Revision Document - Glaciation


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Revision Document - Glaciation

  1. 1. What the Syllabus says about Glaciation, and then some Landforms Syllabus Freeze thaw and the processes of erosion: abrasion and plucking (The processes should be understood in the context of their role in forming the glacial features listed.) The characteristics and formation of corries, aretes, pyramidal peaks, glacial troughs, ribbon lakes, hanging valleys, truncated spurs, boulder clay/till, moraines and drumlins. • Recognise and describe glacial features on ordnance Survey maps and photographs. • The human uses of an upland glaciated area to include farming, forestry and tourism. • The social, environmental and political issues currently affecting upland glaciated areas and the management strategies for contemporary issues re conservation and sustainability e.g. grants for conservation, stewardship schemes, repair and maintenance of footpaths, eco-friendly new developments as examples of contemporary solutions. • The conflicts that arise out of the values and attitudes of different interest groups in upland glaciated areas. • One case study e.g. the Lake District, Alps, Rockies, Himalayas is advised to enable students to describe and explain the human uses of the landscape in upland glaciated areas. Valley Glaciers and Ice Sheets Fresh snow is composed of ice crystals with air spaces. As snow accumulates the snow is compressed, causing the air to be pushed out of the snow below turning it into ice. As snow thickens it moves down slopes due to gravity. Moving ice is called a glacier. There are two main types of glaciers: Valley Glacier - A glacier confined in a valley. It follows the route of a previous river valley starting in upland areas. Examples of existing glaciers are in the Alps and the Himalayas. Ice Sheet - A glacier covering the whole of the land surface over a wide area. The ice can be thick enough to blanket the entire area of a continent such as Antarctica. The processes of glacial erosion are: Plucking - As the glacier moves along the land surface it tears away blocks of rock which had been frozen to the bottom of the glacier as water had entered the joints in the rock and froze. The blocks of rock between the joints are plucked away. Abrasion - Rocks and rock particles held in the bottom of the glacier wear away the rocks over which the glacier passes. They are used as sharp tools of abrasion. Smaller rock particles have the same effect as sandpaper, whereas large sharp- edged rocks cut deep grooves into the rock surface under the glacier called striations. An important factor adding to the 'success' of glaciation is freeze-thaw weathering as this leaves many rocks weak and shattered with cracks making it easier for glaciers to use them as tools for abrasion, as the water gets into cracks into rocks then freezes causing the cracks to expand. Even in the presence of ice outcropping rocks above the surface of the ice are affected by freeze-thaw action as in a cold climate there are many changes above and below freezing point. Valley glaciers are more effective erosion agents compared to ice sheets. The ice touches both the floor and the sides of the valley meaning as there is mroe contact there is more erosion. As valleys usually have steeper gradients these glaciers flow quicker and as there is more melt-water to lubricate the glacier's movement. There is a plentiful supply of rock fragments from shattered rock peaks which are used for abrasion. Although ice sheets cover a wider area and therefore a greater area, they erode much slower. Weaker rocks with may joints are eroded more quickly compared to other rocks no matter the eroding agent. The British Isles was covered in ice sheets from Scandanavia during the 'Pleistocene Ice Age', which covered everywhere except the extreme south of England. In high relief areas such as Snowdonia, heavy snowfall meant the accumulation of snow and ice in hollows on the rocky mountainsides. These were the sources for valley glaciers which flowed down valleys originally eroded by rivers. Over the past 10000 years global warming has occurred meaning that no part of the British Isle lies above the snow line (the line above which snow and ice remain all year). However, Britain still has signs that ice sheets and glaciers were once present, such as drumlins, to name just one example.
  2. 2. Glaciers are so powerful, they have the ability to change landscape features. This ability is greatest in upland areas because that is where the glaciers have been for the greatest amount of time, so therefore more changes can take place as the ice is deeper. Generally, in upland areas, slopes are steeper and peaks are narrower, because that is where glacial erosion is most likely to still be happening. Corrie The first landform to be formed by a glacier is a corrie. This is a circular rock hollow, which are located on the mountain side, with a steep and rocky backwall, generally in excess of 200m high. Corries are ringed by rocks, which lead to sharp ridges; however, the front is open with a rock lip on the surface. After the ice melts, the hollow is filled with a small lake called a tarn. Corries begin where snowfields/névés accumulate below the mountain tops and form ice and grow. Corries actually form through both methods of erosion previously mentioned, but also by other techniques: 1. Freeze-thaw weathering: Frost on the mountain tops and slopes supply loose rocks, otherwise known as scree. Water seeps into the bergschrund crevasse on the headwall and increases the amount of freeze-thaw weathering. It causes the headwall to cut backwards, and makes it steeper. 2. Plucking: Causes the ice sticking to the headwall to pull away blocks of rock as the glacier moves. 3. Abrasion: Scree is embedded in ice and act as tools of erosion for scraping out the bottom of the hollow, by abrasion. 4. Rotational slip movement: Because of this, there is a greater pressure from the ice at the bottom of the headwall and in the base of the hollow than near to the front, where the glacier leaves the corrie in order to move down the valley. A rock lip forms there, as a result of weaker erosion. As aforementioned, when the ice melts, a tarn lake forms in the hollow of the corrie. The rock lip acts as a natural damn at the front of the tarn lake, and this together with the steep slopes ringing the hollow, keeps the tarn lake in place. The tarn lake is often full because they are usually formed in upland areas where there is often more precipitation, as well as a larger amount of run-off down the sides of the corrie. Arête and pyramidal peak An arête is a two-sided sharp-edged ridge, and a pyramidal peak is a three-sided rock. The most famous example of this is the Matterhorn near Zermatt in Switzerland, which has three nearly vertical faces. Both are caused by the cutting back of headwalls of corries on the slopes by freeze-thaw weathering and plucking. An arête is formed when two corries, one on either side of the ridge, cut back until only a narrow piece of rock is left as the ridge top. This is the same method for the formation of a pyramidal peak, however, three corries cut back. All of the peaks are sharpened by frost action. Glacial Erosion: Valley Landforms. A glacial trough is significant landscape feature. These valleys can be hundreds of metres deep with steep rock walls. Waterfalls often cascade from the hanging valleys and on top of the valley sides the land often flattens out; this is known as a high-level bench or an ‘alp’. The floor’s width and flatness contrasts greatly with the steepness of the sides and these valleys are drained by misfit streams which are very small compared to the size of the valley. Ribbon lakes can also be formed on the valley floor covering parts or sometimes the entire floor. They are called ribbon lakes because of their long and thin shape. Landforms of glacial deposition are found in the lower part of the valley, such as terminal moraines.
  3. 3. Formation of valley landforms. The valley glacier widened, deepened and straightened the formerly V-shaped river valley so it became a U-shape and a glacial trough. The ice is in contact with all of the floor and the valley sides so the V- shape is changed to a U-shape because glacial erosion occurs everywhere in the valley where the ice is in contact with the rock. The glacier can't flow freely around corners like a river so it pushes straight forward. This cuts off edges of interlocking spurs to form truncated spurs and straight valley sides. The ice is thicker in the main valley because it is fed by all the glaciers from the tributaries. When the ice melted, those tributaries were left hanging well above the floor. The streams from these hanging valleys fall as waterfalls into the main valley. The glacier picks out weaknesses in rocks, therefore eroding the rocks that are weak and soft more rapidly than hard and resistant rocks. In places where soft and hard rock alternate, rock basins are formed as the soft rock is eroded much quicker and deeper. The hard rock is left as a rock bar. When the ice melts the rock basin becomes filled by water and is known as a ribbon lake. Landforms of Glacial Deposition Valley glaciers and ice sheets erode with so much power that there are large amounts of loose rock available for transport. Glaciers transport enormous loads. Erratic boulders are boulders dropped in an area where it does not belong. All materials transported by glaciers are called moraine. Most of this material is carried near the glacier’s base but some carried on the surface, which show up as dark line of moraine. Piles of materials along the sides are called lateral moraines; those somewhere in the middle of the glacier formed after valley glaciers joined together are called medial moraines. Two separate lateral moraines join together to form one medial moraine. The material for these moraines is broken off from the rocky peaks above by freeze thaw weathering, and it falls down the valley sides onto the top of the ice. However a glacier cannot keep growing forever; it reaches a point where ice loss is greater than the amount of new ice being supplied. Valley glaciers start to melt at lower altitudes due to lower temperatures, only a few reach the sea before having completely melted. As the ice melts it cannot carry as much material. When the glacier reaches the point of overload (load greater than carrying capacity), it must deposit some or all of its material. Any obsticle on its course encourages deposition. The general material deposited by ice is boulder clay. It is clay normally containing numerous boulders of different sizes. The boulders it contains are angular as they have sharp edges, not rounded off as they would if they had be transported in rivers. The materials of boulder clay vary greatly according to what the glacier eroded before it reached the are. Deposits can be more sandy than clayey so it is why geographers prefer to refer to it as glacial till in order to describe all ice deposited materials. As the glacier pushes forward and melts, it leaves a trail of till which forms a hummocky surface of ground moraine. In the lowland areas of south-west Scotland and north-west England, glacier deposition has a landscape of many low hills, each one typically about 30-40 metres high and 300-400 metres long. These hill lie in the same direction and have similar shapes - blunt at one end and tapered at one end and each hill looks like an egg. Each of these is called a drumlin. Drumlins occur in swarms and are said to form a 'basket of eggs' topography, so called because of the appearance of the landscape. Drumlins form when the ice is pushing forwards across a lowland area, but it is overloaded and melting. It does not need much to encourage more deposition as a small obstical such a rock outcrop or mound is sufficient. Most deposition occurs around the upstream end of the obstacle, which forms the
  4. 4. drumlins blunt end. The rest of the boulder clay that is depositied is then moulded into shape around the obstacle by the moving ice to form the tapered end down stream. The drumlin is another landform from which it is possible to detect the direction of ice movement. All the remaing load is dropped and dumped at the glaciers snout (the furthest point reached by the ice). This point is marked by a ridge of boulder clay across the valley or lowlands, running parallel to the ice front, and is called a terminal moraine. Where ice sheets remained stationary for a long time such as in central europe during the main ice advances in the ice age, sufficient boulder clay was deposited to form ridges more than 200 metres high. More typicaly, terminal moraines formed by valley glaciers are between 20 and 40 metres high. Terminal moraines which cross valleys form natural damns behind which river water can pile up to form lakes. These lakes are also long and thin and are called ribbon lakes. Landforms can form using different methods. Human Activities in Upland Glaciated Areas These areas are unattractive for settling people. The natural characteristics of upland areas, due to colder weather leading to a smaller growing season make things difficult to start with. When this is combined with the fact that glaciers have the ability to strip all of the soil off the land, as well as creating steep inaccessible areas of landscape, these areas are often not easy to use. Possibilities for Settlement and Use Farming and Forestry · Valley glaciers widen the valley floor creating large areas of flat and sheltered land for farmers to use · The glacier deposits most of its load on the valley floor making very thick soil · Though the soil content varies some is highly fertile and allows for good growth · The land of the valley floor is used intensively for growing crops · Further away from the farm areas the land can be used only for livestock · The steep slopes are normally used by sheep while other animals such as cattle have to be grazed on the lower parts of the valley · Farming in these areas is known as marginal as the farmer is unlikely to make a huge profit, due to some areas of the upland being unusable · Coniferous trees can be planted on the unusable steep slopes, this is known as diversification and gives the farmers another income stream Tourism · This is another way in which some farmers have diversified in upland glaciated areas · More money can be made from creating camping or caravan sites than using the field for grass · Examples of bed and breakfast facilities can be seen around the lake district and Snowdonia · The farm may also sell its produce if it is based fair away from any shops · Glaciers have created much more attractive and breathtaking scenery that would be seen without their effects and this attracts much more tourism Management in Upland Glaciated Areas
  5. 5. Large numbers of visitors to one area need to be managed as in areas where there are a high number of visitors, known as honeypots, there are worries that need to be addressed such as lessening the damage to the environment as well as stopping conflict between local residents and visitors or between groups of visitors with different interests Glaciers leave the land with thin soil; this means if a large number of people walk on them, they will wear away easily. The erosion of walkers can make scars on the landscape. To reduce this management can change the way a path goes to allow older paths to recover from damage, or to create artificial paths which can better withstand a large number of people walking on them. Other problems may arise between farmers and those visiting the area. A visitor may feel they have a right to roam the area, but this may cause damage to a farmer’s farm which is already struggling to make a profit. This could be in the form of walking through a hay field, dropping some litter or knocking down fences between fields. Equally if they have brought a dog with them this may cause distress to the livestock of the farm. In order to reduce this management methods are to educate people about the country code, keeping people informed about litter dropping and keeping to public foot paths, and equally managing and maintaining farm boundaries People may want to use the areas for different reasons. Some may want to enjoy the peaceful landscape, while others may want to use the lake areas for water skiing or power boating. In order to manage this, certain areas of lake and land need to be divided to allow some activities to only take place in certain areas