Model Answer for 2000 Lithosphere Question
3)
Erosional Features
Hanging Valley
U-Shaped Valley
Roche Moutonnee
Depositional Features
Drumlins
Terminal Moraine
Esker
Select one feature of erosion and one feature of deposition, and explain the
processes involved in the formation of each feature.
U-Shaped Valley:
U-shaped valleys are formed from V-shaped river valleys. As temperatures
drop and snow begins to fall, the water inside the valley would trickle
through the cracks in the rocks. As temperatures fall below zero degrees
celsius, usually at night, the water inside the cracks would turn to ice and
expand. Frozen water can expand by as much as 9%. This causes huge
pressure on the surrounding rock. When temperatures rise again, during the
day, the ice melts, releasing the pressure on the rocks. This continuous
freezing and thawing, a process known as freeze-thaw, causes the rocks to
become very weak, cracks widen and pieces of rock break off.
During glaciation the ice moves down the V-shaped river valley. As it does
so, it tears away huge rocks from the sides and base of the valley. The
glacier freezes onto the surrounding rocks that have been previously
weakened by freeze-thaw, by a process known as plucking. Thus changing
the valley into a U-shape. The erosive power of the glacier overdeepens the
valley floor. The rocks that have become embedded within the glacier
scrape against the valley sides and base, and smoothen it like sandpaper, this
is known as abrasion.
The slopes above the valley have not been steepened and are known as
benches or alps. The valley is now U-shaped in cross section, with very steep
sidewalls, and a flat base. After glaciation, freeze-thaw action continues
with the warming and cooling of temperatures, as well as other agents of
weathering. This causes rocks to weaken and fall to the valley floor as
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scree. The original river channel has altered and so when rivers take over
after glaciation, they meander across the enlarged valley floor. These are
known as mis-fit streams.
Before
Glaciation
During
Glaciation
During
Glaciation
After
Glaciation
Roche Moutonnee:
A roche moutonnee is a mass of resistant rock over which ice passed during
glaciation.
Before glaciation, this was merely a mass of hard, resistant rock. During
glaciation, a glacier moved upvalley and slowly moved over this mound of rock.
The stones and rocks that are embedded in the base of the glacier, known as
ground moraine, scraped the rock on its upvalley slope, this smoothed the
surface of the resistant rock as the glacial moraine scratched the surface
like sandpaper. this process is more commonly known as abrasion. Often
scratch marks known as striations are left on the mass of hard rock
indicating that ice once passed over it.
As the glacier moved over the mound of rock, it tears away rocks from the
mound. This is known as plucking and it occurs when the glacier freezes onto
the rocks and tears them away when it moves downhill due to gravity.
Plucking is done easily on the downvalley slope because the rocks have been
weakened previously by freeze-thaw action. This is where water enters
cracks in the rock. When temperatures fall below freezing the water
expands and turns to ice. Frozen water can expand by as much as 9% and
therefore exerts huge amounts of pressure on the surrounding rocks. As
temperatures rise the ice melts and releases the pressure. This continuous
freezing and thawing causes the surrounding rocks to become very weak, and
therefore can be easily plucked away by the glacier. As a result of this the
downvalley slope is much steeper than the upvalley side. A roche moutonnee
therefore has a smooth, gentle slope facing upvalley and a steep, jagged
slope facing downvalley.
Drumlins:
Drumlins are hummocky deposits of boulder clay and till. They are oval
shaped and are between 100 – 800m in length and between 25 and 100m
high. One end of the drumlin is quite steep, known as the stoss end, while
the other end, known as the lee slope, is longer and more gently sloping.
The stoss end faces upvalley and it is this end that would have faced the ice
flow. Drumlins were formed by the depositing of moraine from a glacier
when it became overloaded with sediment. When the strength of the glacier
reduced, material was deposited. The glacier however must have still been
moving as the shape of the drumlin suggests that it was subject to the
process known as abrasion, which created it streamlined shape. Abrasion
occurs because glaciers have many rocks and stones embedded within the
base of the glacier, these rocks have been torn away from the surrounding
land. The rocks are jagged and scrape against the moraine as it moves over
it. This process is similar to sandpapering, where the rocks rub against each
other and smoothen the surface. Often, this process leaves scratch marks
in the rock called striations.
It is common to find many drumlins grouped together, known as swarms.
Terminal Moraine:
Terminal moraines or end moraines as they are often referred to are ridges
of unsorted material at the snout of the glacier. They mark the furthest
point reached by the ice sheet or glacier.
Terminal moraines form when the ice melts and deposits all the moraine it
was transporting at the front of the glacier. Glaciers can transport huge
amounts of material including rocks, stones and smaller particles, it takes
huge force for a glacier to be able to transport this material. Therefore,
when it begins to lose its strength, for example when it begins to melt or
retreat, it deposits some of this material. At this point the ice is still
moving so material is constantly being added to the terminal moraine. The
longer the ice continues to melt at the same place, the higher the terminal
moraine. Glaciers carry moraine at the sides of the glacier, called lateral
moraine, in the middle of the glacier called medial moraine, inside the ice,
called englacial moraine and at the base of the ice called ground moraine.
Therefore, terminal moraines are completely unsorted, and may extend for
many kilometers.
Esker:
Eskers are long, narrow, sinuous ridges of stratified sand or gravel, which
can be several miles long. The sand and gravel has been picked up and
deposited by water, as eskers are generally regarded as features of fluvioglacial processes.
Towards the end of the ice age, when glaciers and ice sheets began to melt,
rivers and streams would flow inside and under glaciers in ice-walled tunnels.
They are under great pressure from the melting ice and so are able to
transport a lot of sediments. The sediment is deposited along the course of
the sub-glacial river so that, when the ice melts and the meltwater drains
away, the raised bed of the subglacial river is left as an esker.
Eskers are layered because, at different times, the meltwater river would
have differing amounts of energy and so deposits varying amounts of
material of different size