424
Climatic Geomorphology
they fluctuate between 0.05 and 30 klTl 2, with an average of 0.2 kin 2 (Goudie and Thomas,
1985). These authors calculate densities of 100 basins every 100 km 2.
Deflation basins are more commonly developed in poorly consolidated sediments.
Some lithologies, such as the sandstones from the south of Africa, are the most suitable for
the development of these closed depressions. Some rocks quickly disintegrate by saline
weathering, such as some claystones with a high salt content, making deflation easier. The
presence of swelling clays implicates that during dry periods clayey regolith cracks
deeply, and the product of this disintegration can be easily exported by the wind (Price,
1963).
During the Late Pleistocene period desiccation of the great pluvial lakes in the western
USA, a period of intense deflation occurred that caused excavation of the lake bottoms,
mobilization of dune fields and generation of lunettes (Goudie and Wells, 1995). Similar
effects are identified on other continents.
Ancient drainage systems developed in large areas of low relief are suitable places to
generate deflation basins. Excavation of some areas of the creeks and small rivers and
generation of small deflation basins may disconnect the drainage networks, although
paleochannel paths are still identifiable (Bowler, 1986). Dunes can also cut off the
channels, producing deflation basins.
In sandy deserts, there are large surface areas with interdune deflation basins. They are
formed as a consequence of the growth of dune crest fields derived from parabolic dunes.
The development of these dunes implies the formation of deflation basins of an elliptical
shape (Lancaster, 1978).
Also in littoral areas, with more humid conditions, is possible to recognize large fields
of deflation basins. In the Carolina Bays, between Maryland and north of Florida, they
cover a strip parallel to the coast of about 1100 km, and about 500,000 basins have been
identified (Prouty, 1952). They were apparently formed during the Wisconsin glaciation as
the elliptical depressions of giant parabolic dunes, built by northwestern winds that blew
along the sandy surfaces of the coastal plain, when the phreatic level was low. When the
water table rose, depressions were stabilized. After that, during dryer periods, many
lunettes were built (Price, 1968; Goudie and Wells, 1995).
In the leeward margins of the deflation basins sand-clay dunes with a half-moon shape
developed commonly. They are called lunettes. They often reach a height between 10 and
50 m, however in the Sebkha Kelbia, in the centre of Tunisia, their maximum height is
165 m, with a 17-km length and a 9-km width, being the highest lunette in the world. The
surface of these dunes is often colonized by vegetation, which facilitates accretion.
Particles from the bottom of the deflation basin become fragmented into micropolyhedrons because of desiccation, and they are mobilized by winds with a speed higher
than 3 m/s, being able to transport these particles and deposit them in the outer rim of the
closed depression (Perthuisot and Jauzein, 1975). The presence of salt, which blocks
vegetation development in the bottom of the depression, causes the aggregation of clay
particles that will be also exported out of the basin. Pellets of salt and clay that form the
lunette are lixiviated by rain, and salt returns in part to the sebkha bottom; so a cycle of salt
is established (Tricart, 1969).
In some cases in the leeward area several lunettes can occur (Figure 17.14). The most
external dune has a much higher content of quartz, and the rate of clay increases toward the
internal dunes, becoming clayey dunes (Bowler, 1973, 1986). This has been explained
Climatic Geomorphology
they fluctuate between 0.05 and 30 klTl 2, with an average of 0.2 kin 2 (Goudie and Thomas,
1985). These authors calculate densities of 100 basins every 100 km 2.
Deflation basins are more commonly developed in poorly consolidated sediments.
Some lithologies, such as the sandstones from the south of Africa, are the most suitable for
the development of these closed depressions. Some rocks quickly disintegrate by saline
weathering, such as some claystones with a high salt content, making deflation easier. The
presence of swelling clays implicates that during dry periods clayey regolith cracks
deeply, and the product of this disintegration can be easily exported by the wind (Price,
1963).
During the Late Pleistocene period desiccation of the great pluvial lakes in the western
USA, a period of intense deflation occurred that caused excavation of the lake bottoms,
mobilization of dune fields and generation of lunettes (Goudie and Wells, 1995). Similar
effects are identified on other continents.
Ancient drainage systems developed in large areas of low relief are suitable places to
generate deflation basins. Excavation of some areas of the creeks and small rivers and
generation of small deflation basins may disconnect the drainage networks, although
paleochannel paths are still identifiable (Bowler, 1986). Dunes can also cut off the
channels, producing deflation basins.
In sandy deserts, there are large surface areas with interdune deflation basins. They are
formed as a consequence of the growth of dune crest fields derived from parabolic dunes.
The development of these dunes implies the formation of deflation basins of an elliptical
shape (Lancaster, 1978).
Also in littoral areas, with more humid conditions, is possible to recognize large fields
of deflation basins. In the Carolina Bays, between Maryland and north of Florida, they
cover a strip parallel to the coast of about 1100 km, and about 500,000 basins have been
identified (Prouty, 1952). They were apparently formed during the Wisconsin glaciation as
the elliptical depressions of giant parabolic dunes, built by northwestern winds that blew
along the sandy surfaces of the coastal plain, when the phreatic level was low. When the
water table rose, depressions were stabilized. After that, during dryer periods, many
lunettes were built (Price, 1968; Goudie and Wells, 1995).
In the leeward margins of the deflation basins sand-clay dunes with a half-moon shape
developed commonly. They are called lunettes. They often reach a height between 10 and
50 m, however in the Sebkha Kelbia, in the centre of Tunisia, their maximum height is
165 m, with a 17-km length and a 9-km width, being the highest lunette in the world. The
surface of these dunes is often colonized by vegetation, which facilitates accretion.
Particles from the bottom of the deflation basin become fragmented into micropolyhedrons because of desiccation, and they are mobilized by winds with a speed higher
than 3 m/s, being able to transport these particles and deposit them in the outer rim of the
closed depression (Perthuisot and Jauzein, 1975). The presence of salt, which blocks
vegetation development in the bottom of the depression, causes the aggregation of clay
particles that will be also exported out of the basin. Pellets of salt and clay that form the
lunette are lixiviated by rain, and salt returns in part to the sebkha bottom; so a cycle of salt
is established (Tricart, 1969).
In some cases in the leeward area several lunettes can occur (Figure 17.14). The most
external dune has a much higher content of quartz, and the rate of clay increases toward the
internal dunes, becoming clayey dunes (Bowler, 1973, 1986). This has been explained