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A glacier is a river of ice which travels slowly from
the summit of a mountain to the valley below. It is
formed when masses of snow are frozen and pressed
together. for a glacier to form, snowfall during winter
must exceed the melting of snow during summer. Such
conditions only prevail in the mountain and polar
regions. The great weight of a glacier causes it to
move slowly downwards from the snowfield where it is
formed. The movement varies from a few centimeters to a
few meters per day, depending on the slope of the ground
and the presence of obstructions. The movement is
greater in the center and along the surface of the
glacier than at the sides and the bottom. In some
regions, the glacier eventually reaches the sea. Large
chunks of it break off and fall into the sea with a
thundering roar. These chunks of ice float away as
icebergs.
At it source, a glacier is broad and thick; but as it
moves down the mountain, its temperature rises. As a
result, more snow and ice melt and the glacier gradually
decreases in size.
The surface of a glacier is often uneven due to
crevasses or cracks in the ice. These cracks are formed
when the glacier moves over a rough surface. A glacier
also picks up rocks and stones along its route, and
these also make the surface uneven. These rock fragments
are deposited as ridges called moraines when the ice
melts.
Generally, there are three types of glaciers. They
are the continental, valley and piedmont glaciers. The
largest are the continental glaciers. This type of
glacier completely covers the high plains or mountain
regions, examples being the ice-caps of the Antarctic
and Greenland. Valley glaciers are ice streams that flow
down mountain valleys, in the form of narrow ribbons.
Examples of valley glaciers are the Alps in Switzerland
and the Rockies in America. The least common of all
glaciers are the piedmont glaciers. A piedmont glacier
is formed when several glaciers unite at the base of a
mountain and form an extensive glacier.
When glaciers disappear from a valley, they leave
behind characteristic features. The glaciated valley is
always U-shaped. The floor and the sides of the valley
are smooth as the glacier has eroded its sides and
bottom. At the head of the valley, formerly occupied by
the glacier, one may find circular basins cleared of all
loose rocks and debris. These basins are called corries
and are sometimes filled with water to form corrie
lakes.
Along the sides of the glaciated valley, it is quite
possible to find rocks of foreign origin. these rocks
have been carried down by the glacier. When the glacier
disappeared from the valley, these huge rocks called
rocking stones were left behind, lying in rather
precarious positions. The rocking stones are perched so
uncertainly that it may seem to one that they would roll
downhill should someone touch them. |
