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- Glaciation generally gives rise to erosional features in the highlands & depositional features on lowlands
- It erodes its valley by two processes viz. plucking & abrasion.
- Plucking → Glacier freezes the joints & beds of underlying rocks, tears out individual blocks & drags them away.
- Abrasion → Glacier scratches, scrapes, polishes & scours the valley floor with the debris frozen into it.
Characteristic features of Glaciated Highland
Corrie, Cirque or cwm
- The downslope movement of a glacier from its snow covered valley head & the intensive shattering of the upland slopes, tend to produce a depression where neve or firn accumulate
- Plucking & abrasion further deepen the depression into a steep horse shoe shaped basin called Cirque (in French), cwm (in wales) & Corrie (in Scotland)
- There is a rocky ridge at the exit of the corrie & when the ice eventually melts, water collect behind this barrier known as Corrie Lake or tarn
Aretes and Pyramidal Peaks
- When two corries cut back on opposite sides of the mountain, knife edged ridges are formed called aretes
- When three or more cirques cut back together, recession will form an angular horn or pyramidal peak
- At the head of a glacier, where it begins to leave the snowfield of a corrie, a deep vertical crack opens up called a Bergschrund or Rimaye
- This happens in summer when although the ice continues to move out of the corrie, there is no new snow to replace it
- In some cases not one but several such cracks occur which present a major obstacle to climbers
- Further down, where the glacier negotiates a bend or a precipitous slope, more crevasses or cracks are formed
U shaped glacial Troughs & Ribbon lakes
- Glaciers on their downward journey are fed by several corries scratches & grind the bedrock with straightening out any portruding spurs.
- The interlocking spurs are thus blunted to form truncated spurs with floor of the valley deepened.
- Hence, the valley which has been glaciated takes the characterstic U shape, with a wide flat floor & very steep sides.
- After the disappearance of the ice, the deep sections, of these long, narrow glacial troughs may be filled with waterforming Ribbon lakes also known as Trough lakes or Finger Lakes.
- The main valley is eroded much more than the tributary valley as it contains much larger glacier.
- After the ice has been melted, a tributary valley hang above the main valley & plunges down as waterfall. Such Tributary valleys are termed as Hanging valleys.
- Hanging valleys may form a natural head of water for generating hydroelectric power.
Rock Basins and Rock Steps
- A glacier erodes & excavates the bed rock in an irregular manner.
- The unequal excavation gives rise to many rock basins later filled by lakes in valley trough.
- Where a tributary valley joins a main valley, the additional weight of ice in the main valley cuts deeper into the valley floor & deepest at the point of convergence forming rock steps.
- A series of such rock steps may also be formed due to different degrees of resistance to glacial erosion of the bedrocks.
- Moraines are made up of the pieces of rock that are shattered by frost action, imbedded in the glaciers & brought down the valley.
- Those that fall on the sides of the glacier form lateral moraines.
- When two glaciers converge, their inside lateral moraines unite to form a medial moraine.
- The rock fragments which are dragged along, beneath the frozen ice, are dropped when the glacier melts & spread across the floor of the valley as ground moraine.
- The glacier eventually melts on reaching the foot of the valley & the pile of transported materials left behind at the snout is terminal moraine or end moraine.
- The deposition of end moraines may be in several succeeding waves, as the ice may melt back by stages so that a series of recessional moraines are formed.
- If the glacier flows right down to the sea, it drops its load of moraine in the sea.
- If section break off as icebergs, moraine material will only be dropped when they melt
- Where the lower end of the trough is drowned by the sea, it forms a deep, steep side inlet called a Fjord, a typical of Norway & Chilean coast.
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