the erg Chech in southern Argelia (312,000 km 2)
is the largest in this desert. The erg of Rub al Khali, which occupies one-third of the
Arabian Peninsula (Figure 18.3), has a surface area of 560,000 km 2, and the Great SandyGibson Desert, in western Australia, covers a surface area of 630,000 klTl 2.
In some areas it is difficult to distinguish between coastal and desertic dunes. Examples
dealing with this problem occur in the coastal zones of Namibia (Lancaster, 1982;
Lancaster and Ollier, 1983), Pert] (Finkel, 1959) and Oman (Goudie et al., 1987). In some
zones, continental dunes obtain their sand supply in the coastal regions, whereas in other
areas ergs have prograded towards the sea (Pye and Tsoar, 1990).
A hierarchic classification of ergs (Wilson, 1972; Lancaster, 1994) is based on
the identification of three classes with distinct spatial patterns: (1) aeolian ripples (0.1
to 1 m); (2) isolated individual dunes (50 to 500 m); and (3) compound and complex
dunes (> 500 m). Each element in this hierarchy responds to the dynamics of the wind
pattern of the region.
Other large bodies of aeolian sands do not have dune forms and are called sand sheets.
Their surface area ranges from a few square kilometers up to 100,000 K1TI 2 in the eastern
Sahara (Breed et al., 1987). Their development is controlled by vegetation, grain size,
shallow phreatic levels, seasonal floods, and the occurrence of superficial crusts (Kocurek
and Nielson, 1986).