Glaciation generally gives rise to erosional features in the highlands and depositional features on the lowlands, though these processes are not not mutually mu exclusive because a glacier plays a combined role of erosion, transport and deposition throughout its course. A glacier erodes its valley by two processes: plucking and abrasion. By plucking, the glacier freezes the joints and beds of the underlying rocks, tears out individual blocks and drags them away. By abrasion, the glacier scratches, scrapes, polishes and scours the valley floor with the debris frozen into it.
These fragments are powerful ‘tools’ of denudation. Large angular fragments cut deep into the underlying rocks so all glaciated floors bear evidence of striation or scratching. The finer materials smooth and polish the rock surfaces and produce finely ground rock flour. The rate of erosion is determined by several factors such as the velocity of flow, gradient of the slope, the weight of the glacier, the temperature of the ice and the geological structure of the valley.
The characteristic features of a glaciated highland are as follows:
1) Corrie, cirque or cwm: The downslope movement of a glacier from its snow-covered valley-head, and the intensive shattering of the upland slopes, tend to produce a depression where the firn or neve accumulates. The process of plucking operates on the back wall, steepening it and the movement of the ice abrades the floor, deepening the depression into a steep, horse-shoe-shaped basin called a cirque (in French). It is also known as a corrie in Scotland and a cwm in Wales.
There is a rocky ridge at the exit of the corrie and when the ice eventually melts, water collects behind this barrier, to form a corrie lake or tarn.
2) Aretes and pyramidal peaks: Knife-edge ridges may develop as a result of two corries cutting back until they nearly meet. The narrow ridge thus formed is known as an arete.
If a mountain has a number of corrie basins around its flanks, corries will cut by erosion and in time a pinnacle which is shaped like a pyramid will be formed. This is called pyramidal peak, and the Matterhorn on the Swiss-Italian border is a good example.
3) Bergschrund: 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 (in German) or rimaye (in French). 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. The begschrund presents a major obstacle to climbers. Further down where the glacier negotiates a bend or a precipitous slope, more crevasses or cracks are formed.
4) U-shaped glacial trough: The glacier on its downward journey, fed by ice from several corries-like tributaries that join a river, begins to wear away the sides and floor of the valley down which it moves. It scratches and grinds the bedrocks, removing any rock debris and surface soil. It tends to straighten any protruding spurs on its course. The interlocking spurs are thus blunted to form truncated spurs and the floor of the valley is deepened.
A valley which has been glaciated takes a characteristic U-shape, with a wide, flat floor and very steep sides.
After the disappearance of the ice, the overdeepened sections of these long, narrow glacial troughs may be filled with water forming ribbon lakes, such as Loch Ness and Lake Ullswater in Britain. They are sometimes referred to as trough lakes or finger lakes.
5) Hanging valleys: The main valley is eroded much more rapidly than the tributary valleys as it contains a much larger glacier. After the ice has melted, a tributary valley therefore ‘hangs’ above the main valley so that its stream plunges down as a waterfall. Such tributary valleys are termed hanging valleys and may form a natural head of water for generating hydroelectric power.
6) Rock basins and rock steps: A glacier erodes and excavates the bed rock in an irregular manner. The unequal excavation gives rise to many rock basins later filled by lakes in the 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 at the point of convergence, forming a rock step. A series of such rock steps may also be formed due to different degrees of resistance to glacial erosion of the bedrocks.
7) Moraines: Moraines are made up of the pieces of rock that are shattered by frost action, embedded in the glaciers and brought down the valley. Those that fall on the sides of the glacier, mainly screes, 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 and spreads across the floor of the valley as ground moraine. The glacier eventually melts on reaching the foot of the valleys, and the pile of transported materials is left behind at the snout in the terminal moraines or end moraine.
The deposition of the end moraines may be in several succeeding waves, as the ice may melt back by stages so that a series of recessional moraines is formed.
If the glacier flows right down to the sea, it drops its load of moraine in the sea. If sections break off as icebergs, morainic 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-sided inlet called a fiord, typical of the Norwegian and south Chilean coasts.