Icefields and Glaciers
Glaciers, of all our alpine landforms, have had more visual
impact on our landscape than any erosional agent, save water. Look around you,
you’ll see smooth mountain bowls (cirques), wide mountain passes, broad
Despite their immense impact on the landscape, glaciers are a
relative newcomer to the mountain scene. The first buildup of ice occurred
approximately 240,000 years ago and ended 128,000 BP. It was followed by a
period of warmer climate. There were at least 5 subsequent advances that saw
glaciers reclaiming their valleys. In reality, the ice age was not something
that began, and then ended. It was a series of advances followed by warmer Interglacial
Periods. The final advance was quite recent,
beginning approximately 1200 AD, and ending at the turn of the 20th
century. Some scientists claim we may only be in another interglacial period
today and that the ice age may not be over. Only time will tell.
As snow accumulated in small depressions on the side of a
mountain, it eventually began to flow under the force of gravity (see Climatic
Change next page). As it moved, rock and debris falling onto the ice provided a
suitable abrasive that allowed it to scrape and scour its mountain home. As if
flowed out of its depression, it enlarged the basin in which it formed. This
resulted in the smooth glacial bowls visible on many of our peaks. They are
known as cirques, and once the ice melts, they may retain water in the form of a
tiny alpine pond or tarn.
From their cirque nursery, the glaciers flowed downhill until
they reached the valley bottom. Here, like a modern day river, they joined
larger rivers of ice and flowed downstream towards the plains. In some cases the
glaciers may have reached depths in excess of 1,000 metres.
Normally when people think of the ice age, chills run down their
spine as they imagine constant frigid temperatures and endless expanses of ice.
As is often the case, the reality was very different from this icy image. Today,
heavy winter snowfalls test the summer’s warmth, normally melting the snows
away by mid to late July. Today, the climate is in a state of equilibrium. Were
the average annual temperature to drop only a degree or two, as was the case
during the various advances of the Ice Age, a small amount of snow might linger
throughout the summer. This may mark the beginning of a long period of
accumulation. Each summer, the amount of summer snow or neve increases. This is
exactly how the last ice age began. With a slight overall cooling, snows began
to persist through the summer until the layers of snow became so thick that
their own weight compressed the snow on the bottom into ice. It was still not a
glacier, but simply a thick layer of ice.
Our perception of ice is very limited. We tend to think of the
brittle ice that comes out of our home freezer. Under the weight of hundreds,
perhaps thousands, of tonnes of overlying ice, the ice in the mountains begins
to misbehave, and begins to move and flow, slowly, under the force of
gravity—it is now a glacier. Therefore to call an accumulation of ice a
glacier, the ice must be formed by compression, and the ice must flow.
The glaciers in the Rockies have been almost like a yo-yo, moving up and down their valleys for a quarter of a million years. The first advance may have begun as long as 1.9 million years ago. During this period a large ice cap built up west of the Rockies, in the Columbia Mountains, and as the ice filled the Rocky Mountain Trench, it may have escaped eastward over the Rockies. This helped form our major mountain passes: the Yellowhead, Howse, Kicking Horse, and Crowsnest passes. Subsequent advances may have helped deepen these passes.
Beginning 240,000 years ago and ending 128,000 years ago, a
period known as the Great Glaciation took place. Geologists refer to this period
as the Illinoian Glaciation. This was the period of greatest accumulation, and
ice flowed eastward onto the plains. During its peak, they flowed eastward until
they encountered the large continental glacier that was rapidly expanding
westward. As this advance waned, an ice-free corridor was formed between the
retreating mountain glaciers, and the continental glaciers. During subsequent
advances, this corridor was maintained. Scientists believe it formed a travel
route allowing the ancestors of our native Indians to migrate from Asia to North
America. At the time, Alaska was joined to Siberia by a land bridge.
Glaciers always respond to the force of gravity. This constantly
pulls them downhill. Glaciers are also always moving, they never stop. If they
do, they are no longer a glacier. Glaciers must move! Often glaciers are
referred to as either advancing or retreating. Today, most of the glaciers are
of the latter category, that is, they are getting progressively smaller. To
understand how glaciers advance and retreat, we need to look at them a little
Glaciers can be broken into two zones, the zone of accumulation,
and the zone of ablation or melting. Near the uppermost point of the glacier,
the snow accumulates and provides the fresh snow needed to replace the melting
ice at the base of the glacier. This zone is distinguished by its clean
appearance, and bright white snow. Here the snow is high enough, and in a
location cool enough, that it is maintained throughout the year. Later, as fresh
snow compresses it into ice, it will become part of the main glacier.
As you follow the glacier from its source towards its terminus,
you will notice a point at which the snow suddenly loses its bright white
character, and takes on a dirty, rougher appearance. This marks the beginning of
the zone of melting. As the glacier moves downhill, it reaches a point at which
the snow begins to melt. As the fresh snow on the surface melts, the main body
of glacial ice is exposed. Soon meltwater streams appear on the surface and the
glacier begins to feel the heat of summer.
The melting increases towards the toe, and the glacier always
reduces in size over the summer months as melt is at its peak. However even at
these heated times, the glacier continues to move downhill. If, over the course
of the year, the glacier melts more than it advances, we say the glacier is
receding. For instance, if it flows 10 m downhill, yet melts 15 m, than the
glacier is receding. If the situation is reversed, we call it an advancing
glacier. Most, but not all, glaciers in the Rockies are presently receding.
Glaciers always flow downhill under the influence of gravity.
The impression is often that the glacier moves as a unit, almost like a giant
snake slithering down the valley. Scientists have been studying the Athabasca
Glacier for many years now, and have come up with some fascinating details of
how glaciers move. They don’t move as a unit, but rather like a thick liquid.
By placing stakes at regular distances across the surface of the glacier, they
have proven that the centre of the ice moves faster than either side. In a
cross-sectional view, the glacier moves fastest at the surface, and moves
progressively slower as the depth from the surface increases. It moves the
slowest at its contact point with the valley floor.
How fast do they move? The speed of glaciers varies with the
angle of slope, precipitation, thickness, temperature, and many other variables,
but averages range from 10-200 metres/year. Assuming a moderate 54 metres/year,
that would translate to a mere 6 mm/hour—hardly a snail’s pace. According to
one study, a snail is able to move at a blinding 50,000 mm/hour—even a snail
would quickly leave a glacier in the dust.
Glaciers are a fascinating and inviting place to explore.
Despite what may appear as a smooth continuous surface, walking on glaciers can
quickly lead to disaster. Numerous tourists have perished at the toe of the
Athabasca Glacier as they wandered past warning signs, only to end up falling
into a crevasse. It takes very little time to perish from hypothermia in the
frigid grip of a crevasse.
Travel on glaciers should be restricted to experienced glacier
travelers, and should be limited to properly equipped expeditions. The following
list is designed to highlight the absolute minimum safety requirements. It is by
no means meant to provide sufficient information to promote glacial wandering.
Glaciers have helped create the distinctive landscape we see
around us today. They have taken the mountains, and sculpted them into the wide
valleys, sharp ridges and horned peaks we so admire. To accomplish this, they
had a little help from the surrounding peaks. As these constantly moving
conveyor belts of ice flow down their valley, large amounts of rock and debris
fall onto the ice surface from the rocks above. Some rocks merely hitched a ride
on the ice surface, but most become incorporated into the ice. As the ice flows,
the rock debris is scraped and scoured along the mountainside or valley. It is
this abrasive action that allows the glacier to have an impact on its
surroundings. Without these rocks, the glaciers would have little ability to
modify their surroundings.
When exploring glaciated landscapes, look for the telltale signs
of glacial erosion. The bedrock will often exhibit scrapes and scars known as
Glacial Striations. As the debris laden ice moved across the rock, it left scars
which indicate its direction. By examining these striations, we can learn
exactly how ice moved as it advanced down various valleys.
As the ice slowly flows over the mountain landscape, it may also
pick up pieces of bedrock through a process known as plucking. Loose pieces of
rock and debris, may freeze to the base of the glacier, and be ‘plucked’
from the ground as the ice continues to move.
There are many theories regarding the ice age. Some scientists
believe the ice age ended approximately 10,000 years ago, while others believe
we are merely in an interglacial pause. Once we factor in the endless
speculation regarding the Greenhouse Effect, increased volcanic activity, and
global climatic change, we are left with many divergent opinions. The Little Ice
Age only ended at the turn of the 20th century, implying that the
glaciers may not be finished yet. On the other hand, we seem to be experiencing
a warming climate, which may be the result of the infamous Greenhouse Effect.
Which theory is true? Only time will tell. It is likely though that the glaciers
will not disappear altogether. As climate warms, they retreat higher up the
mountain. They find a point at which they reach an equilibrium with the climate.
Steep, cold, north-facing slopes will continue to harbour glaciers for a long,
long time yet.
As the ice retreated, an enormous amount of debris was dumped on
the landscape, adding to the many changes that glacial erosion had already
wrought. The general term till is used
to describe material deposited by glacial ice. It is easily identified by its
unsorted character. Materials deposited by water are generally sorted by size
with the heavier materials depositing first. Glacial material is unceremoniously
dumped, leaving piles of unsorted material. When you look at most gravel cuts in
the Rockies, you quickly notice the mixture of large rocks (many with rounded
edges caused from being carried within glacial ice), mixed with finer material.
When till is deposited in linear ridges, as it is along the
sides and terminus of a valley glacier, we refer to these piles as moraines,
more specifically, lateral moraines
and terminal moraines. Moraines are
found wherever glaciers are found. As the ice moves down the valley, the rock
debris carried within is eventually deposited along the margins of the ice. This
results in moraines. Where two glaciers meet to become one, a moraine may form
at the juncture. This is known as a medial
moraine. Subsequent advances may destroy moraines, so they are often seen as
a record of the most recent glacial retreat.
Till may be deposited in many forms. Beneath continental
glaciers, it is often formed into blunt hill-sides known as drumlins.
The blunt end faces up-glacier, with the smooth tail looking down-glacier.
As streams flow within glaciers, their channels may become
choked with debris. As the ice melts, these deposits appear as linear ridges
that may flow in all directions. More often than not, they follow the length of
the glacier. These sinuous ridges are known as eskers.
When do we use the term crevice? Properly, it should only be
used when referring to cracks found in rock. Glaciers have their own special
kind of crack, the crevasse.
Glacial ice is constantly moving, but although the deepest ice
is acting like a thick liquid, the ice on the surface of the glacier remains
brittle. Since it isn’t under the immense pressure of ice buried in the
glacier, it doesn’t take on the distinctive fluid-like character of glacial
ice. As the glacier flows over obstacles like large rocks or small cliff bands,
the ice surface will reflect this change in topography. While the deeper ice is
able to flow smoothly over the obstacle, the ice on the surface will crack into
long linear crevasses. The term crevasse is applied only to cracks in ice, and
these crevasses can be up to 50 m. deep. For glacier travelers, crevasses form
one of the major hazards. Often hidden by deep snow, an unwary explorer can
Higher on the glacier, where it meets the mountain face at the head of its valley, is a very special crevasse known as a Bergschrund. As the sun warms the rock face faster than the ice, the ice melts away from the rock forming a deep fissure right at the headwall of the ice.
All Material © Ward Cameron 2005