4.3 EOLIAN PROCESSES
109
Modern eolian sediment transport and deposition occurs in three situations. They
are found in the arid desert areas of the world, such as the Sahara. They are found erratically developed around ice caps, where the climate may have considerable precipitation, but this is often seasonal and ice bound. Dunes also occur on the crests of barrier islands and beaches in diverse climates. The bulk of eolian sediments consists of
either traction-deposited sands or suspension-deposited silt. These two types are described next.
4.3.1 Eolian Sedimentation from Traction Carpets
The foundation for the study of sand dunes was laid down in a classic book by Bagnold
(1954), updated in 1979. Additional significant work on the physics of eolian sand transport has been described by Owen (1964), Williams (1964), Glennie (1970, 1987), Wilson
(1972), McKee (1979), Greeley and Iverson (1985), Pye and Lancaster (1993), and Lancaster (1995). These studies describe how sediment, blown by the wind, moves by sliding and saltation just like particles in water. Silt and clay are winnowed from the traction
carpet and carried off in dust clouds. Studies of the threshold velocity needed to commence air movement show that, as with aqueous transport, the threshold velocity increases with increasing grain size. Quartz particles of about 0.10 mm (very fine sand) are
the first to move in a rising wind. Silt and clay need velocities as strong as those for fine
sand to initiate movement (Horikowa and Shen, in Allen, 1970). This is analogous to
the Hjulstrom effect for the threshold of particle movement in aqueous flows.
A relationship between bed form and wind velocity has not been worked out in the
same way that the flow regime concept unifies these variables for aqueous flow. Ripples,
dunes, and plane beds are all common eolian sand bed forms. It is a matter of observation that ripples are blown out on both dunes and plane beds during sandstorms, to be
rebuilt as the wind wanes. The factors that control the areal distribution of sand plains
and sand dune field or sand seas are little understood. Attempts have been made, however, to define a model that integrates wind velocity and direction with net sand flow
paths (Wilson, 1971,1972). Particular attention has been paid to the geometry and genesis of sand dunes. Four main morphological types can be defined (Fig. 4.19).
The most beautiful and dramatic sand dune is the barehan or lunate dune. This is
arcuate in plan, convex to the prevailing wind direction, with the two horns pointing
downwind. Barchans have a steep slip face in their concave downwind side (Fig. 4.19A).
Barchans are lonely dunes; typically they occur in isolation or as outriders around the
edges of sand seas. They generally overlie playa mud or granule deflation surfaces. This
suggests that lunate dunes form where sand is in short supply. They are bed forms of
transportation, not of net deposition. It is unlikely that they are often preserved in the
geological record.
The second type to consider is the stellate, pyramidal, or Matterhorn dune
(Fig. 4.19B). These consist of a series of sinuous, sharp, rising sand ridges, which merge
together in a high peak from which wind often blows a plume of sand, making the dune
look as if it were smoking, a dramatic sight in the middle of a desert. These stellate
dunes are sometimes hundreds of meters high. They often form at the boundary of sand
109
Modern eolian sediment transport and deposition occurs in three situations. They
are found in the arid desert areas of the world, such as the Sahara. They are found erratically developed around ice caps, where the climate may have considerable precipitation, but this is often seasonal and ice bound. Dunes also occur on the crests of barrier islands and beaches in diverse climates. The bulk of eolian sediments consists of
either traction-deposited sands or suspension-deposited silt. These two types are described next.
4.3.1 Eolian Sedimentation from Traction Carpets
The foundation for the study of sand dunes was laid down in a classic book by Bagnold
(1954), updated in 1979. Additional significant work on the physics of eolian sand transport has been described by Owen (1964), Williams (1964), Glennie (1970, 1987), Wilson
(1972), McKee (1979), Greeley and Iverson (1985), Pye and Lancaster (1993), and Lancaster (1995). These studies describe how sediment, blown by the wind, moves by sliding and saltation just like particles in water. Silt and clay are winnowed from the traction
carpet and carried off in dust clouds. Studies of the threshold velocity needed to commence air movement show that, as with aqueous transport, the threshold velocity increases with increasing grain size. Quartz particles of about 0.10 mm (very fine sand) are
the first to move in a rising wind. Silt and clay need velocities as strong as those for fine
sand to initiate movement (Horikowa and Shen, in Allen, 1970). This is analogous to
the Hjulstrom effect for the threshold of particle movement in aqueous flows.
A relationship between bed form and wind velocity has not been worked out in the
same way that the flow regime concept unifies these variables for aqueous flow. Ripples,
dunes, and plane beds are all common eolian sand bed forms. It is a matter of observation that ripples are blown out on both dunes and plane beds during sandstorms, to be
rebuilt as the wind wanes. The factors that control the areal distribution of sand plains
and sand dune field or sand seas are little understood. Attempts have been made, however, to define a model that integrates wind velocity and direction with net sand flow
paths (Wilson, 1971,1972). Particular attention has been paid to the geometry and genesis of sand dunes. Four main morphological types can be defined (Fig. 4.19).
The most beautiful and dramatic sand dune is the barehan or lunate dune. This is
arcuate in plan, convex to the prevailing wind direction, with the two horns pointing
downwind. Barchans have a steep slip face in their concave downwind side (Fig. 4.19A).
Barchans are lonely dunes; typically they occur in isolation or as outriders around the
edges of sand seas. They generally overlie playa mud or granule deflation surfaces. This
suggests that lunate dunes form where sand is in short supply. They are bed forms of
transportation, not of net deposition. It is unlikely that they are often preserved in the
geological record.
The second type to consider is the stellate, pyramidal, or Matterhorn dune
(Fig. 4.19B). These consist of a series of sinuous, sharp, rising sand ridges, which merge
together in a high peak from which wind often blows a plume of sand, making the dune
look as if it were smoking, a dramatic sight in the middle of a desert. These stellate
dunes are sometimes hundreds of meters high. They often form at the boundary of sand