Title: Deserts - Geology and Resources
Author: Walker, A. S.
Language: English
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       U. S. Department of the Interior / U. S. Geological Survey



                                Deserts:
                         Geology and Resources


                            by A. S. Walker

[Illustration: Great Sand Dunes National Monument, Colorado (photograph
by John Keith).]

  _Beauty is before me_
  _And beauty behind me,_
  _Above and below me hovers the beautiful,_
  _I am surrounded by it,_
  _I am immersed in it._
  _In my youth I am aware of it,_
  _And in old age_
  _I shall walk quietly_
  _The beautiful trail._

  _from a Navajo benedictory chant describing the desert_

[Illustration: Cacti dominate the Sonoran Desert vegetation near Tucson,
Arizona (photograph by Peter Kresan).]



                           What Is a Desert?


Approximately one-third of the Earth’s land surface is desert, arid land
with meager rainfall that supports only sparse vegetation and a limited
population of people and animals. Deserts—stark, sometimes mysterious
worlds—have been portrayed as fascinating environments of adventure and
exploration from narratives such as that of Lawrence of Arabia to movies
such as “Dune.” These arid regions are called _deserts_ because they are
dry. They may be hot, they may be cold. They may be regions of sand or
vast areas of rocks and gravel peppered with occasional plants. But
deserts are always dry.

[Illustration: Ripples on a dune in the Gran Desierto, Mexico
(photograph by Peter Kresan).]

Deserts are natural laboratories in which to study the interactions of
wind and sometimes water on the arid surfaces of planets. They contain
valuable mineral deposits that were formed in the arid environment or
that were exposed by erosion. Because deserts are dry, they are ideal
places for human artifacts and fossils to be preserved. Deserts are also
fragile environments. The misuse of these lands is a serious and growing
problem in parts of our world.

[Illustration: DISTRIBUTION OF NON-POLAR ARID LAND (after Meigs, 1953)]

                         [Higher-resolution map]

There are almost as many definitions of deserts and classification
systems as there are deserts in the world. Most classifications rely on
some combination of the number of days of rainfall, the total amount of
annual rainfall, temperature, humidity, or other factors. In 1953,
Peveril Meigs divided desert regions on Earth into three categories
according to the amount of precipitation they received. In this now
widely accepted system, extremely arid lands have at least 12
consecutive months without rainfall, arid lands have less than 250
millimeters of annual rainfall, and semiarid lands have a mean annual
precipitation of between 250 and 500 millimeters. Arid and extremely
arid land are deserts, and semiarid grasslands generally are referred to
as steppes.

[Illustration: Gran Desierto of the Sonoran Desert, Mexico. Surrounding
the dark 25-kilometer-long and 5-kilometer-wide Sierra del Rosario
mountains (upper right) are dunes and sheets of sand.]



                 How the Atmosphere Influences Aridity


We live at the bottom of a gaseous envelope—the atmosphere—that is bound
gravitationally to the planet Earth. The circulation of our atmosphere
is a complex process because of the Earth’s rotation and the tilt of its
axis. The Earth’s axis is inclined 23½° from the ecliptic, the plane of
the Earth’s orbit around the Sun. Due to this inclination, vertical rays
of the Sun strike 23½° N. latitude, the Tropic of Cancer, at summer
solstice in late June. At winter solstice, the vertical rays strike 23½°
S. latitude, the Tropic of Capricorn. In the Northern Hemisphere, the
summer solstice day has the most daylight hours, and the winter solstice
has the fewest daylight hours each year. The tilt of the axis allows
differential heating of the Earth’s surface, which causes seasonal
changes in the global circulation.

[Illustration: The circulation pattern of the Earth’s atmosphere. Most
of the nonpolar deserts lie within the two trade winds belts.]

On a planetary scale, the circulation of air between the hot Equator and
the cold North and South Poles creates pressure belts that influence
weather. Air warmed by the Sun rises at the Equator, cools as it moves
toward the poles, descends as cold air over the poles, and warms again
as it moves over the surface of the Earth toward the Equator. This
simple pattern of atmospheric convection, however, is complicated by the
rotation of the Earth, which introduces the Coriolis Effect.

To appreciate the origin of this effect, consider the following. A stick
placed vertically in the ground at the North Pole would simply turn
around as the Earth rotates. A stick at the Equator would move in a
large circle of almost 40,000 kilometers with the Earth as it rotates.

The Coriolis Effect illustrates Newton’s first law of motion—a body in
motion will maintain its speed and direction of motion unless acted on
by some outside force. Thus, a wind travelling north from the equator
will maintain the velocity acquired at the equator while the Earth under
it is moving slower. This effect accounts for the generally east-west
direction of winds, or streams of air, on the Earth’s surface. Winds
blow between areas of different atmospheric pressures.

The Coriolis Effect influences the circulation pattern of the Earth’s
atmosphere. In the zone between about 30° N. and 30° S., the surface air
flows toward the Equator and the flow aloft is poleward. A low-pressure
area of calm, light variable winds near the equator is known to mariners
as the _doldrums_.

Around 30° N. and S., the poleward flowing air begins to descend toward
the surface in subtropical high-pressure belts. The sinking air is
relatively dry because its moisture has already been released near the
Equator above the tropical rain forests. Near the center of this
high-pressure zone of descending air, called the “Horse Latitudes,” the
winds at the surface are weak and variable. The name for this area is
believed to have been given by colonial sailors, who, becalmed sometimes
at these latitudes while crossing the oceans with horses as cargo, were
forced to throw a few horses overboard to conserve water.

The surface air that flows from these subtropical high-pressure belts
toward the Equator is deflected toward the west in both hemispheres by
the Coriolis Effect. Because winds are named for the direction from
which the wind is blowing, these winds are called the northeast trade
winds in the Northern Hemisphere and the southeast trade winds in the
Southern Hemisphere. The trade winds meet at the doldrums. Surface winds
known as “westerlies” flow from the Horse Latitudes toward the poles.
The “westerlies” meet “easterlies” from the polar highs at about 50-60°
N. and S.

Near the ground, wind direction is affected by friction and by changes
in topography. Winds may be seasonal, sporadic, or daily. They range
from gentle breezes to violent gusts at speeds greater than 300
kilometers/hour.

[Illustration: These dunes in the Algodones Sand Sea of southeastern
California move as much as 5 meters per year. The dunes in this
photograph, looking south, move toward the east (left) (photograph by
Peter Kresan).]

[Illustration: Searles Lake, California (photograph courtesy of
Kerr-McGee, Inc.).]



                           Where Deserts Form


Dry areas created by global circulation patterns contain most of the
deserts on the Earth. The deserts of our world are not restricted by
latitude, longitude, or elevation. They occur from areas close to the
poles down to areas near the Equator. The People’s Republic of China has
both the highest desert, the Qaidam Depression that is 2,600 meters
above sea level, and one of the lowest deserts, the Turpan Depression
that is 150 meters below sea level.

Deserts are not confined to Earth. The atmospheric circulation patterns
of other terrestrial planets with gaseous envelopes also depend on the
rotation of those planets, the tilts of their axes, their distances from
the Sun, and the composition and density of their atmospheres. Except
for the poles, the entire surface of Mars is a desert. Venus also may
support deserts.

[Illustration: The Garlock fault, near the bottom of this Landsat image,
is generally considered to be the geologic border between the Mojave
Desert in the south and the Great Basin Desert in the north. The Great
Basin contains more than 150 discrete desert basins separated by more
than 160 mountain ranges.]



                            Types of Deserts


Deserts are classified by their geographical location and dominant
weather pattern as _trade wind_, _midlatitude_, _rain shadow_,
_coastal_, _monsoon_, or _polar_ deserts. Former desert areas presently
in nonarid environments are _paleodeserts_, and _extraterrestrial_
deserts exist on other planets.


                           Trade wind deserts

The trade winds in two belts on the equatorial sides of the Horse
Latitudes heat up as they move toward the Equator. These dry winds
dissipate cloud cover, allowing more sunlight to heat the land. Most of
the major deserts of the world lie in areas crossed by the trade winds.
The world’s largest desert, the Sahara of North Africa, which has
experienced temperatures as high as 57° C, is a trade wind desert.


                          Midlatitude deserts

Midlatitude deserts occur between 30° and 50° N. and S., poleward of the
subtropical high-pressure zones. These deserts are in interior drainage
basins far from oceans and have a wide range of annual temperatures. The
Sonoran Desert of southwestern North America is a typical midlatitude
desert.


                          Rain shadow deserts

Rain shadow deserts are formed because tall mountain ranges prevent
moisture-rich clouds from reaching areas on the lee, or protected side,
of the range. As air rises over the mountain, water is precipitated and
the air loses its moisture content. A desert is formed in the leeside
“shadow” of the range.

[Illustration: The Sahara of Africa is the world’s largest desert. It
contains complex linear dunes that are separated by almost 6 kilometers
(Skylab photograph).]

[Illustration: A rare rain in the Tengger, a midlatitude desert of
China, exposes ripples and a small blowout on the left. Winds will
shortly cover or remove these features.]

[Illustration: This Landsat image shows the Turpan Depression in the
rain shadow desert of the Tian Shan of China. A sand sea is in the lower
center on the right, but desert pavement, gray in color, dominates this
desert. The few oases in the desert and the vegetation in the mountains
at the top are in red. A blanket of snow separates the vegetation in the
Tian Shan from the rain shadow desert.]


                            Coastal deserts

Coastal deserts generally are found on the western edges of continents
near the Tropics of Cancer and Capricorn. They are affected by cold
ocean currents that parallel the coast. Because local wind systems
dominate the trade winds, these deserts are less stable than other
deserts. Winter fogs, produced by upwelling cold currents, frequently
blanket coastal deserts and block solar radiation. Coastal deserts are
relatively complex because they are at the juncture of terrestrial,
oceanic, and atmospheric systems. A coastal desert, the Atacama of South
America, is the Earth’s driest desert. In the Atacama, measurable
rainfall—1 millimeter or more of rain—may occur as infrequently as once
every 5-20 years.

[Illustration: Crescent-shaped dunes are common in coastal deserts such
as the Namib, Africa, with prevailing onshore winds. Low clouds cover
parts of the Namib in this space shuttle photo.]

[Illustration: High dunes of the Namib Desert near Sossus Vlei
(photograph by Georg Gerster).]

[Illustration: Morning fog moistens the dunes of the Namib coastal
desert (photograph by Georg Gerster).]


                            Monsoon deserts

“Monsoon,” derived from an Arabic word for “season,” refers to a wind
system with pronounced seasonal reversal. Monsoons develop in response
to temperature variations between continents and oceans. The southeast
trade winds of the Indian Ocean, for example, provide heavy summer rains
in India as they move onshore. As the monsoon crosses India, it loses
moisture on the eastern slopes of the Aravalli Range. The Rajasthan
Desert of India and the Thar Desert of Pakistan are parts of a monsoon
desert region west of the range.

[Illustration: The Indus River floodplain, lower left, is the western
border of the Thar Desert. This Landsat image of the monsoon desert
shows small patches of sand sheets in the upper right, with three types
of dunes; some dunes are almost 3 kilometers long.]


                             Polar deserts

Polar deserts are areas with annual precipitation less than 250
millimeters and a mean temperature during the warmest month of less than
10° C. Polar deserts on the Earth cover nearly 5 million square
kilometers and are mostly bedrock or gravel plains. Sand dunes are not
prominent features in these deserts, but snow dunes occur commonly in
areas where precipitation is locally more abundant.

Temperature changes in polar deserts frequently cross the freezing point
of water. This “freeze-thaw” alternation forms patterned textures on the
ground, as much as 5 meters in diameter.

[Illustration: The Dry Valleys of Antarctica have been ice-free for
thousands of years (courtesy of USGS Image Processing Facility,
Flagstaff, Arizona).]


                              Paleodeserts

[Illustration: This aerial photograph of the Nebraska Sand Hills
paleodesert shows a well-preserved crescent-shaped dune (or barchan)
about 60 to 75 meters high (photograph by Thomas S. Ahlbrandt).]

Data on ancient sand seas (vast regions of sand dunes), changing lake
basins, archaeology, and vegetation analyses indicate that climatic
conditions have changed considerably over vast areas of the Earth in the
recent geologic past. During the last 12,500 years, for example, parts
of the deserts were more arid than they are today. About 10 percent of
the land between 30° N. and 30° S. is covered now by sand seas. Nearly
18,000 years ago, sand seas in two vast belts occupied almost 50 percent
of this land area. As is the case today, tropical rain forests and
savannahs were between the two belts.

[Illustration: A dry community of vegetation grows among the dunes of
the Nebraska Sand Hills (photograph by N. H. Darton).]

Fossil desert sediments that are as much as 500 million years old have
been found in many parts of the world. Sand dune-like patterns have been
recognized in presently nonarid environments. Many such relict dunes now
receive from 80 to 150 millimeters of rain each year. Some ancient dunes
are in areas now occupied by tropical rain forests.

The Nebraska Sand Hills is an inactive 57,000-square kilometer dune
field in central Nebraska. The largest sand sea in the Western
Hemisphere, it is now stabilized by vegetation and receives about 500
millimeters of rain each year. Dunes in the Sand Hills are up to 120
meters high.


                        Extraterrestrial deserts

Mars is the only other planet on which we have identified wind-shaped
(_eolian_) features. Although its surface atmospheric pressure is only
about one-hundredth that of Earth, global circulation patterns on Mars
have formed a circumpolar sand sea of more than five million square
kilometers, an area greater than the Empty Quarter of Saudi Arabia, the
largest sand sea on our planet. Martian sand seas consist predominantly
of crescent-shaped dunes on plains near the perennial ice cap of the
north polar area. Smaller dune fields occupy the floors of many large
craters in the polar regions.

[Illustration: This Viking spacecraft image of Mars shows alternating
layers of ice and windblown dust near the north polar cap. Annual and
other periodic climatic changes due to orbit fluctuations may occur on
Mars (courtesy of USGS Image Processing Facility, Flagstaff, Arizona).]

[Illustration: One of the first images taken at the Viking 2 landing
site on Mars shows the pink sky over Utopia and the desert pavement on
the ground (courtesy of NASA).]

[Illustration: Some of the crescent-shaped dunes in this Viking image of
Mars are more than a kilometer wide. The dark material that streaks from
the horn-shaped features may be dust recently blown from the dunes
(courtesy of NASA).]

[Illustration: Natural Bridge, Arches National Monument, Utah
(photograph by Peter Kresan).]



                            Desert Features


Sand covers only about 20 percent of the Earth’s deserts. Most of the
sand is in sand sheets and sand seas—vast regions of undulating dunes
resembling ocean waves “frozen” in an instant of time.

Nearly 50 percent of desert surfaces are plains where eolian
deflation—removal of fine-grained material by the wind—has exposed loose
gravels consisting predominantly of pebbles but with occasional cobbles.

The remaining surfaces of arid lands are composed of exposed bedrock
outcrops, desert soils, and fluvial deposits including alluvial fans,
playas, desert lakes, and oases. Bedrock outcrops commonly occur as
small mountains surrounded by extensive erosional plains.

Oases are vegetated areas moistened by springs, wells, or by irrigation.
Many are artificial. Oases are often the only places in deserts that
support crops and permanent habitation.

[Illustration: Underground channels carry water from nearby mountains
into the Turpan Depression of China. If the channels were not covered,
the water would evaporate quickly when it reached the hot, dry desert
land.]


                                 Soils

Soils that form in arid climates are predominantly mineral soils with
low organic content. The repeated accumulation of water in some soils
causes distinct salt layers to form. Calcium carbonate precipitated from
solution may cement sand and gravel into hard layers called “calcrete”
that form layers up to 50 meters thick.

Caliche is a reddish-brown to white layer found in many desert soils.
Caliche commonly occurs as nodules or as coatings on mineral grains
formed by the complicated interaction between water and carbon dioxide
released by plant roots or by decaying organic material.


                                 Plants

Most desert plants are drought- or salt-tolerant. Some store water in
their leaves, roots, and stems. Other desert plants have long tap roots
that penetrate the water table, anchor the soil, and control erosion.
The stems and leaves of some plants lower the surface velocity of
sand-carrying winds and protect the ground from erosion.

[Illustration: Sparse, very dry, single species vegetation in Death
Valley, California.]

[Illustration: Vegetation amidst the desert pavement of the Sonoran
Desert (photograph by John Olsen).]

Deserts typically have a plant cover that is sparse but enormously
diverse. The Sonoran Desert of the American Southwest has the most
complex desert vegetation on Earth. The giant saguaro cacti provide
nests for desert birds and serve as “trees” of the desert. Saguaro grow
slowly but may live 200 years. When 9 years old, they are about 15
centimeters high. After about 75 years, the cacti are tall and develop
their first branches. When fully grown, saguaro are 15 meters tall and
weigh as much as 10 tons. They dot the Sonoran and reinforce the general
impression of deserts as cacti-rich land.

Although cacti are often thought of as characteristic desert plants,
other types of plants have adapted well to the arid environment. They
include the pea family and sunflower family. Cold deserts have grasses
and shrubs as dominant vegetation.


                                 Water

Rain does fall occasionally in deserts, and desert storms are often
violent. A record 44 millimeters of rain once fell within 3 hours in the
Sahara. Large Saharan storms may deliver up to 1 millimeter per minute.
Normally dry stream channels, called arroyos or wadis, can quickly fill
after heavy rains, and flash floods make these channels dangerous. More
people drown in deserts than die of thirst.

Though little rain falls in deserts, deserts receive runoff from
ephemeral, or short-lived, streams fed by rain and snow from adjacent
highlands. These streams fill the channel with a slurry of mud and
commonly transport considerable quantities of sediment for a day or two.
Although most deserts are in basins with closed, or interior drainage, a
few deserts are crossed by ‘exotic’ rivers that derive their water from
outside the desert. Such rivers infiltrate soils and evaporate large
amounts of water on their journeys through the deserts, but their
volumes are such that they maintain their continuity. The Nile, the
Colorado, and the Yellow are exotic rivers that flow through deserts to
deliver their sediments to the sea.

[Illustration: The Wei River in the Loess Plateau, China (photograph by
I-Ming Chou).]

[Illustration: Running water created this canyon in arid Big Bend
National Park, southwest Texas.]

Lakes form where rainfall or meltwater in interior drainage basins is
sufficient. Desert lakes are generally shallow, temporary, and salty.
Because these lakes are shallow and have a low bottom gradient, wind
stress may cause the lake waters to move over many square kilometers.
When small lakes dry up, they leave a salt crust or hardpan. The flat
area of clay, silt, or sand encrusted with salt that forms is known as a
_playa_. There are more than a hundred playas in North American deserts.
Most are relics of large lakes that existed during the last Ice Age
about 12,000 years ago. Lake Bonneville was a 52,000-square-kilometer
lake almost 300 meters deep in Utah, Nevada, and Idaho during the Ice
Age. Today the remnants of Lake Bonneville include Utah’s Great Salt
Lake, Utah Lake, and Sevier Lake. Because playas are arid land forms
from a wetter past, they contain useful clues to climatic change.

The flat terrains of hardpans and playas make them excellent race tracks
and natural runways for airplanes and spacecraft. Ground-vehicle speed
records are commonly established on Bonneville Speedway, a race track on
the Great Salt Lake hardpan. Space shuttles land on Rogers Lake Playa at
Edwards Air Force Base, California.

[Illustration: The Qaidam Depression in China is the highest desert in
the world. This Landsat image illustrates a salt lake and evaporite
basins in the depression.]



                            Eolian Processes


Eolian processes pertain to the activity of the winds. Winds may erode,
transport, and deposit materials, and are effective agents in regions
with sparse vegetation and a large supply of unconsolidated sediments.
Although water is much more powerful than wind, eolian processes are
important in arid environments.


                             Eolian erosion

Wind erodes the Earth’s surface by deflation, the removal of loose,
fine-grained particles by the turbulent eddy action of the wind, and by
abrasion, the wearing down of surfaces by the grinding action and sand
blasting of windborne particles.

Most eolian deflation zones are composed of _desert pavement_, a
sheetlike surface of rock fragments that remains after wind and water
have removed the fine particles. Almost half of the Earth’s desert
surfaces are stony deflation zones. The rock mantle in desert pavements
protects the underlying material from deflation.

[Illustration: The sand and rock of China’s Turpan Depression resemble
closely those in the view of the Martian surface on page 21.]

[Illustration: The arrow points to shiny black desert varnish on these
rocks of Egypt’s southwest desert (photograph by Carol Breed).]

A dark, shiny stain, called _desert varnish_ or rock varnish, is often
found on surfaces of some desert rocks that have been exposed at the
surface for a long period of time. Manganese, iron oxides, hydroxides,
and clay minerals form most varnishes and provide the shine.

Deflation basins, called _blowouts_, are hollows formed by the removal
of particles by wind. Blowouts are generally small, but may be up to
several kilometers in diameter.

Wind-driven grains abrade landforms. Grinding by particles carried in
the wind creates grooves or small depressions. _Ventifacts_ are rocks
which have been cut, and sometimes polished, by the abrasive action of
wind.

Sculpted landforms, called _yardangs_, are up to tens of meters high and
kilometers long and are forms that have been streamlined by desert
winds. The famous sphinx at Giza in Egypt may be a modified yardang.

    Yardangs of the Lut Desert of Iran. These yardangs are among the
          largest on Earth, with almost 100 meters of relief.

[Illustration: View from Landsat.]

[Illustration: View from high-altitude photograph (photograph by U.S.
Air Force).]

[Illustration: View from low-altitude photograph (photograph by J.T.
Daniels).]


                         Eolian transportation

Particles are transported by winds through suspension, saltation, and
creep.

Small particles may be held in the atmosphere in _suspension_. Upward
currents of air support the weight of suspended particles and hold them
indefinitely in the surrounding air. Typical winds near the Earth’s
surface suspend particles less than 0.2 millimeters in diameter and
scatter them aloft as dust or haze.

_Saltation_ is downwind movement of particles in a series of jumps or
skips. Saltation normally lifts sand-size particles no more than one
centimeter above the ground, and proceeds at one-half to one-third the
speed of the wind. A saltating grain may hit other grains that jump up
to continue the saltation. The grain may also hit larger grains that are
too heavy to hop, but that slowly _creep_ forward as they are pushed by
saltating grains. Surface creep accounts for as much as 25 percent of
grain movement in a desert.

Eolian turbidity currents are better known as _dust storms_. Air over
deserts is cooled significantly when rain passes through it. This cooler
and denser air sinks toward the desert surface. When it reaches the
ground, the air is deflected forward and sweeps up surface debris in its
turbulence as a dust storm.

[Illustration: Saltation moves small particles in the direction of the
wind in a series of short hops or skips.]

Crops, people, villages, and possibly even climates are affected by dust
storms. Some dust storms are intercontinental, a few may circle the
globe, and occasionally they may engulf entire planets. When the Mariner
9 spacecraft arrived at Mars in 1971, the entire planet was enshrouded
in global dust.

[Illustration: Dust storm along the Mohave River near Daggett,
California, October 24, 1919 (photograph by D. G. Thompson).]

Most of the dust carried by dust storms is in the form of silt-size
particles. Deposits of this windblown silt are known as _loess_. The
thickest known deposit of loess, 335 meters, is on the Loess Plateau in
China. In Europe and in the Americas, accumulations of loess are
generally from 20 to 30 meters thick.

Small whirlwinds, called _dust devils_, are common in arid lands and are
thought to be related to very intense local heating of the air that
results in instabilities of the air mass. Dust devils may be as much as
one kilometer high.


                           Eolian deposition

Wind-deposited materials hold clues to past as well as to present wind
directions and intensities. These features help us understand the
present climate and the forces that molded it. Wind-deposited sand
bodies occur as sand sheets, ripples, and dunes.

_Sand sheets_ are flat, gently undulating sandy plots of sand surfaced
by grains that may be too large for saltation. They form approximately
40 percent of eolian depositional surfaces. The Selima Sand Sheet, which
occupies 60,000 square kilometers in southern Egypt and northern Sudan,
is one of the Earth’s largest sand sheets. The Selima is absolutely flat
in some places; in others, active dunes move over its surface.

Wind blowing on a sand surface _ripples_ the surface into crests and
troughs whose long axes are perpendicular to the wind direction. The
average length of jumps during saltation corresponds to the wavelength,
or distance between adjacent crests, of the ripples. In ripples, the
coarsest materials collect at the crests. This distinguishes small
ripples from dunes, where the coarsest materials are generally in the
troughs.

[Illustration: Wind-blown sand moves up the gentle upwind side of the
dune by saltation or creep. Sand accumulates at the brink, the top of
the slipface. When the buildup of sand at the brink exceeds the angle of
repose, a small avalanche of grains slides down the slipface. Grain by
grain, the dune moves downwind.]

Accumulations of sediment blown by the wind into a mound or ridge, dunes
have gentle upwind slopes on the wind-facing side. The downwind portion
of the dune, the lee slope, is commonly a steep avalanche slope referred
to as a _slipface_. Dunes may have more than one slipface. The minimum
height of a slipface is about 30 centimeters.

Sand grains move up the dune’s gentle upwind slope by saltation and
creep. When particles at the brink of the dune exceed the angle of
repose, they spill over in a tiny landslide or avalanche that reforms
the slipface. As the avalanching continues, the dune moves in the
direction of the wind.

Some of the most significant experimental measurements on eolian sand
movement were performed by Ralph Bagnold, a British engineer who worked
in Egypt prior to World War II. Bagnold investigated the physics of
particles moving through the atmosphere and deposited by wind. He
recognized two basic dune types, the crescentic dune, which he called
“barchan,” and the linear dune, which he called longitudinal or “sief”
(Arabic for “sword”).

[Illustration: Sand dunes in Death Valley, California (photograph by
Richard Frear).]

[Illustration: Ripples on a dune in Eureka Valley, California
(photograph by Terrence Moore).]



                             Types of Dunes


A worldwide inventory of deserts has been developed using images from
the Landsat satellites and from space and aerial photography. It defines
five basic types of dunes: _crescentic_, _linear_, _star_, _dome_, and
_parabolic_.

[Illustration: These crescentic dunes of coastal Peru are migrating
toward the left (photograph by John McCauley).]

The most common dune form on Earth and on Mars is the _crescentic_.
Crescent-shaped mounds generally are wider than long. The slipface is on
the dune’s concave side. These dunes form under winds that blow from one
direction, and they also are known as barchans, or transverse dunes.
Some types of crescentic dunes move faster over desert surfaces than any
other type of dune. A group of dunes moved more than 100 meters per year
between 1954 and 1959 in China’s Ningxia Province; similar rates have
been recorded in the Western Desert of Egypt. The largest crescentic
dunes on Earth, with mean crest-to-crest widths of more than 3
kilometers, are in China’s Taklimakan Desert.

Straight or slightly sinuous sand ridges typically much longer than they
are wide are known as _linear dunes_. They may be more than 160
kilometers long. Linear dunes may occur as isolated ridges, but they
generally form sets of parallel ridges separated by miles of sand,
gravel, or rocky interdune corridors. Some linear dunes merge to form
Y-shaped compound dunes. Many form in bidirectional wind regimes. The
long axes of these dunes extend in the resultant direction of sand
movement.

[Illustration: Star dunes, such as these of the Namib, indicate the
winds that formed them blew from many directions (photograph by Georg
Gerster).]

Radially symmetrical, _star dunes_ are pyramidal sand mounds with
slipfaces on three or more arms that radiate from the high center of the
mound. They tend to accumulate in areas with multidirectional wind
regimes. Star dunes grow upward rather than laterally. They dominate the
Grand Erg Oriental of the Sahara. In other deserts, they occur around
the margins of the sand seas, particularly near topographic barriers. In
the southeast Badain Jaran Desert of China, the star dunes are up to 500
meters tall and may be the tallest dunes on Earth.

[Illustration: Linear dunes advance on small playas east of Lake Eyre in
the Simpson Desert of central Australia (photograph by C. Twidale).]

[Illustration: Linear dunes in the Western Desert of Egypt (photograph
by Carol Breed).]

Oval or circular mounds that generally lack a slipface, _dome dunes_ are
rare and occur at the far upwind margins of sand seas.

U-shaped mounds of sand with convex noses trailed by elongated arms are
_parabolic dunes_. Sometimes these dunes are called U-shaped, blowout,
or hairpin dunes, and they are well known in coastal deserts. Unlike
crescentic dunes, their crests point upwind. The elongated arms of
parabolic dunes follow rather than lead because they have been fixed by
vegetation, while the bulk of the sand in the dune migrates forward. The
longest known parabolic dune has a trailing arm 12 kilometers long.

[Illustration: Small crescentic dunes occur on the crests of these
complex dome dunes of Saudi Arabia’s Empty Quarter (photograph by Elwood
Friesen).]

[Illustration: Ripples and horns of this crescentic dune in Egypt
indicate that the dune is moving right to left (photograph by John
Olsen).]

Occurring wherever winds periodically reverse direction, _reversing
dunes_ are varieties of any of the above types. These dunes typically
have major and minor slipfaces oriented in opposite directions.

All these dune types may occur in three forms: _simple_, _compound_, and
_complex_. Simple dunes are basic forms with a minimum number of
slipfaces that define the geometric type. Compound dunes are large dunes
on which smaller dunes of similar type and slipface orientation are
superimposed, and complex dunes are combinations of two or more dune
types. A crescentic dune with a star dune superimposed on its crest is
the most common complex dune. Simple dunes represent a wind regime that
has not changed in intensity or direction since the formation of the
dune, while compound and complex dunes suggest that the intensity and
direction of the wind has changed.

[Illustration: The northern Mojave Desert. The Landsat Thematic Mapper
(TM) acquires data in seven bands of the electromagnetic spectrum. On
this image, white and yellow colors indicate rocks rich in clay minerals
and limonite in rocks of red and yellow hues. The large concentrations
of limonite and clays may indicate mineral deposits exposed at the
surface or buried up to several thousand feet below it (photograph
courtesy of Melvin Podwysocki).]



                      Remote Sensing of Arid Lands


The world’s deserts are generally remote, inaccessible, and
inhospitable. Hidden among them, however, are hydrocarbon reservoirs,
evaporites, and other mineral deposits, as well as human artifacts
preserved for centuries by the arid climate. In these harsh
environments, the information and perspective required to increase our
understanding of arid-land geology and resources often depends on
remote-sensing methods. Remote sensing is the collection of information
about an object without being in direct physical contact with it.

Remote-sensing instruments in Earth-orbit satellites measure radar,
visible light, and infrared radiation. Radar imaging systems provide
their own source of electromagnetic energy, so they can operate at any
time of day or night. Additionally, clouds and all but the most severe
storms are transparent to radar.

The first Shuttle Imaging Radar System (SIR-A), flown on the U.S. space
shuttle _Columbia_ in 1981, recorded images that show buried fluvial
topography, faults, and intrusive bodies otherwise concealed beneath
sand sheets and dunes of the Western Desert in Egypt and the Sudan. Most
of these features are not visible from the ground. The radar signal
penetrated loose dry sands and returned images of buried river channels
not visible at the surface. These images helped find new archeologic
sites and sources of potable water in the desert. These “radar rivers”
are the remnants of a now vanished major river system that flowed across
Africa some 20 million years before the development of the Nile River
system. Radar imagery also is a powerful tool for exploring for placer
mineral deposits in arid lands.

In 1972, the United States launched the first of a group of unmanned
satellites collectively known as Landsat. Landsat satellites carry
sensors that record “light,” or portions of the electromagnetic
spectrum, as it reflects off the Earth. Landsat acquires digital data
that are converted into an image.

[Illustration: The Landsat simulated true color mosaic (left) shows the
Selima Sand Sheet covering all but rocky areas of the Sahara Desert in
Sudan. On the right, a 50-kilometer-wide strip of Shuttle Imaging Radar,
SIR-A, is placed over the Landsat mosaic to reveal old stream channels
and geologic structures like these. Structures that are otherwise
invisible under the surface sands are potential sources of water, placer
minerals, ancient artifacts, and information on changes of climate in
arid areas (courtesy of USGS Image Processing Facility, Flagstaff).]

The scarcity of vegetation makes spectral remote sensing especially
effective in arid lands. Rocks containing limonite, a hydrous iron
oxide, may be identified readily from Landsat Multispectral Scanner
data. The Landsat Thematic Mapper (TM) has increased our ability to
detect and map the distribution of minerals in volcanic rocks and
related mineral deposits in arid and semiarid lands.

More than a million images of Earth have been acquired by the Landsat
satellites. A Landsat image may be viewed as a single band in
black-and-white, or as a combination represented by three colors, called
a color composite. The most widely used Landsat color image is called a
false-color composite because it reproduces the infrared band (invisible
to the naked eye) as red, the red band as green, and the green band as
blue. Healthy vegetation in a false-color composite is red.

Desert studies still are hampered in many regions by lack of accurate
climate data. Most desert weather stations are in oases surrounded by
trees and buildings and have been subjected to many location and
elevation changes throughout the life of the station. Data from oases do
not reflect conditions from the surrounding desert. A wide variety of
instruments has been used to record measurements over varying lengths of
time and in different formats, making data difficult to interpret and
compare.

To overcome some of these problems in deserts of the American Southwest,
the U.S. Geological Survey (USGS) established its Desert Winds Project
to measure in a standard format several key meteorologic characteristics
of arid lands. Project scientists have successfully established
instrument stations to measure wind-speed, including peak gusts, which
alter the landforms the most. A station recorded a windstorm near
Vicksburg, Arizona, for example, with peak gusts of almost 150
kilometers per hour. Using low-maintenance, automatic, solar-powered
sensors, the stations also measure wind direction, precipitation,
humidity, soil and air temperatures, and barometric pressure at specific
heights above the surface. Data are sampled at 6-minute intervals and
transmitted every 30 minutes to a Geostationary Operational
Environmental Satellite (GOES). From GOES, the data are transmitted to
the USGS laboratory in Flagstaff, Arizona.

The Desert Winds Project’s investigators combine analyses of data with
detailed geologic field studies and repetitive remote-sensing coverage
in order to investigate and understand the long-term changes produced by
wind in deserts of differing geologic and climatic types.

[Illustration: This geometeorologic station of the Desert Winds Project
measures wind speed and direction, soil and air temperature, and
precipitation and humidity in the Great Basin Desert (photograph by
Carol Breed).]

[Illustration: Nitrate workings in a broad valley in the Atacama Desert
of northern Chile, where saline-cemented surficial deposits formed near
a playa lake (photograph by George Ericksen).]



                      Mineral Resources in Deserts


Some mineral deposits are formed, improved, or preserved by geologic
processes that occur in arid lands as a consequence of climate. Ground
water leaches ore minerals and redeposits them in zones near the water
table. This leaching process concentrates these minerals as ore that can
be mined. Of the 15 major types of mineral deposits in the Western
Hemisphere formed by action of ground water, 13 occur in deserts.

Evaporation in arid lands enriches mineral accumulation in their lakes.
Playas may be sources of mineral deposits formed by evaporation. Water
evaporating in closed basins precipitates minerals such as gypsum, salts
(including sodium nitrate and sodium chloride), and borates. The
minerals formed in these evaporite deposits depend on the composition
and temperature of the saline waters at the time of deposition.

Significant evaporite resources occur in the Great Basin Desert of the
United States, mineral deposits made forever famous by the “20-mule
teams” that once hauled borax-laden wagons from Death Valley to the
railroad. Boron, from borax and borate evaporites, is an essential
ingredient in the manufacture of glass, ceramics, enamel, agricultural
chemicals, water softeners, and pharmaceuticals. Borates are mined from
evaporite deposits at Searles Lake, California, and other desert
locations. The total value of chemicals that have been produced from
Searles Lake substantially exceeds $1 billion.

The Atacama Desert of South America is unique among the deserts of the
world in its great abundance of saline minerals. Sodium nitrate has been
mined for explosives and fertilizer in the Atacama since the middle of
the 19th century. Nearly 3 million metric tons were mined during World
War I.

Valuable minerals located in arid lands include copper in the United
States, Chile, Peru, and Iran; iron and lead-zinc ore in Australia;
chromite in Turkey; and gold, silver, and uranium deposits in Australia
and the United States. Nonmetallic mineral resources and rocks such as
beryllium, mica, lithium, clays, pumice, and scoria also occur in arid
regions. Sodium carbonate, sulfate, borate, nitrate, lithium, bromine,
iodine, calcium, and strontium compounds come from sediments and
nearsurface brines formed by evaporation of inland bodies of water,
often during geologically recent times.

[Illustration: This open-pit mine in the Sonoran Desert near Ajo,
Arizona, has exposed an elliptical copper deposit about 1,000 meters
long and 750 meters wide. The copper ore mined here is in a bed that
averages 150 meters in thickness (photograph by Peter Kresan).]

The Green River Formation of Colorado, Wyoming, and Utah contains
alluvial fan deposits and playa evaporites created in a huge lake whose
level fluctuated for millions of years. Economically significant
deposits of trona, a major source of sodium compounds, and thick layers
of oil shale were created in the arid environment.

[Illustration: Trona mine at Searles Lake, California (photograph by
John Keith).]

Some of the more productive petroleum areas on Earth are found in arid
and semiarid regions of Africa and the Mideast, although the oil
reservoirs were originally formed in shallow marine environments. Recent
climate change has placed these reservoirs in an arid environment.

Other oil reservoirs, however, are presumed to be eolian in origin and
are presently found in humid environments. The Rotliegendes, a
hydrocarbon reservoir in the North Sea, is associated with extensive
evaporite deposits. Many of the major U.S. hydrocarbon resources may
come from eolian sands. Ancient alluvial fan sequences may also be
hydrocarbon reservoirs.

[Illustration: The Sahelian drought that began in 1968 was responsible
for the deaths of between 100,000 and 250,000 people, the disruption of
millions of lives, and the collapse of the agricultural bases of five
countries (photograph by Daniel Stiles, UNEP).]



                            Desertification


The world’s great deserts were formed by natural processes interacting
over long intervals of time. During most of these times, deserts have
grown and shrunk independent of human activities. Paleodeserts, large
sand seas now inactive because they are stabilized by vegetation, extend
well beyond the present margins of core deserts, such as the Sahara. In
some regions, deserts are separated sharply from surrounding, less arid
areas by mountains and other contrasting landforms that reflect basic
structural differences in the regional geology. In other areas, desert
fringes form a gradual transition from a dry to a more humid
environment, making it more difficult to define the desert border.

[Illustration: Overgrazing has made the Rio Puerco Basin of central New
Mexico one of the most eroded river basins of the American West and has
increased the high sediment content of the river (photograph by Terrence
Moore).]

[Illustration: Linear dunes of the Sahara Desert encroach on Nouakchott,
the capital of Mauritania. The dunes border a mosque at left (photograph
by Georg Gerster).]

These transition zones have very fragile, delicately balanced
ecosystems. Desert fringes often are a mosaic of microclimates. Small
hollows support vegetation that picks up heat from the hot winds and
protects the land from the prevailing winds. After rainfall the
vegetated areas are distinctly cooler than the surroundings. In these
marginal areas, human activity may stress the ecosystem beyond its
tolerance limit, resulting in degradation of the land. By pounding the
soil with their hooves, livestock compact the substrate, increase the
proportion of fine material, and reduce the percolation rate of the
soil, thus encouraging erosion by wind and water. Grazing and the
collection of firewood reduces or eliminates plants that help to bind
the soil.

This degradation of formerly productive land—_desertification_—is a
complex process. It involves multiple causes, and it proceeds at varying
rates in different climates. Desertification may intensify a general
climatic trend toward greater aridity, or it may initiate a change in
local climate.

Desertification does not occur in linear, easily mappable patterns.
Deserts advance erratically, forming patches on their borders. Areas far
from natural deserts can degrade quickly to barren soil, rock, or sand
through poor land management. The presence of a nearby desert has no
direct relationship to desertification. Unfortunately, an area
undergoing desertification is brought to public attention only after the
process is well underway. Often little or no data are available to
indicate the previous state of the ecosystem or the rate of degradation.
Scientists still question whether desertification, as a process of
global change, is permanent or how and when it can be halted or
reversed.

[Illustration: Camels and other animals trample the soil in the semiarid
Sahel of Africa as they move to water holes such as this one in Chad
(photograph courtesy of the U.S. Agency for International Development).]


                                Problem

Desertification became well known in the 1930’s, when parts of the Great
Plains in the United States turned into the “Dust Bowl” as a result of
drought and poor practices in farming, although the term itself was not
used until almost 1950. During the dust bowl period, millions of people
were forced to abandon their farms and livelihoods. Greatly improved
methods of agriculture and land and water management in the Great Plains
have prevented that disaster from recurring, but desertification
presently affects millions of people in almost every continent.

[Illustration: Off-road vehicles significantly increase soil loss in the
delicate desert environment of the western United States. In a few
seconds, soils that took hundreds of years to develop can be destroyed
(photograph by Terrence Moore).]

Increased population and livestock pressure on marginal lands has
accelerated desertification. In some areas, nomads moving to less arid
areas disrupt the local ecosystem and increase the rate of erosion of
the land. Nomads are trying to escape the desert, but because of their
land-use practices, they are bringing the desert with them.

It is a misconception that droughts cause desertification. Droughts are
common in arid and semiarid lands. Well-managed lands can recover from
drought when the rains return. Continued land abuse during droughts,
however, increases land degradation. By 1973, the drought that began in
1968 in the Sahel of West Africa and the land-use practices there had
caused the deaths of more than 100,000 people and 12 million cattle, as
well as the disruption of social organizations from villages to the
national level.

While desertification has received tremendous publicity by the political
and news media, there are still many things that we don’t know about the
degradation of productive lands and the expansion of deserts. In 1988
Ridley Nelson pointed out in an important scientific paper that the
desertification problem and processes are not clearly defined. There is
no consensus among researchers as to the specific causes, extent, or
degree of desertification. Contrary to many popular reports,
desertification is actually a subtle and complex process of
deterioration that may often be reversible.

[Illustration: Goat seeks food in the sparsely vegetated Sahel of Africa
(photograph courtesy of the U.S. Agency for International Development).]


                           Global monitoring

In the last 25 years, satellites have begun to provide the global
monitoring necessary for improving our understanding of desertification.
Landsat images of the same area, taken several years apart but during
the same point in the growing season, may indicate changes in the
susceptibility of land to desertification. Studies using Landsat data
help demonstrate the impact of people and animals on the Earth. However,
other types of remote-sensing systems, land-monitoring networks, and
global data bases of field observations are needed before the process
and problems of desertification will be completely understood.


                             Local remedies

At the local level, individuals and governments can help to reclaim and
protect their lands. In areas of sand dunes, covering the dunes with
large boulders or petroleum will interrupt the wind regime near the face
of the dunes and prevent the sand from moving. Sand fences are used
throughout the Middle East and the United States, in the same way snow
fences are used in the north. Placement of straw grids, each up to a
square meter in area, will also decrease the surface wind velocity.
Shrubs and trees planted within the grids are protected by the straw
until they take root. In areas where some water is available for
irrigation, shrubs planted on the lower one-third of a dune’s windward
side will stabilize the dune. This vegetation decreases the wind
velocity near the base of the dune and prevents much of the sand from
moving. Higher velocity winds at the top of the dune level it off and
trees can be planted atop these flattened surfaces.

[Illustration: Straw grids (one of which is shown above) and vegetation
irrigated by water from the Yellow River stabilize dunes in this part of
China’s Tengger Desert (shown below) and protect a nearby railroad from
windblown sand.]

[Illustration: Tengger Desert]

Oases and farmlands in windy regions can be protected by planting tree
fences or grass belts. Sand that manages to pass through the grass belts
can be caught in strips of trees planted as wind breaks 50 to 100 meters
apart adjacent to the belts. Small plots of trees may also be scattered
inside oases to stabilize the area. On a much larger scale, a “Green
Wall,” which will eventually stretch more than 5,700 kilometers in
length, much longer than the famous Great Wall, is being planted in
northeastern China to protect “sandy lands”—deserts believed to have
been created by human activity.

More efficient use of existing water resources and control of
salinization are other effective tools for improving arid lands. New
ways are being sought to use surface-water resources such as rain water
harvesting or irrigating with seasonal runoff from adjacent highlands.
New ways are also being sought to find and tap groundwater resources and
to develop more effective ways of irrigating arid and semiarid lands.
Research on the reclamation of deserts also is focusing on discovering
proper crop rotation to protect the fragile soil, on understanding how
sand-fixing plants can be adapted to local environments, and on how
grazing lands and water resources can be developed effectively without
being overused.

[Illustration: From wasteland to vineyard. Ground water and underground
channels help this vineyard flourish on land reclaimed from desert
pavement in China’s Turpan Depression.]

If we are to stop and reverse the degradation of arid and semiarid
lands, we must understand how and why the rates of climate change,
population growth, and food production adversely affect these
environments. The most effective intervention can come only from the
wise use of the best earth-science information available.



                           Selected Readings


  Bagnold, R. A., 1941, The physics of blown sand and desert dunes:
  Methuen, London, 265 p. (A classic treatise concerning the origin and
  evolution of dunes.)

  Breed, C. S., and others, 1979, Regional studies of sand seas, using
  Landsat (ERTS) imagery: _in_ McKee, E. D., ed., A study of global sand
  seas: U.S. Geological Survey Professional Paper 1052, p. 305-397. (A
  study of selected sand seas based on analysis of remote sensing
  images, surface wind summaries, and available literature.)

  Cook, R. U., and Warren, Andrew, 1973, Geomorphology in deserts:
  University of California Press, Berkeley, California, 374 p. (Examines
  the nature of landforms, soils, and geomorphological processes in the
  world’s deserts.)

  Eigeland, Tor, and others, 1982, The desert realm: National Geographic
  Society, Washington, 304 p. (A well illustrated discussion of deserts
  of America, Africa, Asia, and Australia.)

  Ericksen, G. E., 1983, The Chilean nitrate deposits: American
  Scientist, v. 71, p. 366-374. (A discussion of the origin of the
  Chilean nitrate deposits which has puzzled scientists for more than
  100 years.)

  Gerster, Georg, 1960, Sahara-desert of destiny: Coward-McCann, New
  York, 302 p. (How plants, animals, and people survive in the Sahara.)

  Greeley, Ronald, and Iversen, J. D., 1985, Wind as a geological
  process on Earth, Mars, Venus and Titan: Cambridge University Press,
  New York, 333 p. (Expands the classic work of Bagnold to discuss
  eolian processes in a planetary context. Describes the processes on
  all moons and terrestrial planets with atmospheres.)

  Hare, F. K., 1983, Climate on the desert fringe: _in_ Gardner, Ritz,
  and Scoging, Helen, eds., Mega-geomorphology: Clarendon Press, Oxford,
  p. 134-151. (The margins of many deserts are affected by tension
  between society and environment. This paper summarizes the climatology
  of arid zones.)

  MacMahon, James A., 1985, Deserts: Alfred A. Knopf, Inc., New York,
  640 p. (An Audubon Society Nature Guide to the deserts of the United
  States, and their inhabitants.)

  McCauley, J. F., and others, 1984, Remote monitoring of processes that
  shape desert surfaces: The Desert Winds Project: U.S. Geological
  Survey Bulletin 1634, 19 p. (Describes a new study on collecting
  weather data from solar-powered data-collection platforms in deserts.
  The data are relayed by a GOES satellite to the USGS in Flagstaff,
  Arizona, and converted to graphic form.)

  Meigs, Peveril, 1953, World distribution of arid and semi-arid
  homoclimates: _in_ Reviews of research on arid zone hydrology: Paris,
  United Nations Educational, Scientific, and Cultural Organization,
  Arid Zone Programme-1, p. 203-209. (Classifies arid lands according to
  precipitation.)

  Nelson, R., 1988, Dryland management: the desertification problem:
  Environmental Department Working Paper No. 8, Washington: World Bank,
  42 p. (An excellent review of the present state of knowledge
  concerning desertification.)

  Tolba, M. K., 1984, Desertification is stoppable: Arid Lands
  Newsletter No. 21, p. 2-9. (A discussion of the problems involved in
  preventing desertification and reclaiming arid lands.)

  Walker, A.S., 1986, Eolian geomorphology: _in_ Short, N.M., and Blair,
  R.W., eds., Geomorphology from space: a global overview of regional
  landforms: NASA SP-486, p. 447-520 (a brief review of desert
  processes).

  Warren, A. and Agnew, C., 1988, An assessment of desertification and
  land degradation in arid and semi-arid areas: International Institute
  for Environment and Development, Drylands Programme, Paper 2, London:
  IIED, 103 p. (An evaluation of land degradation problems.)



The metric units used in this publication can be converted to English
units by using the approximate conversions given below:

  Length

  1 kilometer              0.6 of a mile
  1 meter                   39.37 inches
  1 centimeter                  0.4 inch
  1 millimeter                 0.04 inch

  Area

  1 sq. kilometer          0.04 sq. mile
  1 sq. meter              1.2 sq. yards
  1 sq. centimeter        0.155 sq. inch

  Temperature

  To convert °Celsius to °Fahrenheit, multiply °C by 1.8 and add 32.
  To convert °Fahrenheit to °Celsius, subtract 32 from °F and divide the
          result by 1.8.

[Illustration: Landsat image shows complex linear and crescentic dunes
in the northeastern Taklimakan Desert of China.]

_In this desert there are a great many evil spirits and also hot winds;
those who encounter them perish to a man. There are neither birds above
nor beasts below. Gazing on all sides as far as the eye can reach in
order to mark the track, no guidance is to be obtained save from the
rotting bones of dead men, which point the way._

   _Explorer Fa Xian describing the Taklimakan Desert of China about 400
                                                                   A.D._



This publication is one of a series of general interest publications
prepared by the U.S. Geological Survey to provide information about the
earth sciences, natural resources, and the environment. To obtain a
catalog of additional titles in the series “General Interest
Publications of the U.S. Geological Survey,” write:

    U.S. Geological Survey
    Branch of Distribution
    P.O. Box 25286
    Denver, CO 80225

★ U. S. GOVERNMENT PRINTING OFFICE : 1992 0-332-326 QL 2


As the Nation's principal conservation agency, the Department of the
Interior has responsibility for most of our nationally-owned public
lands and natural and cultural resources. This includes fostering wise
use of our land and water resources, protecting our fish and wildlife,
preserving the environmental and cultural values of our national parks
and historical places, and providing for the enjoyment of life through
outdoor recreation. The Department assesses our energy and mineral
resources and works to assure that their development is in the best
interests of all our people. The Department also promotes the goals of
the Take Pride in America campaign by encouraging stewardship and
citizen responsibility for the public land and promoting citizen
participation in their care. The Department also has a major
responsibility for American Indian reservation communities and for
people who live in Island Territories under U.S. Administration.

  Cover Photographs:

[Illustration: Granite Mountain in the Great Basin Desert (photograph by
Terrence Moore).]

[Illustration: Sonoran Desert (photograph by Peter Kresan).]

[Illustration: Zabriskie Point in Death Valley, California (photograph
by Peter Kresan).]

[Illustration: Artists Point in Monument Valley (photograph by Peter
Kresan).]

[Illustration: Death Valley, California (photograph by Cecil
Stoughton).]

[Illustration: Cacti in the Sonoran Desert (photograph by John Olson).]

[Illustration: Back Cover]


                          Transcriber’s Notes


--Corrected a few typographical errors.

--Slightly displaced photographs and captions for better display on
  scrolling eBook viewers.





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