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CHAPTER 19 • GL ACIAL SYSTEM S AN D L AN DFOR M S
(b) Maximum glaciation
(a) Preglacial fluvial topography
Horn
Arête
Tarn
Hanging
valley
Glacial
trough
Ci
Continental Glaciers
● FIGURE
19.20
(a) Preexisting mountain stream valleys provide the path of least resistance toward lower elevations, and (b) are therefore preferred locations
for advancing alpine glaciers. (c) After the ice disappears, the tremendous geomorphic work accomplished by the alpine glaciers is evident in
the distinctive erosional landforms created by the moving ice.
How do the valley profiles change from preglacial to postglacial
times?
Braided meltwater streams laden with sediment commonly
issue from the glacier terminus ( ● Fig. 19.24). The sediment,
called glacial outwash, is deposited beyond the terminal moraine, with larger rocks deposited first, followed downstream by
progressively finer particles. Often resembling an alluvial fan confined by valley walls, this depositional form left by braided streams
is called a valley train. Valleys in glaciated regions may be filled
to depths of a few hundred meters by outwash or by deposits from
moraine-dammed lakes.
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In terms of their size and shape, continental glaciers, which consist of
ice sheets or the somewhat smaller ice caps, are very different from
alpine glaciers. However, all glaciers share certain characteristics and
processes, and much of what we have discussed about alpine glaciers
also applies to continental glaciers.The geomorphic work of the two
categories of glaciers differs primarily in scale, attributable to the
enormous disparity in size between continental and alpine glaciers.
Ice sheets and ice caps are shaped somewhat like a convex
lens in cross section, thicker in the center and thinning toward
the edges. They flow radially outward in all directions from where
the pressure is greatest, in the thick, central zone of accumulation,
to the surrounding zone of ablation ( ● Fig. 19.25). Like all glaciers, ice sheets and ice caps advance and retreat by responding
to changes in temperature and snowfall. As with alpine glaciers
flowing down preexisting stream valleys, movement of advancing continental glaciers takes advantage of paths of least resistance
found in preexisting valleys and belts of softer rock.
Existing Continental Glaciers
Glaciers of all categories currently cover about 10% of Earth’s
land area. In area and ice mass, alpine glaciers are almost insignificant compared to the huge ice sheets of Greenland and Antarctica,
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USGS/Bruce F. Molnia
USGS
C O N T I N E N TA L G L A C I E R S
(a)
● FIGURE
19.21
USGS/Austin Post
The alpine glacial topography of the north-central Chugach Mountains,
Alaska, includes numerous cirques, arêtes, and horns as well as deposits
of alpine glaciation.
J. Petersen
© Matt Ebiner
(b)
● FIGURE
19.22
Glacial till, here deposited by an alpine glacier in the Sierra
Nevada, consists of an unsorted, unstratified, rather jumbled
mass of gravel, sand, silt, and clay.
Why does till have these disorganized characteristics?
which account for 96% of the area occupied by glaciers today. Ice
caps currently exist in Iceland, on the arctic islands of Canada and
Russia, in Alaska, and in the Canadian Rockies.
The Greenland ice sheet covers the world’s largest island with
a glacier that is more than 3 kilometers (2 mi) thick in the center.
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(c)
● FIGURE
19.23
(a) Lateral moraines on the Kenai Peninsula of Alaska clearly mark the
former position of the side margins of a glacier, thereby indicating its
width as well. (b) Where two valley glaciers flow together, their adjoining lateral moraines merge into a medial moraine of the larger glacier
that they create. Medial moraines comprise the characteristic stripes that
appear on these glaciers in the Alaska Range. (c) The glacial deposits of
an end moraine form a ridge that rims the position of the terminus of
this glacier in Pakistan.
What can we learn from studying moraines?
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CHAPTER 19 • GL ACIAL SYSTEM S AN D L AN DFOR M S
not float into heavily used shipping lanes, these extensive tabularshaped Antarctic icebergs are not as much of a hazard to navigation
( ● Fig. 19.29). They do, however, add to the problem of access to
Antarctica for scientists. The huge wall of the ice shelf itself, the
massive, broken-up sea ice, and the extreme climate combine to
make Antarctica inaccessible to all but the hardiest individuals and
equipment. This icy continent serves, however, as a natural laboratory for scientists from many countries to study Antarctic glaciology, climatology, and ecology.
USGS
Pleistocene Glaciation
● FIGURE
19.24
These braided stream channels carry meltwater and large amounts
of outwash derived from glaciers in Denali National Park, Alaska.
Why are braided channels often associated with the deposition
of glacial outwash?
● FIGURE
19.25
This diagram shows how ice in an ice sheet flows outward and
downward from the center where the glacier is thickest.
How is this manner of ice flow different from and similar to that
of a valley glacier?
The only land exposed in Greenland is a narrow, mountainous
coastal strip ( ● Fig. 19.26). Where the ice reaches the ocean, it
usually does so through fjords. These flows of ice to the sea resemble alpine glaciers and are called outlet glaciers. The action
of waves and tides on the outlet glaciers breaks off huge ice masses
that float away. The resulting icebergs are a hazard to vessels in the
North Atlantic shipping lanes south of Greenland. Tragic maritime
disasters, such as the sinking of the Titanic, have been caused by
collisions with these large irregular chunks of ice, which are ninetenths submerged and thus mostly invisible to ships. Today, with
iceberg tracking by radar, satellites, and the ships and aircraft of the
International Ice Patrol, these sea disasters are minimized.
The Antarctic ice sheet covers some 13 million square kilometers (5 million sq mi), almost 7.5 times the area of the Greenland ice sheet ( ● Fig. 19.27). As in Greenland, little land is exposed in Antarctica, and the weight of the 4.5-kilometer- (nearly
3 mi) thick ice in some interior areas has depressed the land well
below sea level. Where the ice reaches the sea, it contributes to
the ice shelves, enormous flat-topped plates of ice attached to
land along at least one side ( ● Fig. 19.28). These ice shelves are
the source of icebergs in Antarctic waters, which do not have the
irregular surface form of Greenland’s icebergs. Because they do
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The Pleistocene Epoch of geologic time was an interval of
great climate change that began about 2.4 million years ago
and ended around 10,000 years before the present. There were
a great number of glacial fluctuations during the Pleistocene,
marked by numerous major advances and retreats of ice over
large portions of the world’s landmasses. It is a common misconception that the continental ice sheets originated, and remain
thickest, at the poles. When the Pleistocene glaciers advanced,
ice spread outward from centers in Canada, Scandinavia, and
eastern Siberia, as well as Greenland and Antarctica, while alpine glaciers spread to lower elevations. At their maximum extent in the Pleistocene, glaciers covered nearly a third of Earth’s
land surface ( ● Fig. 19.30). At the same time, the extent of sea
ice (frozen sea water) expanded equatorward. In the Northern
Hemisphere, sea ice was present along coasts as far south as Delaware in North America and Spain in Europe. Between each
glacial advance, a warmer time called an interglacial occurred,
during which the enormous continental ice sheets, ice caps,
and sea ice retreated and almost completely disappeared. Studies of glacial deposits and landforms have revealed that within
each major glacial advance, many minor retreats and advances
occurred, reflecting smaller changes in global temperature and
precipitation. During each advance of the ice sheets, alpine
glaciers were much more numerous, extensive, and massive in
highland areas than they are today, but their total extent was still
dwarfed by that of the continental glaciers.
North America and Eurasia experienced major glacial expansion during the Pleistocene. In North America, ice sheets extended as far south as the Missouri and Ohio Rivers and covered
nearly all of Canada and much of the northern Great Plains, the
Midwest, and the northeastern United States. In New England,
the ice was thick enough to overrun the highest mountains, including Mount Washington, which has an elevation of 2063 meters (6288 ft). The ice was more than 2000 meters (6500 ft) thick
in the Great Lakes region. In Europe, glaciers spread over most of
what is now Great Britain, Ireland, Scandinavia, northern Germany,
Poland, and western Russia. The weight of the ice depressed the
land surface several hundred meters. As the ice receded, the land
rose by isostatic rebound. Measurable isostatic rebound still raises
elevations in parts of Sweden, Canada, and eastern Siberia by up
to 2 centimeters (1 in.) per year and it may cause Hudson Bay and
the Baltic Sea to emerge someday above sea level. Should Greenland and Antarctica lose their ice sheets, their depressed central
land areas would also rise to reach isostatic balance.
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NASA/Goddard Space Flight Center/MODIS
C O N T I N E N TA L G L A C I E R S
Arctic
Circle
1
66 −
N
2
0
0
(a)
● FIGURE
300 km
300 mi
(b)
19.26
The Greenland ice sheet. (a) Except for the mountainous edges, the Greenland ice sheet almost completely
covers the world’s largest island. Ice thickness is more than 3000 meters (10,000 ft) and depresses the
bedrock below sea level. In this satellite view, several outlet glaciers flow seaward from the ice sheet to
the east coast of the island. (b) The extent of the Greenland ice sheet.
The geomorphic effects of the last major glacial advance,
known in North America as the Wisconsinan stage, are the most
visible in the landscape today. The glacial landforms created during the Wisconsinan stage, which ended about 10,000 years ago,
are relatively recent and have not been destroyed to any great extent by subsequent geomorphic processes. Consequently, we can
derive a fairly clear picture of the extent and actions of the ice
sheets, as well as alpine glaciers, at that time.
Where did the water locked up in all the ice and snow come
from? Its original source was the oceans. During the periods of
glacial advance, there was a general lowering of sea level, exposing
large portions of the continental shelf and forming land bridges
across the present-day North, Bering, and Java Seas. The most recent Pleistocene melting and glacial retreat raised sea level a similar amount—about 120 meters (400 ft). Evidence for this rise in
sea level can be seen along many coastlines around the world.
NASA
Continental Glaciers and
Erosional Landforms
● FIGURE
19.27
The Antarctic ice sheet. The world’s largest ice sheet covers an area larger
than the United States and Mexico combined, and has a thickness of
more than 4500 meters (14,000 ft). This satellite mosaic image covers
the whole south polar continent. Notice that most of Antarctica is ice-covered
(white and blue on the image). The only rocky areas (darker areas on the
image) are the Antarctic Peninsula and the Transantarctic Mountains. Large
ice shelves flow to the coastline, the largest being the Ross Ice Shelf.
The image is oriented with the Greenwich Meridian at the top.
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Ice sheets and ice caps erode the land through plucking and
abrasion, carving landscape features that have many similarities
to those of alpine glaciers, but on a much larger scale. Erosional
landforms created by ice sheets are far more extensive than those
formed by alpine glaciation, stretching over millions of square kilometers of North America, Scandinavia, and Russia. As ice sheets
flowed over the land, they gouged Earth’s surface with striations,
enlarged valleys that already existed, scoured out rock basins, and
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CHAPTER 19 • GL ACIAL SYSTEM S AN D L AN DFOR M S
G EO G R A P H Y ’ S S PAT I A L SC I E N C E P E R S P EC T I V E
The Driftless Area—A Natural Region
T
he Driftless Area, a region in the
upper midwestern United States,
mainly covers parts of Wisconsin,
but also sections of the adjacent states of
Iowa, Minnesota, and Illinois. This region
was not glaciated by the last Pleistocene
ice sheets that extended over the rest of
the northern Midwest, so it is free of glacial
deposits from that time. Drift is a general
term for all deposits of glacial ice and its
meltwater, thus the regional name is linked
to its geomorphic history. There is some
debate about whether earlier Pleistocene
glaciations covered the region, but the
most recent glacial advance did not override this locality.
The landscape of the Driftless Area is
very different from the muted terrain and
low rounded hills of adjoining glaciated
terrain. Narrow stream valleys, steep bluffs,
caves, sinkholes, and loess-covered hills
produce a scenic landscape with many
loess, both derived from the surrounding
glaciers and their sediments.
The Driftless Area has a unique landscape because of its isolation, an unglaciated “island” or “peninsula” almost
completely surrounded by extensive glaciated regions. The rocks, soils, terrain, relief,
vegetation, and habitats contrast strongly
with the nearby drift-covered terrains and
provide a glimpse of what the preglacial
landscapes of this midwestern region may
have been like. The Driftless Area also
provides distinctive habitats for flora and
fauna that do not exist in the adjacent
glaciated terrain. Unique and unusual aspects of its topography and ecology attract
ecotourists and offer countless opportunities for scientific study. For these reasons,
there are many protected natural lands in
the Driftless Area, an excellent example
of a natural region defined by its physical
geographic characteristics.
landforms that would not have survived
erosion by a massive glacier.
In some parts of the United States,
lobes of the ice sheet extended as
much as 485 kilometers (300 mi)
farther south than the latitude of the
Driftless Area. Why this region remained
glacier-free is a result of the topography
directly to the north. A highland of resistant rock, called the Superior Upland,
caused the front of the glacier to split
into two masses, called lobes, diverting
the southward-flowing ice around this
topographic obstruction.
The diverging lobes flowed into two
valleys that today hold Lake Superior and
Lake Michigan. These two lowlands were
oriented in directions that channeled the
glacial lobes away from the Driftless Area.
Although not overridden by these glaciers,
the Driftless Area did receive some outwash deposits and a cover of wind-deposited
90°
95°
85°
C A N
A
D
A
MINNESOTA
Wisconsinan
terminal
moraines
45°
WISCONSIN
MICHIGAN
Driftless
Area
IOWA
40°
Wisconsinan
terminal
moraines
ILLINOIS
INDIANA
OHIO
io n
Illinoian
terminal
moraines
at
MISSOURI
Southern extent
0 50 100 KILOMETERS
outh
S
0
of g
laci
ati o n
50 100 MILES
xte n
ne
er
t of
gl a
ci
KENTUCKY
A map of the Midwest shows that the Driftless Area was virtually
surrounded by the advancing Pleistocene ice sheets yet was
not glaciated.
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NASA/GSFC/LaRC/JPL, MISR Team
© Josh Landis, National Science Foundation
C O N T I N E N TA L G L A C I E R S
(a)
(b)
● FIGURE
19.28
(a) Forming a coastal buffer of ice between the open ocean and the ice-covered continent, Antarctica’s flattopped ice shelves are impressively beautiful. (b) In recent years, the ice shelves have experienced significant
destruction, in some cases generating huge icebergs, as with the breakup of the Ross Ice Shelf in 2000. This
image records the subsequent collapse of Antarctica’s Larsen B Ice Shelf, which occurred in 2002.
● FIGURE
19.29
Hajo Eicken /Alfred Wegener Institute for Polar and Marine Research
Antarctica’s flat-topped (tabular) icebergs, here with penguin passengers,
look quite different from the irregularly shaped icebergs of the Northern
Hemisphere.
What portion of an iceberg is hidden below the ocean surface?
● FIGURE
19.30
Glacial ice coverage in the Northern Hemisphere was extensive during
the Pleistocene. Glaciers up to several thousand meters thick covered
much of North America and Eurasia.
What might be a reason for some areas that were very cold during
this time, such as portions of interior Alaska and Siberia, being
ice-free?
1
66−
°N
2
Arctic Circle
45° N
45° N
1
1
23−
°N
2
0°
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1
66−
°N
2
23−
°N
2
Equator
0°
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CHAPTER 19 • GL ACIAL SYSTEM S AN D L AN DFOR M S
glaciation. These landforms generally range up to about 60 meters
(200 ft) in height ( ● Fig. 19.33). The last major Pleistocene glacial
advance through New England left its terminal moraine running
the length of New York’s Long Island and created the offshore
islands of Martha’s Vineyard and Nantucket, Massachusetts. Glacial retreat left recessional moraines forming both Cape Cod and
the rounded southern end of Lake Michigan. Both types of end
moraine are usually arc-shaped and convex toward the direction of ice flow. Their pattern and placement indicate that the ice
sheets did not maintain an even front but spread out in tongueshaped lobes channeled by the underlying terrain ( ● Fig. 19.34).
The positions of terminal and recessional moraines provide more
evidence than simply the direction of ice flow. Examining the
characteristics of deposited materials helps us detect the sequence of
advances and retreats of each successive ice sheet.
NASA
Till Plains In the zone of ice sheet deposition, massive
● FIGURE
19.31
Seen from the space shuttle, Lake Manicouagan on the Canadian Shield in Quebec occupies a circular depression that continental glaciers erosionally enhanced in rocks surrounding an
asteroid impact structure. Although the overall relief in the region
is low, note how erosion has exploited fractures to make linear
valleys and lakes. The annular (ring-shaped) lake has a diameter
of approximately 70 kilometers (43 mi).
smoothed off existing hills. The ice sheets removed most of the
soil and then eroded the bedrock below. Today, these ice-scoured
plains are areas of low, rounded hills, lake-filled depressions, and
wide exposures of bedrock ( ● Fig. 19.31).
When ice sheets expand, they cover and totally disrupt the
former stream patterns. Because the last glaciation was so recent
in terms of landscape development, new drainage systems have
not had time to form a well-integrated system of stream channels. In addition to large expanses of exposed gouged bedrock,
ice-scoured plains are characterized by extensive areas of standing
water, including lakes, marshes, and muskeg (poorly drained areas
grown over with vegetation that form in cold climates).
Continental Glaciers
and Depositional Landforms
The sheer disparity in scale causes depositional landforms of ice
sheets and ice caps to differ from those of alpine glaciers. Although terminal and recessional moraines, ground moraines, and
glaciofluvial deposits are produced by both categories of glaciers,
retreating continental glaciers leave significantly more extensive
versions of these features than alpine glaciers do ( ● Fig. 19.32 and
Map Interpretation: Continental Glaciation).
End Moraines Terminal and recessional end moraines deposited by Pleistocene ice sheets comprise substantial belts of
low hills and ridges on the land in areas affected by continental
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amounts of poorly sorted glacial till accumulated, often to depths
of 30 meters (100 ft) or more to form till plains. Because of the
uneven nature of deposition from the wasting ice, the topographic
configuration of land covered by till varies from place to place. In
some areas, the till is too thin to hide the original contours of
the land, while in other regions, thick deposits of till make broad,
rolling plains of low relief. Small hills and slight depressions, some
filled with water, characterize most till plains, reflecting the uneven glacial deposition. Some of the best agricultural land of the
United States is found on the gently rolling till plains of Illinois
and Iowa. The young, dark-colored, grassland soils (mostly mollisols) that developed on the till are extremely fertile.
Outwash Plains Beyond the belts of hills that represent
terminal and recessional moraines lie outwash plains composed
of meltwater deposits. These extensive areas of relatively low relief consist of glaciofluvial deposits that were sorted as they were
transported by meltwater from the ice sheets. Outwash plains,
which may cover hundreds of square kilometers, are analogous to
the valley trains of alpine glaciers.
Small depressions or pits, called kettles, mark some outwash
plains, till plains, and moraines. Kettles represent places where
blocks of ice were originally buried in glacial deposits. When the
blocks of ice eventually melted, they left surface depressions, and
many kettles now contain kettle lakes ( ● Fig. 19.35). For example, most of Minnesota’s famous 10,000 lakes are kettle lakes.
Some kettles occur in association with alpine glacial deposits, but
the vast majority are found in landscapes that were occupied by
ice sheets or ice caps.
Drumlins A drumlin is a streamlined hill, often about
0.5 kilometer (0.3 mi) in length and less than 50 meters (160 ft)
high, molded in glacial drift on till plains ( ● Fig. 19.36a). The
most conspicuous feature of drumlins is the elongated, streamlined shape that resembles half an egg or the convex side of a teaspoon. The broad, steep end faces in the up-ice direction, while
the gently sloping tapered end points in the direction that the ice
flowed; thus the geometry of a drumlin is the reverse of that of
roches moutonnées. Drumlins are usually found in swarms, called
drumlin fields, with as many as a hundred or more clustered together.
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C O N T I N E N TA L G L A C I E R S
Moraine
B id d
Interlobate moraine
Drumlins
End
Delta kame
● FIGURE
19.32
Landscape alteration by continental glacier sedimentation creates (a) depositional features associated with ice
stagnation at the edge of the glacier, and (b) landforms resulting from further modification of the ice-marginal
terrain as the glacier retreats.
How important is liquid water in creating the landforms shown here?
Opinions among scientists differ regarding the origin of drumlins,
particularly with respect to the relative importance of ice versus
meltwater processes in their formation. Drumlins are well developed in Canada, in Ireland, and in the states of New York and
Wisconsin. Boston’s Bunker Hill, a drumlin, is one of America’s
best-known historical sites.
kilometers is more typical of esker length. Most eskers probably
formed by meltwater streams flowing in ice tunnels at the base
of ice sheets. Eskers are prime sources of gravel and sand for
the construction industry. Being natural embankments, they are
frequently used in marshy, glaciated landscapes as highway and
railroad beds. Eskers are especially well developed in Finland,
Sweden, and Russia.
Eskers An esker is a narrow and typically winding ridge
composed of glaciofluvial sands and gravels (Fig. 19.36b). Some
eskers are as long as 200 kilometers (130 mi), although several
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Kames Roughly conical hills composed of sorted glaciofluvial
deposits are known as kames. Kames may develop from sediments
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CHAPTER 19 • GL ACIAL SYSTEM S AN D L AN DFOR M S
that accumulated in glacial ice pits, in crevasses, and
among jumbles of detached ice blocks. Like eskers,
kames are excellent sources for mining sand and
gravel and are especially common in New England.
Kame terraces are landforms resulting from accumulations of glaciofluvial sand and gravel along
the margins of ice lobes that melted away in valleys
of hilly regions. Examples of kame terraces can be
seen in New England and New York.
© John S. Shelton
Erratics Large boulders scattered in and on
● FIGURE
19.33
The hilly topography of an end moraine deposited by a continental ice sheet. This aerial
photo is of an end moraine on the Waterville Plateau in eastern Washington.
What makes the terrain at the left of the photo appear bumpier compared to the
smoother surface of the plain at the right?
the surface of glacial deposits or on glacially
scoured bedrock are called erratics if the rock
they consist of differs from the local bedrock
( ● Fig. 19.37). Moving ice is capable of transporting large rocks very far from their source. The source
regions of erratics can be identified by rock types,
which provide evidence of the direction of ice flow.
Erratics are known to be of glacial origin because
● FIGURE
19.34
Glacial deposits are widespread in the Great
Lakes region.
Lake Superior
Lake Huron
Lak
eM
ichig
an
Why do the many end moraines have such a
curved pattern?
rie
eE
k
La
Principal glacial deposits
in the Great Lakes Region
Till plains
Drift deposited
Principal glacial d
during middle
End moraines
in the Great Lake
and late
Outwash plains and
Wisconsinan
T trains
valley
Drift deposited
glaciation
during
middle
Glacial
lake deposits
E
and late
O
Undifferentiated
drift
Wisconsinan
ofvearlier glaciations
glaciation
G
Driftless
regions
U
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C O N T I N E N TA L G L A C I E R S
Lake
Ice
Glacial deposits
Glacial deposits
(b)
© Galen Rowell/CORBIS
NASA/Visible Earth/Jesse Allen, Earth Observatory
(a)
545
(c)
● FIGURE
(d)
19.35
J. Petersen
© Henry Kyllingstad/Photo Researchers
Kettle lakes form (a) where large, discrete blocks of ice buried or partly buried in glacial deposits (b) melt away
leaving a depression. (c) This kettle, near the headwaters of the Thelon River in the Northwest Territories,
Canada, formed in an area of ground moraine deposits, as can be seen by the hummocky topography.
(d) Numerous large kettle lakes dot part of Siberia due to Pleistocene continental glaciation.
(a)
● FIGURE
(b)
19.36
Drumlins and eskers can be identified by their distinctive shapes. (a) Drumlins, such as this one in Montana, are
streamlined hills elongated in the direction of ice flow. (b) An esker near Albert Lea, Minnesota, illustrates the
form of these ridges of meltwater sediments deposited in a tunnel under the ice.
What economic importance do eskers have?
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● FIGURE
19.37
This huge boulder, being examined by a class on a field trip to Yellowstone
National Park, is a glacial erratic, transported and deposited by a glacier.
NASA
© Robert B. Jorstad
CHAPTER 19 • GL ACIAL SYSTEM S AN D L AN DFOR M S
● FIGURE
19.38
A satellite image shows the Finger Lakes of New York that occupy linear,
glacially eroded basins excavated during the Pleistocene. A Pleistocene
ice sheet glaciated the entire region shown here.
they are marked by glacial striations, and are found only in glaciated terrain. Erratics can occur in association with alpine glaciers,
but they are best known and more impressive when deposited by ice
sheets, which have moved boulders weighing hundreds of tons over
hundreds of kilometers. In Illinois, for example, glacially deposited
erratics have come from source regions as far away as Canada.
What characteristics of the bedrock caused ice to form these narrow
lake basins?
Glacial Lakes
A recurring theme in this chapter has been the association of lakes
with glaciated terrain. Many, many thousands of glacially created lakes exist in depressions within deposits of the continental
glaciers that once covered much of North America and Eurasia.
The Pleistocene ice sheets created numerous other lake basins by
erosion, scooping out deep elongated basins along zones of weak
rock or along former stream valleys. New York’s beautiful Finger
Lakes are excellent examples of lakes in elongated, ice-deepened
basins ( ● Fig. 19.38). Lakes are also common in areas that were
impacted by erosion and deposition from alpine glaciers. Lakes
in ice-free cirques and glacial troughs are commonly contained
on one side by end moraines, as in the case of Washington’s Lake
Chelan and Lakes Maggiore, Como, and Garda in the Italian Alps
( ● Fig. 19.39). There is evidence for many other glacial lakes that
no longer exist. The glaciolacustrine (from glacial, ice; lacustrine, lake)
deposits of these ancient lakes prove their former existence and size.
Some lakes developed while the Pleistocene ice was present
where glacial deposition disrupted the surface drainage, or a glacier prevented depressions from being drained of meltwater.These
lakes usually accumulated where water became trapped between
a large end moraine and the ice front, or where the land sloped
toward, instead of away from, the ice front. In both situations,
ice-marginal lakes filled with meltwater. They drained and
ceased to exist when the retreat of the ice front uncovered an
outlet route for the water body.
During their existence, fine-grained sediment accumulated
on the floors of these ice-marginal lakes, filling in topographic
irregularities. As a result of this sedimentation, extremely flat
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From Mortara G., Mercalli L., 2002–Il lago epiglaciale “Effimero” sul ghiacciaio del Belvedere, Macugnaga, Monte Rosa. Nimbus, n. 23–24, p. 10–17.
What does this erratic illustrate about the ability of flowing ice to
modify the terrain?
● FIGURE
19.39
After withdrawal of the ice, end moraines often contribute to the formation of closed depressions in which glacial lakes accumulate.
surfaces characterize glacial plains where they consist of glaciolacustrine deposits. An outstanding example of such a plain is
the valley of the Red River in North Dakota, Minnesota, and
Manitoba. This plain, one of the flattest landscapes in the world,
is of great agricultural significance because it is well suited to
growing wheat. The plain was created by deposition in a vast
Pleistocene lake held between the front of the receding continental ice sheet on the north and moraine dams and higher topography to the south.This ancient body of water is named Lake
Agassiz for the Swiss scientist who early on championed the
theory of an ice age. The Red River flows northward eventually
into the last remnant of Lake Agassiz, Lake Winnipeg, which
occupies the deepest part of an ice-scoured and sedimentfilled lowland.
Another ice-marginal lake in North America produced
much more spectacular landscape features, but not in the area of
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547
© Jeff Schmaltz, MODIS Rapid Response Team, NASA/GSFC
GLACIAL LAKES
● FIGURE
19.40
Much of eastern Washington State (tan colored) is called the channeled
scablands because of the gigantic, largely abandoned river channels that
cross the region (dark gray). These channels represent huge outpourings
of water that occurred when glacial ice damming large lakes in the Pleistocene failed, releasing an amount of water perhaps ten times the flow
of all rivers of the world today.
(c) Glacial retreat (post-Valderan)
● FIGURE
the lake itself. In northern Idaho, a glacial lobe moving southward from Canada blocked the valley of a major tributary of the
Columbia River, creating an enormous ice-dammed lake known
as Lake Missoula. This lake covered almost 7800 square kilometers
(3000 sq mi) and was 610 meters (2000 ft) deep at the ice dam.
On occasions when the ice dam failed, Lake Missoula emptied
in tremendous floods that engulfed much of eastern Washington.
The racing floodwaters scoured the basaltic terrain, producing Washington’s channeled scablands consisting of intertwining
steep-sided troughs (coulees), dry waterfalls, scoured-out basins, and
other features quite unlike those associated with normal stream
erosion, particularly because of their gigantic size ( ● Fig. 19.40).
The Great Lakes of the eastern United States and Canada
make up the world’s largest lake system. Lakes Superior, Michigan,
Huron, Erie, and Ontario occupy former river valleys that were
vastly enlarged and deepened by glacial erosion. All the lake basins
except that of Lake Erie have been gouged out to depths below
sea level and have irregular bedrock floors lying beneath thick
blankets of glacial till. The history of the Great Lakes is exceedingly complex, resulting from the back-and-forth movement of
the ice front that produced many changes of lake levels and overflow in varying directions at different times ( ● Fig. 19.41).
(d) Postglacial Great Lakes
19.41
The Great Lakes of North America formed as the ice sheet receded at the close of the Pleistocene Epoch.
Name and locate the five Great Lakes.
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