Earth Science 7.1B Glaciers
Glacial Erosion

Glaciers are natures
bulldozers. Their massive
force tears rock from valley
floors and walls carrying the
rock fragments down the
valleys as they travel.

Unlike streams that drop
sediments when they slow;
glaciers carry their
sediments until they
Because of this, many landscapes were
eventually melt.
changed by glaciers in the last ice age.
How Glaciers Erode

Glaciers erode the land in two
ways:
 Plucking
 Abrasion

Plucking occurs when rock
surfaces beneath glaciers
break up as melted water from
the glacier penetrates the
cracks. When the water
refreezes, it expands and
pries rocks loose. As the
glacier flows over these
fractured rocks, it loosens and
lifts the rocks and carries them
with the ice flow, plucking
them from the Earth.
How Glaciers Erode

In abrasion, the load of rock
and ice combined acts like
sandpaper as it slides over
the surface. The rock
fragments act to smooth
and polish the rock
beneath, scraping away the
surface. This leaves telltale
lines going in one direction
showing the direction of ice
flow.

This pulverized rock
produced by this scraping is
called rock flour.
How Glaciers Erode
These marks are called glacial striations. They are the long
scratch marks left by the dragging of stones and boulders by
the glaciers as they moved, abrading the surface.
How Glaciers Erode: rate of erosion
As with all agents of erosion; the
rate of erosion is controlled by
several factors.
1: rate of glacier’s movement
2: thickness of the ice
3: shape, hardness, and amount
of rock fragments in the ice at
the glacier’s front edge
4: the type of surface and
hardness of the rock beneath the
glacier
Grooves in the rock cut by glacial abrasion
Landforms created by Glacial Erosion
Glaciers are responsible
for a variety of erosional
landscape features.
Among these are
•Glacial troughs
•Hanging valleys
•Cirques
•Aretes
•Horns
Glacial Troughs
Before a glacier moves
through a mountain valley,
the valley is usually a deep
V shaped trough from the
downcutting of streams.
When a glacier moves
through a mountain valley,
it widens, deepens, and
straightens this valley.
The once V shaped valley
becomes a U shaped
glacial trough.
Hanging Valleys
The amount of glacial erosion
depends on the thickness of the
ice.
Main glaciers cut deep U shaped
valleys that are deeper than the
those carved by smaller side
glaciers that feed into the main
glacier.
When all the ice recedes, the
valleys from these smaller side
glaciers is left higher up than the
floor of the main glacier. These
side valleys are called hanging
valleys.
Cirques
A cirque is a bowl shaped
depression at the head of a glacial
valley that is surrounded on three
sides by steep rock walls.
Cirques form at the head of a
glacier, in the zone of
accumulation. Glaciers carve
cirques by plucking rock from the
side walls and bottom. The glacier
acts as a means to move this
plucked debris down the mountain
as the glacial ice moves from top
to bottom.
Aretes and Horns
Other mountain landscape features
result from these glaciers, side
glaciers, and cirques.
Aretes are snaking, sharp-edged
ridges produced when cirques form
on either side of a divide. As the
cirques grow, the divide separating
them is ground down, reduced to a
sharp edge narrow divide.
Horns are pyramid shaped peaks
produced when several cirques
surround a mountain.
Swiss Matterhorn
Glacial Deposits

Glaciers transport huge
loads of debris as they
slowly advance across the
land.

When glaciers begin to
melt, they deposit these
large amounts of materials
they carry.

These deposited sediments
give us features ranging
from the rocky fields of New
England to the formation of
Cape Cod.
Glacial Drift

The term glacial drift is used
to include all sediments of
glacial origin; no matter
where, how, or what form
they were deposited in.

There are two types of
glacial drift:
 Till
 Stratafied drift
Till

Till is material deposited
directly by the glacier,
deposited as the glacier
melts and drops it’s load of
rock debris.

Because everything is
dropped at once, till is
usually an unsorted mixture
of rocks made up of all
sizes.
Course glacial till
Stratified Drift

Stratified drift is sediment laid
down by glacial meltwater.

Stratified drift contains
particles that are sorted by
size and weight of the debris.

Some deposits of debris come
from streams coming directly
from the glacier.

Stratified drift often consists of
sand and gravel because
meltwater cannot carry large
boulders and the finer
sediments remain suspended
and are carried farther away
by the waters flow.
Depositional features

Glaciers are
responsible for a
variety of
depositional features
including:
 Moraines
 Outwash plains
 Kettles
 Drumlins
 Eskers
Depositional features: Lateral Moraines

Lateral Moraines:
The sides of a glacial
valley gather large
amounts of debris
from the inside walls.

Lateral moraines are
ridges that form
along the sides of
glacial valleys when
the glaciers melt and
leave the material it
has gathered.
Depositional features: End Moraines



When the snow and ice
accumulate at the head of the
glacier at the same rate the
glacier melts, the glacier stays
stationary, balanced in it’s
glacial budget.
End Moraines form when
glaciers stay stationary for long
periods of time.
Within the glacier, the ice still
flows. This flow still carries rock
debris to the foot of the glacier
like a conveyer belt. Here the
debris builds up and produces
End Moraines.
Depositional Features: Ground Moraines

Ground moraines form when
glaciers begin to recede.

The glacier front continues to
deliver debris with the ice
movement but instead of
creating a ridge (as in the end
moraine) the retreating foot of
the glacier deposits the debris
as a rock strewn plain.

This rock strewn plain is a
ground moraine.
Depositional features: Terminal Moraines

Glaciers can periodically retreat
and than find balance again and
remain stationary for a long time
period.

A glacier will form an end
moraine when stationary than
create ground moraines in it’s
periods of retreat.

This pattern can repeat many
times before the glacier
completely melts.
Depositional features: Terminal Moraines

The farthest end moraine
created in this pattern of
stopping and retreating is called
the terminal moraine.

End moraines that form when
the ice front occasionally
becomes stationary during it’s
retreat are recessional end
moraines.
Depositional features: Outwash Plains

At the same time that end moraines
are forming, streams of fast moving
meltwater emerge from the bases
of glaciers.

This water is so full of fine
sediment it almost looks like milk.

Once it leaves the glacier, the
water slows and drops it’s sediment
in a broad ramp-like accumulation
downstream from the end moraine.
This sediment ramp resulting from
the retreating ice sheet is called an
outwash plain.
Depositional features: Kettle ponds and lakes

You can often find depressions;
ponds and small lakes called
kettles.

Kettles form when blocks of
stagnant ice become buried in
drift and eventually melt. This
melting leaves pits in the glacial
sediment resulting in small
ponds and lakes.
Kettle ponds
Depositional features: Drumlins and Eskers

Moraines are not the only
landscape features formed by
glacial deposits.

Drumlins are long elongated
hills composed of glacial till.
Drumlins are taller and steeper
on one end; ranging in height
from 15 to 60 meters and .4 to .8
kilometers long.

The steep side of the hill faces
the direction the glacier came
from and the gentle slope side
the direction the ice moved
toward.
Depositional features: Drumlins and Eskers

Eskers are snakelike ridges
composed of sand and gravel
that were deposited by streams
once flowing in tunnels beneath
glaciers.

They can be several meters
high and many kilometers long.

Many eskers are mined for the
sand and gravel they contain.
Glaciers of the Ice Age

Many of the deposits and
landscape features that
define North America
today are the product of
the glaciers that covered
this continent during the
last ice age.

During the last ice age,
glaciers covered almost
30% of the earth’s land.
North America, Siberia,
and Europe were
extensively covered.
Glaciers of the Ice Age


The ice sheets greatly
affected the drainage
patterns of major rivers in
North America.
Before the glaciers:
 The Missouri River
flowed north toward
Hudson Bay
 The Mississippi flowed
through central Illinois
 The Great lakes did not
exist
Glaciers of the Ice Age

The sheets of glacial ice
also triggered changes in
the climates of North
America beyond their
edges.

Regions that are arid
today became cooler and
wetter. This change in
climate at the time resulted
in the formation of large
lakes in regions of Nevada
and Utah.
Key Concepts :

Many landscapes were changed by the widespread
glaciers of the last ice age.

Glaciers are responsible for a variety of erosional
landscape features; such as hanging valleys, cirques,
aretes, and horns.

Glacial drift applies to all sediments of glacial origin; no
matter how, where, or what form they are deposited.
There are two types; till and stratified drift.

Glaciers are responsible for a variety of depositional
features, including moraines, outwash plains, kettles,
drumlins, and eskers.