Unsuccessfulwell Perchedwater table successful wellSpring.docx

Unsuccessful
well Perched
water table successful well
Spring

Main water table
to 50 percent of the sediment's total volume. Pore space depends
on the size and shape of the grains; how they are packed
together; the degree of sorting; and in sedimentary rocks, the
amount of cementing material. Most igneous and metamorphic
rocks, as well as some sedimentary rocks, are composed of
tightly interlocking crystals so the voids between grains may be
negligible. In these rocks, fractures must provide the voids.
96 chapter 3 Landscapes Fashioned by Water

)6 chapter 3 Landscapes Fashioned by Water
Zone of saturation
Unsaturated zo
ne
figure
3.30
This diagram illustrates the relative positions of many features
associated with subsurface water.
Several factors contribute to the irregular surface of the water
table. One important influence is the fact that groundwater
moves very slowly. Because of this, water tends to "pile up"
beneath high areas between stream valleys. If rainfall were to
cease completely, these water "hills" would slowly subside and
gradually approach the level of the adjacent valleys. However,
new supplies of rainwater are usually added often enough to
prevent this. Nevertheless, in times of extended drought, the
water table may drop enough to dry up shallow wells. Other
causes for the uneven water table are variations in rainfall and
permeability of Earth materials from place to place.
Factors Influencing the Storage and Movement of Groundwater
The nature of subsurface materials strongly influences the rate
of groundwater movement and the amount of groundwater that
can be stored. Two factors are especially important— porosity
and permeability.
Porosity
Water soaks into the ground because bedrock, sediment, and
soil contain countless voids or openings. These openings are
similar to those of a sponge and are often called pore spaces.
The quantity of groundwater that can be stored depends on the
porosity of the material, which is the percentage of the total

volume of rock or sediment that consists of pore spaces. Voids
most often are spaces between sedimentary particles, but also
common are joints, faults, cavities formed by the dissolving of
soluble rock such as limestone, and vesicles (voids left by gases
escaping from lava).
Variations in porosity can be great. Sediment is commonly quite
porous, and open spaces may occupy 10 percent
Permeability
Porosity alone cannot measure a material's capacity to yield
groundwater. Rock or sediment may be very porous and still
prohibit water from moving through it. The permeability of a
material indicates its ability to transmit a fluid. Groundwater
moves by twisting and turning through interconnected small
openings. The smaller the pore spaces, the slower the
groundwater moves. If the spaces between particles are too
small, water cannot move at all. For example, clay's ability to
store water can be great, owing to its high porosity, but its pore
spaces are so small that water is unable to move through it.
Thus, we say that clay is impermeable.
Aquitards and Aquifers
Impermeable layers such as clay that hinder or prevent water
movement are termed aquitards (aqua = water, tard = slow). In
contrast, larger particles, such as sand or gravel, have larger
pore spaces. Therefore, water moves with relative ease.
Permeable rock strata or sediments that transmit groundwater
freely are called aquifers ("water carriers"). Aquifers are
important because they are the water-bearing layers sought after
by well drillers.
Groundwater Movement
The movement of most groundwater is exceedingly slow, from
pore to pore. A typical rate is a few centimeters per day. The
energy that makes the water move is provided by the force of
gravity. In response to gravity, water moves from areas where
the water table is high to zones where the water table is lower.
DID YOU KNOW?

Because of its high porosity, excellent permeability, and great
size, the Ogallala Formation, the largest aquifer in the United
States, accumulated huge amounts of groundwater—enough
freshwater to fill Lake Huron.
Water table
3.31 Arrows indicate groundwater movement through uni
formly
5
material.The looping curves may be thought of as a compromise
:he downward pull of gravity and the tendency of water to move
eas of reduced pressure.
Springs
97
figure
3.32 Thousand Springs along the Snake River in
HagermanValley, Idaho. (Photo by David Frazier)jans that
water usually gravitates toward a stream chan-e, or spring.
Although some water takes the most direct >wn the slope of the
water table, much of the water fol-ng, curving paths toward the
zone of discharge. gurc 3.3 > shows how water percolates into a
stream 11 possible directions. Some paths clearly turn up-
apparently against the force of gravity, and enter h the bottom
of the channel. This is easily explained: ■eper you go into the
zone of saturation, the greater ter pressure. Thus, the looping
curves followed by n the saturated zone may be thought of as a
compro-etween the downward pull of gravity and the ten-of
water to move toward areas of reduced pressure.
Sculpturing Earth's Surface Groundwater
s have aroused the curiosity and wonder of people for inds of
years. The fact that springs were (and to some ■ still are) rather
mysterious phenomena is not difficult lerstand, for here is water

flowing freely from the d in all kinds of weather in seemingly
inexhaustible r but with no obvious source. Today, we know
that the : of springs is water from the zone of saturation and e
ultimate source of this water is precipitation. Vhenever the
water table intersects Earth's surface, a il outflow of
groundwater results, which we call a Springs such as the one
pictured in figure 3.32 form
an aquitard blocks the downward movement of iwater and forces
it to move laterally. Where the perme-xl (aquifer) outcrops, a
spring or several springs result. Another situation that can
produce a spring is illus-
in Figure 3.30. Here an aquitard is situated above the water
table. As water percolates downward, a portion i intercepted by
the aquitard, thereby creating a local-;one of saturation and a
perched water table. Springs, ver, are not confined to places
where a perched water ;reates a flow at the surface. Many
geologic situations o the formation of springs because
subsurface condi-/ary greatly from place to place.
Hot Springs
By definition, the water in hot springs is 6° to 9°C (10° to 15°F)
warmer than the average annual air temperature for the
localities where they occur. In the United States alone, there are
well over 1000 such springs.
Temperatures in deep mines and oil wells usually rise with
increasing depth, an average of about 2°C per 100 meters (1°F
per 100 feet). Therefore, when groundwater circulates at great
depths, it becomes heated. If it rises to the surface, the water
may emerge as a hot spring. The water of some hot springs in
the eastern United States is heated in this manner. The great
majority (more than 95 percent) of the hot springs (and geysers)
in the United States are found in the West. The reason for such
a distribution is that the source of heat for most hot springs is
cooling igneous rock, and it is in the West that igneous activity
has occurred most recently.
Geysers
Geysers are intermittent hot springs or fountains in which

columns of water are ejected with great force at various
intervals, often rising 30 to 60 meters (100 to 200 feet) into the
air. After the jet of water ceases, a column of steam rushes out,
often with a thunderous roar. Perhaps the most famous geyser in
the world is Old Faithful in Yellowstone National Park, which
erupts about once each hour (figure 3.33). Geysers are also
found in other parts of the world, notably New Zealand and
Iceland. In fact, the Icelandic word geysa, to gush, gives us the
name geyser.
Geysers occur where extensive underground chambers exist
within hot igneous rocks. As relatively cool groundwater enters
the chambers, it is heated by the
97
DID YOU KNOW?
Many people think that Old Faithful Geyser in Wyoming'sN
stone National Park erupts so reliably—every hour ( hour—that
you can set your watch by it. So goes the lege it's not true.Time
spans between eruptions vary from ab minutes to more than 90
minutes, and have generally inc over the years thanks to
changes in the geyser's plumbing.

figure 3.33 A wintertime eruption of Old Faithful, one of the
world's most famous geysers. During a typical eruption it emits
as much as 45,000 liters (almost 12,000 gallons) of hot water
and steam. (Photo by Art Director & Trip/Alamy)
The most common method for removing groundwate well, a hole
bored into the zone of saturation. Wells s small reservoirs into
which groundwater migrates ar which it can be pumped to the
surface. The use of wel back many centuries and continues to be
an important: of obtaining water. By far the single greatest use
of this the United States is irrigation for agriculture. More thar
cent of the groundwater used each year is for this purf dustrial
uses rank a distant second, followed by the i used by homes in
cities and rural areas.
The water-table level may fluctuate considerat ing the course of
a year, dropping during dry seas< rising following periods of
precipitation. Therefon sure a continuous supply of water, a
well must p< below the water table. Whenever a substantial am
water is withdrawn from a well, the water table aro well is
lowered. This effect, termed drawdown, d< with increasing
distance from the well. The result pression in the water table,
roughly conical in shape as a cone of depression (figure 3.34).
For most si mestic wells, the cone of depression is negligible. E
when wells are used for irrigation or for industrial p the

withdrawal of water can be great enough to crea wide and steep
cone of depression that may subs lower the water table in an
area and cause nearby wells to become dry. Figure 3.34
illustrates this situc
surrounding rock. At the bottom of the chamber, the water is
under great pressure because of the weight of the overlying
water. This great pressure prevents the water from boiling at the
normal surface temperature of 100°C (212°F). For example, at
the bottom of a 300-meter (1000-foot) water-filled chamber,
water must attain a temperature of nearly 230°C (450°F) before
it will boil. The heating causes the water to expand, with the
result that some is forced out at the surface. This loss of water
reduces the pressure on the remaining water in the chamber,
which lowers the boiling point. A portion of the water deep
within the chamber quickly turns to steam and causes the geyser
to erupt. Following the eruption, cool groundwater again seeps
into the chamber, and the cycle begins anew.
In most wells, water cannot rise on its own. If waf encountered
at 30 meters (100 feet) depth, it remai level, fluctuating perhaps
a meter or two with sea; and dry periods. However, in some
wells, water ris times overflowing at the surface.
The term artesian is applied to any situation groundwater rises
in a well above the level where • dally encountered. For such a
situation to occur, two ( must exist (figure 3.35): (1) Water
must be confi
Pressure surface (level to which water will rise)Water tank
Well
Water is pumped into • tank

Pressure moves water through pipe
figure 3.36 City water systems can be considered artificial
artesian systems.
aquifer that is inclined so that one end is exposed at the surface,
where it can receive water; and (2) aquitards both above and
below the aquifer must be present to prevent the water from
escaping. Such an aquifer is called a confined aquifer. When
such a layer is tapped, the pressure created by the weight of the
water above will force the water to rise. If there were no
friction, the water in the well would rise to the level of the
water at the top of the aquifer. However, friction reduces the
height of this pressure surface. The greater the distance from
the recharge area (area where water enters the inclined aquifer),
the greater the friction and the smaller the rise of water.
In Figure 3.35, Well 1 is a no flowing artesian well, because at
this location the pressure surface is below ground level. When
the pressure surface is above the ground and a well is drilled
into the aquifer, a flowing artesian well is created (Well 2,
Figure 3.35).
Artesian systems act as "natural pipelines," transmitting water
from remote areas of recharge great distances to the points of
discharge. In this manner, water that fell in central Wisconsin
years ago is now taken from the ground and used by
communities many kilometers to the south in Illinois. In South
Dakota, such a system brings water from the western Black
Hills eastward across the state.
IN VIR1 Or GRJ
On a different scale, city water systems may be considered
examples of artificial artesian systems (figure 3.36). The water
tower, into which water is pumped, may be considered the area
of recharge, the pipes the confined aquifer, and the faucets in
homes the flowing artesian wells.
J MENTAL PRC

As with many of our valuable natural resources, groundwater is
being exploited at an increasing rate. In some areas, overuse
threatens the groundwater supply. In other places, groundwater
withdrawal has caused the ground and everything resting upon it
to sink. Still other localities are concerned with the possible
contamination of their groundwater supply.
Treating Groundwater as a Nonrenewable Resource
Many natural systems tend to establish a condition of equili
rium. The groundwater system is no exception. The wat table's
height reflects a balance between the rate of wat added by
precipitation and the rate of water removed by d charge and
withdrawal. An imbalance will either raise or low the water
table. A long-term drop in the water table can occui there is
either a decrease in recharge due to prolonged droug or an
increase in groundwater discharge or withdrawal
For many people, groundwater appears to be an en lessly
renewable resource, for it is continually replenished 1 rainfall
and melting snow. But in some regions, groundwal has been and
continues to be treated as a nonrenewable resoui because the
amount of water available to recharge the aquii is significantly
less than the amount being withdrawn.
The High Plains, a relatively dry region that exten from the
western Dakotas to western Texas, provides o example of an
extensive agricultural economy that is large dependent on
irrigation (figure 3.37). Underlying abc 111 nv'lion acres
(450,000 square kilometers or 174,000 squc miles) in parts of
eight states, the High Plains Aquifer is o of the largest and most
agriculturally significant aquifers the United States. It accounts
for about 30 percent of groundwater withdrawn for irrigation in
the country. In t southern part of this region, which includes the
Texas p? handle, the natural recharge of the aquifer is very slow
a the problem of declining groundwater levels is acute. In fe in
years of average or below-average precipitation, rechai is
negligible because all or nearly all of the meager rainfal
returned to the atmosphere by evaporation and transpirati<
Therefore, where intense irrigation has been practiced an

extended period, depletion of groundwater can be seve Declines
in the water table at rates as great as I meter per yi have led to
an overall drop of between 15 and 60 meters ' and 200 feet) in
some areas. Under these circumstances, it c be said that the
groundwater is literally being "mined." Eva pumping were to
cease immediately, it would take thousar of years for the
groundwater to be fully replenished.
Groundwater depletion has been a concern in the H: Plains and
other areas of the West for many years, but i worth pointing out
that the problem is not confined to t part of the country.
Increased demands on groundwater sources have overstressed
aquifers in many areas, not jus arid and semiarid regions.
Land Subsidence Caused by Groundwater Withdrawal
As you will sec later in this chapter, surface subsidence < result
from natural processes related to groundwa However, the
ground may also sink when water is pumt from wells faster than
natural recharge processes can place it. This effect is
particularly pronounced in areas derlain by thick layers of loose
sediments. As watei withdrawn, the water pressure drops and
the weight of overburden is transferred to the sediment. The
grec
131
132
131
Wind Erosion
figur
e 4.27
Satellite image of a portion of Death Valley, California, a
classic Basin and Range landscape. Shortly before this image
was taken in February 2005, heavy rains led to the formation of

a playa lake—the pool of greenish water on the basin floor. By
May 2005, the lake had reverted to a salt-covered playa.
(NASA)With the ongoing erosion of the mountain mass and the
accompanying sedimentation, the local relief continues to
diminish. Eventually, nearly the entire mountain mass is gone.
Thus, by the late stages of erosion, the mountain areas are
reduced to a few large bedrock knobs (called inselbergs)
projecting above the sediment-filled basin.
DID YOU KNOW?
The Atacama Desert of Chile is the world's driest desert. This
narrow belt of arid land extends for about 1200 km (750 mi)
along South America's Pacific Coast (see Figure 4.22). It is said
that some portions of the Atacama have not received rain for
more than 400 years! One must view such pronouncements
skeptically. Nevertheless, for places where records have been
kept.Arica, Chile, in the northern part of the Atacama, has
experienced a span of 14 years without measurable rainfall.
Each of the stages of landscape evolution in an arid climate
depicted in Figure 4.26 can be observed in the Basin and Range
region. Recently uplifted mountains in an early stage of erosion
are found in southern Oregon and northern Nevada. Death
Valley, California, and southern Nevada fit into the more
advanced middle stage, whereas the late stage, with its
inselbergs, can be seen in southern Arizona.
v-W Sculpturing Earth's Surface slm&t Deserts
Moving air, like moving water, is turbulent and able to pick up
loose debris and transport it to other locations. Just as in a
river, the velocity of wind increases with height above the
surface. Also like a river, wind transports fine particles in
suspension while heavier ones are carried as bed load (figure
4.28). However, the transport of sediment by wind differs from

that by running water in two significant ways. First, wind's
lower density compared to water renders it less
132 chapter 4 Glacial and Arid Landscapes
Saudi Arabia
Sudan
Eritrea
Yemen
figure
4.29 A dust storm blackens the Colorado sky in this historic
image from the Dust Bowl of the 1930s. (P
hoto courtesy U.S.DA/Natural Resources Conservation Service)
figure 4.28
a.
This satellite image shows thick plumes of dust from the Sahara
Desert blowing across the Red Sea on June 30,2009. Such dust
storms are common in arid North Africa. In fact, this region is
the largest dust source in the world. Satellites are an excellent t
ool for studying the transport of dust on a global scale.They
show that dust storms can cover huge areas and that dust can be
trans
ported great distances. (NASA)
b.
The bed load carried by wind consists of sand grains, many of
which move by bouncing along
the surface. Sand never travels far from the surface, even when
winds are very strong. (Photo by imagebroker/Alamy)capable of
picking up and transporting coarse materials. Second, because
wind is not confined to channels, it can spread sediment over
large areas, as well as high into the atmosphere.
Compared to running water and glaciers, wind is a relatively
insignificant erosional agent. Recall that even in deserts, most
erosion is performed by intermittent running water, not by the
wind. Wind erosion is more effective in arid lands than in

humid areas because in humid places moisture binds particles
together and vegetation anchors the soil. For wind to be an
effective erosional force, dryness and scanty vegetation are
important prerequisites. When such circumstances exist, wind
may pick up, transport, and deposit great quantities of fine
sediment. During the 1930s, parts of the Great Plains
experienced vast dust storms (figure 4.29). The plowing under
of the drought, exposed the land to wind erosion and led to the
area being labeled the Dust Bowl.
Deflation, Blowouts, and Desert Pavement
One way that wind erodes is by deflation, the lifting and
removal of loose material. Wind can suspend only fine sediment
such as clay and silt. Larger grains of sand are rolled or skipped
along the surface (a process called saltation) and comprise the
bed load. Particles larger than sand are usually not transported
by wind. Deflation sometimes is difficult to notice because the
entire surface is being lowered at the same time, but it can be
significant.
The most noticeable result of deflation in some places is
shallow depressions called blowouts (figure 4.30). In the Great
Plains region, from Texas north to Montana, thousands of
blowouts can be seen. They range from small dimples less than
1 meter (3 feet) deep and 3 meters (10 feet) wide to depressions
that are more than 45 meters (150 feet > deep and several
kilometers across.
In portions of many deserts, the surface is characterized by a
layer of coarse pebbles and cobbles that are too large to be
moved by the wind. This stony veneer, called
DID YOU KNOW?
Deserts do not necessarily consist of mile after mile of drifting
sand dunes. Surprisingly, sand accumulations represent only a
sma percentage of the total desert area. In the Sahara, dunes
cove-only one-tenth of its area. The sandiest of all deserts, the

Arabian, is one-third sand covered.
Wind Erosion I 33
133
Blowout
b. ~- W 1 - ■ ^^^^^mmmvI''SmWKStKKK
*e 4.30 a. Blowouts are depressions created by deflation. Land
that is dry and largely unprotected by anchoring • station is
particularly susceptible. b. In this example, deflation has
removed about 4 feet of soil—the distance from the i- s
outstretched arm to his feet. (Photo courtesy of U.S.DA/Natural
Resources Conservation Service)
Deflation
Deflation
Desert pavement
Desert pavement established, deflation ends
'0
Deflation begins
Deflation continues to remove finer particles
Time
blown away. Eventually, a continuous cover of coarse particles

remains.
Studies have shown that the process depicted in Figure 4.31A is
not an adequate explanation for all environments in which
desert pavement exists. As a result, an alternate explanation was
formulated and is illustrated in figure 4.3ib. This hypothesis
suggests that pavement develops on a surface that initially
consists of coarse pebbles. Over time, protruding cobbles trap
fine wind-blown grains that settle and sift downward through
the spaces between the larger surface stones. The process is
aided by infiltrating rainwater.
Once desert pavement becomes established, a process that might
take hundreds of years, the surface is effectively protected from
further deflation if left undisturbed. However, as the layer is
only one or two stones thick, the passage of vehicles or animals
can dislodge the pavement and expose the fine-grained material
below. If this happens, the surface is no longer protected from
deflation.
Wind Abrasion
M
b
i
Wind-blown silt accumulates and sifts downward through coarse
particles
Weathered pebbles and cobbles on bedrock
3.
desert pavement,
may form as deflation lowers the surface by removing sand and
silt from poorly sorted materials. As
figure 4.31
A
illustrates, the concentration of larger particles at the surface
gradually increases as the finer particles are
■ft

Silt continues to accumulate and lift desert pavement
figure 4.31 Two models of desert pavement formation. a. This
model portrays an area with poorly sorted surface deposits.
Coarse particles gradually become concentrated into a tightly
packed layer as deflation lowers the surface by -emoving sand
and silt Here desert pavement is the result of wind erosion. b.
This model shows the formation of desert pave-•nent on a
surface initially covered with coarse pebbles and cobbles.Wind-
blown dust accumulates at the surface and gradually sifts
downward through spaces between coarse particles. Infiltrating
rainwater aids the process.This depositional process -aises the
surface and produces a layer of coarse pebbles and cobbles
underlain by a substantial layer of fine sediment
Like glaciers and streams, wind erodes in part by abrasion. In
dry regions as well as along some beaches, windblown sand will
cut and polish exposed rock surfaces. Abrasion is often given
credit for accomplishments beyond its actual capabilities. Such
features as balanced rocks that stand high atop narrow pedestals
and intricate detailing on tall pinnacles are not the results of
abrasion by windblown sand. Sand is seldom lifted more than a
meter above the surface, so the wind's sandblasting effect is
obviously limited in vertical extent. But in areas prone to such
activity, telephone poles have actually been cut through near
their bases. For this reason, collars are often fitted on the poles
to protect them from being "sawed" down.
figure 4.33 a.
The White Point dunes near Preston Mesa.Arizona. Strong
winds move sand up the more gentle windward slopes. As sand
accumulates near the dune crest, the slope becomes steeper.
(Photo by Michael Collier)
b.
Eventually, some of the sand slides down the

slip
face.
(Photo by Christopher Liu/ChinaStock Photo Library)WIND'
DEPOSITS
Sculpturing Earth's Surface sIMIIEe Deserts
Although wind is relatively unimportant in carving erosional
features, significant depositional landforms are created by the
wind in some regions. Accumulations of windblown sediment
are particularly conspicuous in the world's dry lands and along
many sandy coasts. Wind deposits are of two distinctive types:
(1) extensive blankets of silt, called loess, that once were
carried in suspension, and (2) mounds and ridges of sand from
the wind's bed load, which we call dunes.
Loess
A.In some parts of the world the surface topography is mantled
with deposits of windblown silt, called loess. Dust storms
deposited this material over thousands of years. When loess is
breached by streams or road cuts, it tends to maintain vertical
cliffs and lacks any visible layers, as you can see in figure
4.32a.
The distribution of loess worldwide indicates two primary
sources for this sediment: deserts and glacial deposits of
stratified drift. The thickest and most extensive loess deposits
occur in western and northern China
figure 4.32 a.This vertical loess bluff near the Mississippi River
in southern Illinois is about 3 meters (10 feet) high. (Photo by
James E. Patterson) b. In parts of China, loess has sufficient
structural strength to permit the excavation of cavelike
dwellings. (Photo by Betty Crowell) C.This satellite image from
November 1.2006, shows streamers of windblown dust moving
southward into the Gulf of Alaska. It illustrates a process
similar to the one that created many loess deposits in the

American Midwest during the Ice Age. Fine silt is produced by
the grinding action of glaciers and then transported beyond the
margin of the ice by running water and deposited. Later, the
fine silt is picked up by strong winds and later deposited as
loess. (NASA)
135
Wind Deposits 135
(figure 4.32b). They were blown there from the extensive desert
basins of central Asia. Accumulations of 30 meters (100 feet)
are not uncommon, and thicknesses of more than 100 meters
(300 feet) have been measured. It is this fine, buff-colored
sediment that gives the Yellow River (Huang He) its name.
In the United States, deposits of loess are significant in many
areas, including South Dakota, Nebraska, Iowa, Missouri, and
Illinois, as well as portions of the Columbia Plateau in the
Pacific Northwest. Unlike the deposits in China, which
originated in deserts, the loess in the United States and Europe
is an indirect product of glacia-tion. Its source is deposits of
stratified drift. During the retreat of the ice sheets, many river
valleys were choked with glacial sediment. Strong winds
sweeping across the barren floodplains picked up the finer
sediment and dropped it as a blanket on areas adjacent to the
valleys ( )•
Sand Dunes
Like running water, wind releases its load of sediment when its
velocity falls and the energy available for transport diminishes.
Thus, sand begins to accumulate wherever an obstruction across
the path of the wind slows its movement. Unlike deposits of
loess, which form blanketlike layers over broad areas, winds
commonly deposit sand in mounds or ridges called dunes (figure
4.33).
DID YOU KNOW?

The highest dunes in the world are located along the southwest
coast of Africa in the Namib Desert (see the chapter opening
photo). In places, these huge dunes reach heights of 300 to 350
m (1000 to 1167 ft).The dunes at Great Sand Dunes National
Park in southern Colorado are the highest in North America,
rising more than 210m (700 ft) above the surrounding terrain.
As moving air encounters an object, such as a clump of
vegetation or a rock, the wind sweeps around and over it,
leaving a shadow of more slowly moving air behind the obstacle
as well as a smaller zone of quieter air just in front of the
obstacle. Some of the sand grains moving with the wind come to
rest in these wind shadows. As the accumulation of sand
continues, it forms an increasingly efficient wind barrier to trap
even more sand. If there is a sufficient supply of sand and the
wind blows steadily long enough, the mound of sand grows into
a dune.
Many dunes have an asymmetrical profile, with the leeward
(sheltered) slope being steep and the windward slope more
gently inclined. Sand is rolled up the gentle slope on the
windward side by the force of the wind. Just beyond the crest of
the dune, the wind velocity is reduced and the sand
accumulates. As more sand collects, the slope
A. and B. Dunes commonly have an asymmetrical shape.The
steeper leeward side is called the slip face. Sand grains
deposited on the slip face create the cross beddin
g of the dunes.
c.
Over time, a complex pattern develops. Also notice that when
dunes are buried and become part of the sedimentary record, the

cross bedded structure is preserved.
d.
Cross beds are an obvious characteristic of the Navajo
Sandstone in Zion
National Park, Utah. (Photo by Dennis Tasa)
steepens and eventually some of it slides under the pull of
gravity. In this way, the leeward slope of the dune, called the
slip face, maintains a relatively steep angle. Continued sand
accumulation, coupled with periodic slides down the slip face,
results in the slow migration of the dune in the direction of air
movement (Figure 4.33).
As sand is deposited on the slip face, it forms layers inclined in
the direction the wind is blowing. These sloping layers are
called cross beds. When the dunes are eventually buried under
layers of sediment and become part of the sedimentary rock
record, their asymmetrical shape is destroyed, but the cross beds
remain as a testimony to their origin. Nowhere is cross bedding
more prominent than in the sandstone walls of Zion Canyon in
southern Utah (figure 4.34).
THE CHAPTER IN REVIEW
■•• A glacier is a thick mass of ice originating on the land from
the compaction and recrystallization of snow. It shows evidence
or past or present movement. Today, valley, ot alpine glaciers,
are found in mountain areas where they usually follow valleys
originally occupied by streams. Ice sheets exist on a much
larger scale, covering most of Greenland and Antarctica. Other
categories include ice caps and piedmont glaciers'.
» Glaciers move in part by flowing. On the surface of a glacier,
ice is brittle. However, below about 50 meters (165 feet),
pressure is great and ice behaves like a plastic material and

flows. A second important mechanism of glacial movement
consists of the whole ice mass slipping along the ground.
· Glaciers form in areas where more snow falls in winter than
melts during summer. Snow accumulation and ice formation
occur in the zone of accumulation. Beyond this area is the zone
of wastage, where there is a net loss to the glacier. The glacial
budget is the balance, or lack of balance, between accumulation
at the upper end of the glacier and loss at the lower end.
· Glaciers erode land by plucking (lifting pieces of bedrock out
of place) and abrasion (grinding and scraping of a rock surface).
Erosional features produced by valley glaciers include glacial
troughs, hanging valleys, cirques, arites, horns, and fiords.
- Any sediment of glacial origin is called drift. The two distinct
types of glacial drift are (a) till, which is material deposited
directly by the ice. and (b) stratiified drifi, which is sediment
laid down by meltwater from a glacier.
The most widespread features created by glacial deposition are
layers or ridges of till, called moraines. Associated with valley
glaciers are lateral moraines, formed along the sides of the
valley, and medial moraines, formed between two valley
glaciers that have joined. End moraines, which mark the former
position of the front of a glacier, and ground moraines,
undulating layers of till deposited as the ice front retreats, are
common to both valley glaciers and ice sheets.
Perhaps the most convincing evidence for several glacial
advances during the Ice Age is the widespread existence of
multiple layers of drift on land and an uninterrupted record of
climate cycles preserved in seafloor sediments. In addition to
massive erosional and depositional work, other effects of Ice
Age glaciers included the forced migration of animals, changes
in river courses, adjustment of the crust by rebounding after the
removal of the immense load of ice, and climate changes caused
by the existence of the glaciers themselves. In the sea, the most
far-reaching effect of the Ice Age was the worldwide change in
sea level that accompanied each advance and retreat of the ice
sheets.

Unsuccessfulwell Perchedwater table successful wellSpring.docx

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