Glaciers have
played an important role in the shaping of landscapes
in the middle and high latitudes and in alpine environments.
Their ability to erode soil and rock, transport sediment, and deposit sediment is
extraordinary. During the last glacial period more
than 50 million square kilometers of land surface were
geomorphically influenced by the presence of glaciers.
Glacial Erosion
Two major erosional processes
occur at the base of a glacier.
First, at the base of a glacier, large amounts of loose rock and sediment are
incorporated into the moving glacial ice by partial melting
and refreezing. The second process of erosion involves
the abrasive action
of the held rock and sediment held by the ice on the
surface underneath the glacier. This abrasive process
is known as scouring. Scouring creates a variety
of features. The most conspicuous feature of scouring
is striations (Figure
10af-1). Striations appear as scratches of various
size on rock surfaces. In some cases, abrasion can polish
the surface of some rock types smooth. This geomorphic
feature is known as glacial
polish. The abrasive action of scouring also
produces a fine clay-sized sediment that
is often transported away from the glacier by meltwater.
As a result of this process, glacial meltwater can have
a light, cloudy appearance, and is called glacial
milk.
Figure
10af-1: Glacial
striations, Lac Blanchet, Canada. (Source: Natural
Resources Canada - Terrain Sciences Division
- Canadian Landscapes). |
The second major erosional process
that occurs at the base of a glacier is plucking.
Plucking is the process of particle detachment by moving
glacial ice. In this process, basal ice freezes in rock
surface cracks. As the main body of the glacial ice moves
material around the ice in the cracks is pulled and plucked
out. The intensity of the plucking process is greatest
on the lee-side of
rock mounds. When combined with glacial abrasion, the
action of plucking on rock mounds produces a unique asymmetrical
feature known as roche moutonnee. Roche moutonnee
are smooth on the side of ice advancement and steep and
jagged on the opposite side.
Glaciers generally
flow over the land surface along a path of least resistance.
The flow of an alpine
glacier into a valley, causes the glacier to
rapidly advance producing a swollen tongue of ice at
the glacier's snout,
known as a lobe.
As the lobe moves down the valley it often encounters
the lobes of other glaciers from connecting valleys.
The glacier grows in size with addition of the flow of
connected sub-valleys. The following image illustrates
one of these networks of connected alpine glaciers
(Figure 10af-2).
Figure 10af-2: Merging
alpine glaciers viewed from above (Source: NASA).
A number of distinct erosional features
can be observed in mountainous regions that have experienced the effects of
glaciation. Much of this erosion is exerted on the bottoms and sides of alpine
valleys that guide the flow of glaciers. This erosion causes the bottom and
the sides of any glaciated valley to become both wider and deeper over time.
Glacial erosion also results in a change in the valley's cross-sectional shape.
Glacial valleys tend to have a pronounced U-shape that contrasts sharply with
V-shape valleys created by stream erosion. Small adjoining feeder valleys entering
a large valley in a glaciated mountainous region tend to have their floors
elevated some distance above the level of the main valley's floor. Geomorphologist
call this landform a hanging
valley. Hanging valleys develop because of two reasons: 1) larger,
more massive glaciers create greater erosion and subsequently a deeper valley,
and 2) some valleys have seen more glacier ice pass through them which also
results in more erosion and a deeper valley. Many hanging valleys are also
the sites of sensational waterfalls.
Some of the other features associated with
glacier erosion in alpine regions are cirques, horns,
and arêtes (Figure 10af-3). Cirques are
the bowl shaped depressions found at the head of glacial
valleys. For most alpine glaciers, cirques are the areas
in the alpine valleys where snow first accumulated and
was modified into glacial ice. The glaciers that occupy
cirques are called cirque
glaciers. Horns are
pyramidal peaks that form when several cirques chisel
a mountain from three or more sides. The most famous
horn is the Matterhorn found in the Swiss Alps. Arêtes are
the narrow serrated ridges found in glaciated alpine
areas. Arêtes form when two opposing cirques back
erode a mountain ridge.
Figure
10af-3: Features
associated with alpine glaciation. |
Talus and
other foot-slope deposits are also common in a
glaciated valley. Because of the enhancement of freeze-thaw processes
bedrock in alpine areas is weathered by
the growth of ice crystals. This type of weathering shatters
the bedrock into sharp angular fragments that accumulate
at the bottom of rock slopes as talus. Much of the debris
carried by an alpine glacier comes from valley sides
where talus accumulates.
The erosional
landforms produced by continental
glaciers are usually less obvious than those
created by alpine glaciers. Like alpine glaciers,
the movement of continental glaciers followed topographic
trends found in the landscape. Continental ice sheets
were very thick, between 1000 to 3000 meters. The
mass of these glaciers covered all but the highest
features and had extremely strong erosive power.
Much of the Canadian Shield shows the effects
of abrasion and gouging which created glacial
polish and striations on
bedrock surfaces. In some areas, continental ice
sheets produced huge U-shaped valleys from previously
V-shaped stream valleys. In other areas, erosion
by the continental ice sheets scooped out large shallow
basins, many of which exist today as lakes. Many
of the lakes on the Canadian Shield, including those
of the Great Lakes, were created by glacial erosion.
Glacial Deposition
A large part of the surface of a glacier
is covered with a coating of sediment and rock debris.
This is especially prevalent near the snout of the glacier,
where most of the ice has been lost to ablation and sediment
is left behind. Sediment is added to glacial ice in two
ways. Large quantities of sediment are picked up by abrasion and plucking at
the base of the ice. In alpine areas, sediment is added
to the surface of the glacier from the valley walls through
various types of mass
movement. Much of the debris that is added to
the ice of the glacier is eventually delivered to the
snout because of the continual forward flow of glacial
ice. From the snout this material can be placed directly
from the ice or it can be deposited through the action
of flowing meltwater. Geomorphologists call
the later deposits glaciofluvial deposits.
The technical term used to describe material deposited
by the ice is called till or moraine.
All glacial deposits are by and large known as glacial
drift.
Till is
a heterogeneous combination of unstratified sediments
ranging in size from large boulders to minute particles
of clay.
When till is deposited along the edge of a glacier it
tends to form irregular hills and mounds known as moraines.
A terminal
moraine is a deposit that mark, the farthest
advance of a glacier. Moraine deposits created during
halts in the retreat of
the glacier are called recessional moraines.
The debris that falls from valley side slopes can be
concentrated in a narrow belt and cause a deposit known
as a lateral moraine (Figure 10af-4).
When two glaciers flow together, two lateral moraines
can merge to form an interior belt of debris, called
a medial
moraine (Figure 10af-5). A till
plain is a large, relatively flat plain of till
that forms when a sheet of ice becomes detached from
the main body of the glacier and melts in place. Sometimes
the sediments in a till plain can contain large boulders.
If these boulders are
transported a great distance from their place of origin,
they are called erratics (Figure
10af-6).
Figure
10af-4: Lateral
moraines along the sides of the Salmon Glacier,
British Columbia, Canada
(Photo © Trudy
Kavanagh). |
Figure
10af-5: Medial
moraine down the center of the Salmon Glacier,
British Columbia, Canada
(Photo © Trudy
Kavanagh). |
Figure
10af-6: Glacial
erratic near Point Lake, Northwest Territories.
Glacial erratics are large pieces of rock
that have been transported away from their
source areas by moving glacial ice sheets.
(Source: Natural
Resources Canada - Terrain Sciences Division
- Canadian Landscapes). |
Glaciofluvial deposits
are generally quite stratified and less sorted in terms
of particle size. Outwash deposits
are formed when sand is eroded, transported, and deposited
by meltwater streams from the glacier's snout and nearby
till deposits to areas in front of the glacier. Outwash
plain develops when there are a great number
of meltwater streams depositing material ahead of the
glacier (Figure 10af-7).
Figure
10af-7: Glacier
snout and outwash plain, Bylot Island, Canada.
(Source: Natural
Resources Canada - Terrain Sciences Division
- Canadian Landscapes). |
Glaciofluvial deposits are also associated with the
melting of stagnant ice at the front of the glacier.
Where sediment rich water flows into a crevasse or
depression in the ice, a conical-shaped pile of sand
and gravel, known as a kame,
can form (Figure 10af-8). Many kames are often
found on or at the edge of moraines.
Figure
10af-8: Kame,
La Bluff, Ile de la Grande Entree, Canada.
This kame was deposited during the main stage
of the last glaciation when the Laurentide
ice sheet filled the Gulf of St. lawrence.
(Source: Natural
Resources Canada - Terrain Sciences Division
- Canadian Landscapes). |
Glaciers can also contain sinuous flows
of meltwater that occur in ice tunnels at the base of
the ice. The beds of these sub-surface glacial streams
are composed of layers of sand and gravel.
When the ice melts from around the meltwater tunnels,
the beds of sand and gravel are deposited on the Earth's
surface as long twisting ridges known as eskers (Figure 10af-9).
Figure
10af-9: Esker
near Lac du Sauvage, Northwest Territories.
The slightly curving thin ridge in the centers
of this photo is the esker. The flat region
in the foreground to the left of the esker
was formed by glacial outwash. (Source: Natural
Resources Canada - Terrain Sciences Division
- Canadian Landscapes). |
Another feature of continental glaciation
are hill shaped deposits of till known as drumlins (Figure
10af-10). Drumlins often occur in large numbers
across areas of New York and Wisconsin, USA and Ontario,
Canada. The streamline shape of these glacial features
resembles a extended teaspoon laying bowl down. The
gently sloping tapered end of the drumlin points in
the general direction the glacier traveled. Drumlins
also come in a variety of dimensions. Lengths can range
from 100 to 5000 meters and heights can sometimes exceed
200 meters. A couple theories exist to explain their
formation. The most excepted theory suggests they form
when saturated ground sediments oozes up into hollows
at the base of an advancing glacier. The sediment is
then stretched out and molded into a streamline form
as the ice moves forward.
Figure
10af-10: Drumlin
field in northwestern Manitoba. These features
are made of till and are formed at the
base of a glacial ice sheet. The long axis
of this feature aligns with the direction
of glacial movement. (Source: Natural
Resources Canada - Terrain Sciences Division
- Canadian Landscapes). |
When glaciers are rapidly retreating,
numerous blocks of ice can become detached from the main
body of the glacier. If glacial drift is then placed
around the ice, a depression on the surface called a kettle
hole can be created when the ice melts
. Kettle holes are commonly found on moraine and outwash
plain deposits. Large kettle holes that reach below the
water table can form into lakes. The photograph below
shows some kettle lakes in glaciofluvial outwash complex
located in the Northwestern District of Mackenzie, Northwest
Territories (Figure 10af-10). Some kettle holes
develop into wetlands such as bogs, swamps, and marshes.
Figure
10af-11: Kettle
Lakes, Northwest Territory, Canada. (Source: Natural
Resources Canada - Terrain Sciences Division
- Canadian Landscapes). |