A.
B.
CHAPTER 12 Deserts and Wind
300
As sand is deposited on the slip face, layers form that are
inclined in the direction the wind is blowing. These sloping layers
are called cross beds. When the dunes are eventually buried under
other layers of sediment and become part of the sedimentary rock
record, their asymmetrical shape is destroyed, but the cross beds
remain as testimony to their origin. Nowhere is cross-bedding
more prominent than in the sandstone walls of Zion Canyon in
southern Utah (FIGURE 12.18).
Types of Sand Dunes
Dunes are not just random heaps of windblown sediment. Rather,
they are accumulations that usually assume surprisingly consistent
patterns. A broad assortment of dune forms exists, generally simplified to a few major types for discussion. Of course, gradations
exist among different forms as well as irregularly shaped dunes that
do not fit easily into any category. Several factors influence the
FIGURE 12.17 A. Dunes composed of gypsum sand at White Sands
National Monument in southeastern New Mexico. These dunes are
gradually migrating from the background (top) toward the foreground
(bottom) of the photo. Strong winds move sand up the more gentle
windward slopes. As sand accumulates near the dune crest, the slope
steepens and some of the sand slides down the steeper slip face.
B. Sand sliding down the steep slip face of a dune in White Sands
National Monument, New Mexico. (Photos by Michael Collier)
D I D Y O U K N O W ?
Migrating dunes can lead to costly problems. For
example, to keep a portion of Highway 95 near
Winnemucca, Nevada, open to traffic, sand must be
removed about three times a year. Each time between
1500 and 4000 m
3 of sand are removed. Attempts at
stabilizing the dunes with vegetation have failed.
form and size that dunes ultimately
assume. These include wind direction and
velocity, availability of sand, and the
amount of vegetation present. Six basic
dune types are shown in FIGURE 12.19, with
arrows indicating wind directions.
BARCHAN DUNES. Solitary sand dunes
shaped like crescents with tips pointing
downwind are called barchan dunes
(Figure 12.19A). These dunes form where
supplies of sand are limited and the surface is relatively flat, hard, and lacking vegetation. They migrate slowly with the wind
at a rate of up to 15 meters per year. Their
size is usually modest, with the largest barchans reaching heights
of about 30 meters while the maximum spread between their
horns approaches 300 meters. When the wind direction is nearly
constant, the crescent form of these dunes is nearly symmetrical.
However, when the wind direction is not perfectly fixed, one tip
becomes larger than the other.
TRANSVERSE DUNES. In regions where the prevailing winds are
steady, sand is plentiful, and vegetation is sparse or absent, the
dunes form a series of long ridges that are separated by troughs
and oriented at right angles to the prevailing wind. Because of this
orientation, they are termed transverse dunes (Figure 12.19B).
Typically, many coastal dunes are of this type. In addition, transverse dunes are common in many arid regions where the extensive
surface of wavy sand is sometimes called a sand sea. In some parts
of the Sahara and Arabian deserts, transverse dunes reach heights
of 200 meters, are 1 to 3 kilometers across, and can extend for
distances of 100 kilometers or more.
B.
CHAPTER 12 Deserts and Wind
300
As sand is deposited on the slip face, layers form that are
inclined in the direction the wind is blowing. These sloping layers
are called cross beds. When the dunes are eventually buried under
other layers of sediment and become part of the sedimentary rock
record, their asymmetrical shape is destroyed, but the cross beds
remain as testimony to their origin. Nowhere is cross-bedding
more prominent than in the sandstone walls of Zion Canyon in
southern Utah (FIGURE 12.18).
Types of Sand Dunes
Dunes are not just random heaps of windblown sediment. Rather,
they are accumulations that usually assume surprisingly consistent
patterns. A broad assortment of dune forms exists, generally simplified to a few major types for discussion. Of course, gradations
exist among different forms as well as irregularly shaped dunes that
do not fit easily into any category. Several factors influence the
FIGURE 12.17 A. Dunes composed of gypsum sand at White Sands
National Monument in southeastern New Mexico. These dunes are
gradually migrating from the background (top) toward the foreground
(bottom) of the photo. Strong winds move sand up the more gentle
windward slopes. As sand accumulates near the dune crest, the slope
steepens and some of the sand slides down the steeper slip face.
B. Sand sliding down the steep slip face of a dune in White Sands
National Monument, New Mexico. (Photos by Michael Collier)
D I D Y O U K N O W ?
Migrating dunes can lead to costly problems. For
example, to keep a portion of Highway 95 near
Winnemucca, Nevada, open to traffic, sand must be
removed about three times a year. Each time between
1500 and 4000 m
3 of sand are removed. Attempts at
stabilizing the dunes with vegetation have failed.
form and size that dunes ultimately
assume. These include wind direction and
velocity, availability of sand, and the
amount of vegetation present. Six basic
dune types are shown in FIGURE 12.19, with
arrows indicating wind directions.
BARCHAN DUNES. Solitary sand dunes
shaped like crescents with tips pointing
downwind are called barchan dunes
(Figure 12.19A). These dunes form where
supplies of sand are limited and the surface is relatively flat, hard, and lacking vegetation. They migrate slowly with the wind
at a rate of up to 15 meters per year. Their
size is usually modest, with the largest barchans reaching heights
of about 30 meters while the maximum spread between their
horns approaches 300 meters. When the wind direction is nearly
constant, the crescent form of these dunes is nearly symmetrical.
However, when the wind direction is not perfectly fixed, one tip
becomes larger than the other.
TRANSVERSE DUNES. In regions where the prevailing winds are
steady, sand is plentiful, and vegetation is sparse or absent, the
dunes form a series of long ridges that are separated by troughs
and oriented at right angles to the prevailing wind. Because of this
orientation, they are termed transverse dunes (Figure 12.19B).
Typically, many coastal dunes are of this type. In addition, transverse dunes are common in many arid regions where the extensive
surface of wavy sand is sometimes called a sand sea. In some parts
of the Sahara and Arabian deserts, transverse dunes reach heights
of 200 meters, are 1 to 3 kilometers across, and can extend for
distances of 100 kilometers or more.