1. Chapter 19: Glacial
Modification of Terrain
McKnight’s Physical Geography:
A Landscape Appreciation,
Tenth Edition, Hess

2. Glacial Modification of Terrain
• The Impact of Glaciers on the Landscape
• Glaciations of the Past and Present
• Types of Glaciers
• Glacier Movement and Formation
• The Effects of Glaciers
• Continental Ice Sheets
• Mountain Glaciers
• The Periglacial Environment
• Causes of the Pleistocene
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3. The Impact of Glaciers on the
Landscape
• Snowpack over years turns to ice
• Ice mass motion under gravity grinds anything in its
path
• Glaciation increases erosion rate on a mountain by at
least 10 times to an unglaciated mountain
• Modifies flat landscapes as well
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4. Types of Glaciers
• Continental ice sheets
– Exist in nonmountainous areas
– Antarctica and Greenland only
two
– Outlet glaciers
• Mountain glaciers
– Highland icefields: ice sheets
that submerge most underlying
topography; valley and piedmont
glaciers
– Alpine glaciers: develop
individually instead of part of ice
field, cirque glaciers
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Figure 19-2

5. Glaciations Past and Present
• Glacial ice volume has varied considerably over last few
million years
• Evidence left behind allows scientists to determine the
chronology of past glaciations
• Pleistocene glaciation
– Began at least 2.59 million years ago
– Last major ice retreat occurred only 9000 years ago
– Dominant environmental characteristic was refrigeration of
high-latitude and high-elevation areas
– Consistent alterations of glacial and interglacial periods
– Wisconsin glacial stage marked end
– At peak, 1/3 of total land covered in ice
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6. Glaciations Past and Present
• Pleistocene glaciation (cont.)
– Laurentide ice and the Driftless Area
• Indirect effects of Pleistocene glaciation
– Periglacial processes
• Periglacial zone—zone where ice never existed but glacial factors
affected the landscape such as erosion from ice melt, solifluction
• Sea-level changes—buildup of ice on continents led to less drainwater on
continents and brought about a lowering of sea levels
• Crustal depression—the weight of the ice on the continents caused
continents to sink, ice melt allowed for continental rebound
• Pluvial developments—considerable runoff results in increased moisture,
leading to increased precipitation and less evaporation. Developed many
lakes, including the Great Salt Lake (formed from Lake Bonneville)
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7. Glaciations Past and Present
• Maximum extent of the Pleistocene glaciation
7© 2011 Pearson Education, Inc.Figure 19-5a

8. Glaciations Past and Present
• Contemporary glaciation
– Limited ice cover today (about
10% of total land surface)
– 96% of the total ice cover is
Greenland and Antarctica
– Antarctic ice cap
• Consists of two unequal sections
separated by Transantarctic
mountains
• West Antarctica has a few “dry
valleys”
– Greenland ice cap
– North American glaciers
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Figure 19-8

9. Glaciations Past and Present
• Climate change related to
contemporary glaciation
– Retreating of polar ice caps
– Shrinking ice caps an
indicator of a warming
climate
– Antarctic ice shelves
breaking
– Higher flow rates of outlet
glaciers
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Figure 19-10

10. Glacier Formation and Movement
• Require balance between
accumulation and ablation
• Snow begins as crystallized
water vapor
• Compressed to granular form
• More compression causes
granules to coalesce, névé/firn
• Further compression results in
glacial ice
• Ablation and accumulation
zones
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Figure 19-11

11. Glacier Formation and Movement
• Glacier “flow” is orderly sliding
of ice molecules
• Ice under extreme pressure
deforms instead of slipping
• Meltwater contributes surface
for glacier to slide on
• Flow in response to overlying
weight
• Plastic flow and basal slip
• Glacier flow versus glacier
advance
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Figure 19-12

12. The Effects of Glaciers
• Erosion by glaciers
– Volume and speed determine
success of glacial erosion
– Erosive power of moving ice
slightly larger than that of water
– Glacial plucking—picking up of
rock material through refreezing
of meltwater
– Glacial abrasion—bedrock
worn down by rock debris
embedded in glacier
– Subglacial meltwater erosion
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Figure 19-15

13. The Effects of Glaciers
• Transportation by glaciers
– Glaciers effective to move
large rock pieces
– Typically move glacial flour
– Most rock material
transported along base
of the ice
– Remaining glacial ice free
of rock debris
– Role of flowing water on
moving ice, melt streams
– Cracks in ice in which
streams run—moulins
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Figure 19-16

14. The Effects of Glaciers
• Deposition by glaciers
– Glaciers move lithospheric
material from one region to
another in a vastly different
form
– Material moved by glaciers—
drift
– Till—rock debris deposited
by moving or melting ice
– Large boulders that are
different from surrounding
local bedrock, glacial erratics
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Figure 19-18

15. The Effects of Glaciers
• Deposition of meltwater
– Large portion of debris carried
by glaciers deposited or
redeposited by meltwater
– Subglacial streams from
glaciers carry sedimentary
material
– Glaciofluvial deposition
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Figure 19-17

16. Continental Ice Sheets
• Ice sheets third most
extensive feature on the
planet
• Development and flow of
ice sheets
– Pleistocene ice sheets
originated in midlatitudes
and subpolar regions
– Ice flowed outward from
center of accumulation
– Ice sheets ebbed and flowed
with changing climate
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Figure 19-19

17. Continental Ice Sheets
• Erosion by ice sheets
– Principal topography from ice sheet is gently undulating
surface
– Valley bottoms created from moving ice
– Roche mountonnée, stoss side versus lee side
– Postglacial landscape has low relief but is not absolutely flat
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Figure 19-20

18. Continental Ice Sheets
• Deposition by Ice Sheets
– Irregular, uneven surface of
deposition, till plain
– Moraines—land consisting
primarily of till
– Three types of moraines
• Terminal moraine—marks
outermost limit of glacial advance
• Recessional moraine—positions
where ice front is stabilized
• Ground moraine—large quantities
of till laid down from under a
glacier instead of from its edge,
kettles
– Drumlins 18© 2011 Pearson Education, Inc.
Figure 19-21
Figure 19-24

19. Continental Ice Sheets
• Glaciofluvial features
– Deposition of debris by ice-sheet
meltwater produces features,
composed of stratified drift
– Composed of gravel, sand, silt
since meltwater is incapable of
moving larger material
– Outwash plains
– Valley trains
– Eskers
– Kames
– Lakes very common
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Figure 19-26

20. Mountain Glaciers
• Mountain glacier
development and flow
– Usually form in sheltered
depressions near heads of
stream valleys
• Erosion by mountain glaciers
– Basic landform in glaciated
mountains is the cirque
– Marks the location where an
alpine glacier originated
– Shifting equilibrium line
generate quarrying action,
bergschrund formation
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Figure 19-29

21. Mountain Glaciers
• Erosion by mountain glaciers
(cont.)
– Quarried fragments from
cirque carried away when ice
flows out of cirque
– Cirque ice melts away,
depression that holds water
is a tarn
– Several cirques cut back into
interfluve result in spine of
rock, an arête
– Cols and horns
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Figure 19-31

22. Mountain Glaciers
• Erosion in the valleys
– Some glaciers never leave
cirques
– Principle erosive work is to
deepen, steepen, and widen
valley
– U-shaped glacial troughs
– Glacial steps result from
differences in rock resistance
– Small cliffs and small lakes,
paternoster lakes
– Hanging valleys
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Figure 19-38

23. Mountain Glaciers
• Deposition by mountain
glaciers
– Continental ice sheets
more responsible for
deposition than mountain
glaciation
– Moraines primary
deposition mechanism
– Lateral moraines
– Medial moraines
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Figure 19-41

24. Mountain Glaciers
• Distribution of moraines around a valley glacier
24© 2011 Pearson Education, Inc.Figure 19-42

25. The Periglacial Environment
• Periglacial—on the
perimeter of glaciation
• Permafrost presence
• Frozen ground exists in
Alaska, Canada, Russia
• Extends to great depths
• Patterned ground
• Proglacial lakes
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Figure 19-45

26. Causes of the Pleistocene
Glaciations
• What initiates ice ages?
• Any plausible theory must
account for four main
characteristics
– Ice accumulation is in both
hemispheres but is non-
uniform
– Concurrent development of
pluvial conditions in dryland
areas
– Multiple ice advance and
retreat cycles
– Eventual total deglaciation
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Figure 19-46

27. Causes of the Pleistocene
Glaciations
• Cold versus warm climate for glaciation
• Role of Milankovitch cycles
• Variations in solar output
• Variations in carbon dioxide in atmosphere
• Changes in continental positions
• Atmospheric circulations
• Tectonic upheaval
• Are we still in an ice age?
27© 2011 Pearson Education, Inc.
Figure 19-47

28. Summary
• Glaciers impact the landscape through ice mass motion
and associated erosion
• There are two primary well known eras for glaciation, the
Pleistocene and contemporary glaciation
• During the Pleistocene, ice occupied a third of the total
land mass of the Earth
• There were four indirect effects of the Pleistocene
glaciation
• Antarctica and Greenland make up a large percentage
of the contemporary glaciation
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29. Summary
• There are two primary types of glaciers, continental ice
sheets and mountain glaciers
• Glacier formation involves the process of converting
snow to ice through intense pressure and snow
accumulation
• Glaciers move via sliding along a land surface;
meltwater helps enhance the ability of glaciers to move
• Glaciers have two primary erosive effects
• Glaciers are capable of transporting large rock material
as well as glacial flour
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30. Summary
• Glaciers deposit material through their transport as well
as meltwater
• Continental ice sheets have a unique set of erosive and
depositional characteristics
• Moraines are glacier-deposited landforms that consist
entirely or largely of till
• Glaciofluvial features play an important role in the
distribution of deposited glacier material
• Mountain glaciers have limited erosive and depositional
characteristics
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31. Summary
• Valley effects of mountain glaciers can drastically alter
the landscape in these regions
• The region surrounding a glacier that is modified by the
glacier but not under it is called the periglacial
• There are numerous unique characteristics of the
periglacial environment
• The exact causes of ice ages, including the Pleistocene,
are unknown, though many theories hypothesize about
the different effects that could have contributed
• It is unknown if we are still in an ice age
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