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Glaciation

Glaciation

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Glaciers form when the climate becomes cold enough for precipitation to fall as snow. The weight of the new snowfall turns the underlying snow to ice. When the ice moves downhill under the force of gravity, it’s called a glacier.

Glaciers are like huge JCBs because the can:

Erode the upland area (digging)image
Move the material down the valleys (transportation)
Deposit the material in the lowlands (dump material)

Much of the scenery we have in Britain is due to Glaciation which happened many thousands of years ago.

The Lake District is a text book example of a landscape full of glacial landforms.

Glacial erosion

There are three main processes that cause glacial erosion. These processes may work together or on their own, depending on conditions at the time. The are freeze thaw, abrasion and plucking.

Freeze thaw weathering or frost shattering

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Water – eg from rainfall or melting snow and ice – becomes trapped in a crack or joint in the rock.
If the air temperature drops below freezing, the water will freeze and expand by 9-10 per cent putting pressure on the rock.
The ice will melt when the temperature rises above freezing.
If this process happens repeatedly, the rock will weaken and eventually shatter into angular fragments.
This material, called moraine, then freezes into the glacial ice. As it scrapes along the valley floor and sides, it widens and deepens the valley through erosion.
It is most effective where the temperature fluctuates around 0°C, eg on north-facing high altitude slopes in Snowdonia.

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Plucking:

Plucking happens when rocks and stones around the glacier become frozen to the base or sides of the ice. They are then attached to the ground (that doesn’t move) and the glacier (which is moving). One of two things will happen; EITHER the rock is so strongly attached to the ground that the ice breaks off it as the glacier moves, OR the ice pulls or plucks the rock out of the ground. When the ice wins, plucking has occurred.

Abrasion:

Abrasion is wearing something down by rubbing it against something else. Sandpaper and kitchen scouring pads work by abrasion.

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In glaciers abrasion occurs then rocks and stones are picked up by the glacier (by plucking or from moraine that falls onto the surface of the glacier and works its way down to the bottom), and are rubbed against the bedrock at the bottom and side of the glacier as it moves. This turns the ice from a smooth surface into something like a gigantic scouring pad. As the glacier moves the rocks scour the bedrock and erode it away. Scratch marks, or striations, show where rocks have been scraped over the surface.

Glacial erosion landforms

The following landforms are found in glaciated areas: Corries, aretes & pyramidal peaks, glacial troughs, and hanging valleys.

Corries

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Corries have deep, rounded hollows with a steep back wall and a rock basin
Snow accumulates in hollows on hillsides, especially in hollows on hillsides with a less sunny north and east facing aspect.
Snow turns into ice and then the ice moved downhill
Freeze-thaw and plucking loosened and removed material from the back of the hollow creating a steep back wall
Moraine dragged along the base of the glacier, deepened the floor of the hollow by abrasion and formed a rock basin
A rock lip was left where the rate of erosion decreased
The lip was often heightened by the deposition of moraine
When the ice melted the rock lip and moraine acted as a natural dam to meltwater.

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Aretes & pyramidal peaks (horns)

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An arete is a knife-edged ridge often found at the back of a corrie or separating two glaciated valleys. Aretes are often extremely narrow features. A typical arete forms when erosion in two back-to-back corries causes the land in between to become even narrower. If three or more corries have formed on a mountain, erosion may lead to the formation of a single peak rather than a ridge. This is called a pyramidal peak
Glacial troughs (U-shaped valley) and hanging valleys

The diagram to the left shows the changes down a river valley before and after glaciation.
Glaciers form in river valleys. These are generally v-shaped. As the glacier moves down the valley it creates a valley which is more u-shaped.
This leaves a valley which is steep-sided, wide and relatively flat-bottomed.
Abrasion is the key agent of erosion in this process. The moving glacier grinds into the base and sides of the valley over a period of many hundreds of years.
The glacier is unable to flow past the previous interlocking spurs and simple cuts through them, forming steep-edged truncated spurs.

On the side of the main valley are smaller valleys which feed into the main valley. The main valley is eroded more quickly and deeper than the tributary valleys. This leaves the tributary valleys at a much higher level than that of the main valley. The tributary valleys are then called hanging valleys and often end in spectacular waterfalls which flow into the main valley.

Hanging valley

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This is the name givens to a former river tributary that would have originally joined the river at the same height as the main valley
When a glacier fills a former river valley it is at a much higher level and therefore any tributary glaciers will join the main glacier also at a higher level
Once the glacier has eroded it leaves the tributary valley hanging at the side of the main valley
If a stream enters the hanging valley it plunges over the edge as a waterfall.

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Before, during and after glaciation

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The diagram above indicates that glacial landscapes have two distinct landforms. One which is produced by erosion and the other produced due to deposition.

Glacial deposition landforms

Glaciers erode and transport huge amounts of material. The material size ranges from small particles to rocks the size of houses. Eventually all this material will be dumped (deposited) when the glacier melts. This deposited material is called moraine, till or boulder clay.

Moraine
This is the material produced by glacial erosion. The material tends to be unsorted (it contains really huge boulders and at the same time a fine powder called glacial flour). It also tends to be very angular, as the processes that form the material involve freezing and shattering. There are different types of moraine including:

Ground moraine – is moraine spread all over the ground as a glacier retreats up valley in warmer times
Terminal moraine – are rocks deposited in a ridge at the maximum advance of the ice
Lateral Moraine – are ridges of moraine that come from the valley sides and run parallel to those valley sides
Medial Moraine – this is a ridge of rocks running down the middle of a valley formed by two lateral moraines from two glaciers coming together.

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Erratics
Unlike river deposits that are often sorted into different sizes, all glacial deposits are angular and mixed up (unsorted). The extreme of this can be seen in erratics. These are large rocks or boulders that are often found on their own, rather than in piles. They are unusual shapes, unusually large and of a rock type uncommon to the area they have been dumped.

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Drumlins
Drumlins are elongated hills of glacial deposits. They can be 1 km long and 500 metres wide, often occurring in groups. A group of drumlins is called a drumlin swarm. These would have been part of the debris that was carried along and then accumulated under the ancient glacier. The long axis of the drumlin indicates the direction in which the glacier was moving.

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Ribbon lake
A ribbon lake is a long and narrow, finger-shaped lake, usually found in a glacial trough. Its formation begins when a glacier moves over an area containing alternate bands of hard and soft bedrock. The sharp-edged boulders that are picked up by the glacier and carried at the bottom of the glacier erode the softer rock more quickly by abrasion, thus creating a hollow called a rock basin. On either side of the rock basin, the more resistant rock is eroded less and these outcrops of harder rock are known as rock bars, which act as dams between which rainwater may accumulate after the retreat of the ice age, filling up the rock basin and creating a ribbon lake.

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Human Uses of Glacial Areas

Agriculture
Upland glaciated areas are not particularly conducive to farming,with their steep slopes, high precipitation, low temperatures and relatively thin, poor soils. In the Alps some sheep farming occurs.

The glacial valley floor is very valuable farming land, because it is sheltered, flat and well irrigated. The soils are variable but can be very fertile in areas of clay deposits. The farming is still mainly pastoral,although some areas can be used for arable farming.

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Tourism & Recreation
The scenery of a glaciated area is spectacular, with it’s high mountain pastures, wide valley floor, crashing waterfalls and sharp arête’s. Many people love walking in the Swiss Alps purely to see this scenery.
For the more adventurous traveller, glacial valleys provide some fantastic rock climbing opportunities, as well as being perfect for things like hang-gliding.
Glaciers also can be very useful to the winter skiing industry, by guaranteeing that the resort will have some skiing, even if the winter snowfall is very poor. Some glaciers in North America offer all-year-round skiing. The ribbon lakes are perfect for recreational activities such as water-skiing, sailing and fishing, as well as many other water-based activities. The prime example of this is the many ribbon lakes of the Lake District.

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Hydro-Electric Power
As seen in many parts of the Alps, glaciated valleys provide a perfect opportunity for the production of hydroelectric power. Their steep sides, high precipitation and low population density make them ideal places for dams to be built and reservoirs created (often by just increasing the area already filled by a ribbon lake).

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Management
Management is needed in glacial areas to cope with many demands and pressures:
Glacial valleys are very attractive to tourists, and so methods have to be put in place to protect the environment from damage.

This includes trying to prevent soil erosion by introducing artificial paths or by diverting popular routes to allow the old paths time to recover.
Some places have become ‘honeypot’ sites, which attract a huge number of tourists every year. By promoting other similar areas, the local authorities can try to alleviate the pressure on certain very popular places.
Conflicts can also occur between local farmers or residents and tourists. The increased traffic, footpath erosion and the problems of family dogs worrying sheep all have led to conflicts between the local people and the visitors. Speed limits for boats have been introduced in some of the most popular lakes, such as Windermere. These are aimed at protecting local wildlife and preventing too much disruption to those enjoying the tranquil nature of the area.
Tourist facilities, such as visitor centres and parking areas have been introduced to try to cater for the influx of visitors, without taking over local villages and towns. These also provide a good base from which to try to educate people on how to treat the countryside when the visit.

2 Comments
  1. Pierce Anderson permalink

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