The most diagnostic features of glacial erosion occur in areas of high relief and precipitation. Most of the evidence of glacial erosion available is in the shape of landforms which are presumed to have arisen largely by erosion. Some of the major features produced by glacial erosion are as follows:
1. The Cirque
This is a French term for amphitheater shaped basins commonly located at the head of a glaciated valley. These are steep- sided semicircular depressions often with their floors over deepened to rock basins. In different countries they are variously termed, such as Kar in German, Cwm in Welsh, Corrie in Scotch, Botn and Kjedel in Scandinavia.
These bowl-shaped depressions are excavated mainly by frost action. A cirque begins to form, beneath a snow bank or snow-field just above the snow-line, by snow accumulation in small erosion rills or other chance depressions in the slope of a mountain.
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The rock surface beneath and around the snow bank is gradually broken up and deepens y freeze-thaw and mass-wasting, while the rock surface is broken up by the freezing of water, the smaller rock particles are carried away down slope by melt water during thaws.
Such shattering by freeze- thaw action is thought to be responsible for the overall enlargement of the depression. This process of quarrying of rocks by frost action is called nivation.
The steep head wall (or back wall) seems to be produced largely by the shattering of the rocks in freeze-thaw alternations. The steep headwall is relatively free of talus at its base. It may be a kilometre or so in height.
With more active excavation of the floor the depression grows larger and ice starts accumulating to greater thickness. When enough ice gets accumulated, it flows down slope as a glacier. Thus cirques are the main sources of supply of valley-glaciers.
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After the glacier melts, a small lake known as Tarn usually occupies the depression. A cirque basin usually terminates at a bedrock riser called its threshold. A small glacier in a large cirque is indicative of the waning stages of glaciation.
2. Aretes
As already described, the cirques grow steadily larger due to frost shattering and plucking. The headward erosion gradually consumes the preglacial uplands. In this process, two adjacent cirques along the opposite slopes of a mountain may begin to coalesce which results in a typical jagged knifelike ridge, known as an arete or comb.
They are often referred to as razor-edged- ridges, serrate-ridges, or saw-toothed-ridges.
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3. Horn
This is a sharp, pyramidal peak produced due to growth and enlargement of three or more cirques together by head ward erosion.
The up land being consumed by cirque-erosion from several sides is reduced to a number of aretes radiating from the central summit. In due course the aretes themselves are wom back and the central mass remains as a pyramidal peak.
4. Col
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It is a depression formed along the arete at a place where the headwalls of two opposed cirques intersect each other.
5. Glacial Troughs
Valley glaciers reshape their valleys by widening, deepening and straightening them. Most glacial valleys were originally stream-cut valleys. The typical V-shaped cross section of the stream-cut valleys are transformed to a wide, deep, flat-floored and steep-sided U- shaped valley, known as Glacial-trough.
The long profiles of the glacial valleys is irregular and ungraded which exhibits a series of step like forms and deep basins cut from the bed-rock. The step-like forms are known as glacial-step or glacial- stairway.
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Each step consists of three components known as a riser, which marks the down valley end of each step; a riegel which is a sort of rock bar at the ton of a riser; and a tread, which is relatively a flat surface of a step.
The steps and basins are believed to develop because of the differential resistance of the rocks composing valley-floor to glacial erosion. Riegels develop where the bed rocks are massive, unjointed and highly resistant to erosion, whereas basins are formed in highly fractured and jointed zones of the bed rock which offers least resistance.
6. Hanging Valleys
Tributary glaciers also carve out U-shaped troughs. Tributary glaciers generally contain less ice than the main valley glaciers and accordingly in the latter case the floor is more deepened than the tributary glaciers meeting it from the sides.
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After a period of prolonged glaciation the tributary valley appears to hang above the floor of the main-valley occupied by the larger glacier. Such tributary valleys are called hanging-valleys. The junctions of such valleys are the sites of waterfalls.
7. Truncated Spurs
As already explained, the rock-studded bottom of the glacier can abrade the valleys both laterally and vertically in a more effective manner. Besides, glaciers are less easily deflected by obstacles than rivers and the flow of ice is streamline.
The sharp and acute bends which occur along the course of stream-valleys are straightened up by glacial abrasion and in the process it cuts projecting spurs producing blunt triangular facets, known as truncated-spurs or facetted-spurs.
8. Glacial boulders
The rock fragments entrapped in the glacial ice, get abraded, rounded and their surface polished and striated during the course of the glacier movement. Such rounded blocks are called glacial-boulders.
9. Glacial Scars
These are small-scale erosional features produced by the abrasive action of the glacier. Rock debris entrapped by the glacier during its movement are dragged under great pressure over the bed rock.
The sharp points and edges of the rock debris produce characteristic scratches, gougings and grooves in the solid rock. The process of glacial abrasion also produces polished and facetted surfaces. These erosional features are collectively referred to as glacial scars.
10. Roches Moutonnees
These are also known as Sheep-back rocks, formed due to glacial abrasion. When a little hill of rock or small elevation is encountered on the way of the glacier, it is not usually worn away completely.
The side facing the direction of glacier movement becomes gentle, smooth and striated, while the opposite side remains rough, rugged and steep. The gentle sloping up-hill side is also called the stoss-side and the steep down-hill side is called the lee-side. While abrasion is pronounced on the stoss-side, plucking is prominent on the lee-side.
These forms are somewhat elongated and longitudinal asymmetrical. From a distance a group of such features often look like sheep lying down or resemble the curls on a lawyer’s wig. Rocks thus shaped are called roches-mountonnees.
11. Fiords
These are deep glacial troughs which have been eroded below sea-level or are submerged to a depth below sea-level (even though they are formed at higher levels).
As a result the U-shaped valley may continue right out into the sea, which are subsequently occupied by sea-water itself, and become inland stretches or arms of sea.
Within fiords, the glaciers come in contact with the sea-water where blocks of ice break off from the glacier and float on the sea as icebergs. This process of wastage of glacier is known as Calving.