The action of glacier as an important agent of erosion, transportation and deposition of materials is confined to mountainous and temperate regions of the world.
TERMS ASSOCIATED WITH GLACIERS
- Ice: This refers to solid form of water, resulting from freezing when the temperature is persistently below zero degrees Celsius.
- Glacier: Meaning moving ice, refers to a very large accumulation of ice (frozen sea or river) in motion.
- Glaciation: This refers to the wearing away of the earth surface by glacier.
- Snow: This refers to frozen water vapour which falls in form of crystals through the atmosphere.
- Snow-line: This is the lower limit of perpetual snow cover on a mountain.
- Snow field: This refers to the region or area permanently under snow cover.
ACTION OF GLACIER EROSION
Glacier erosion is carried out in the following ways:
- Sapping: This is the breaking up of rocks of alternate freezing and thawing of water at the bottom of cracks between a mass of ice and the sides and floor of a valley or the sides of a mountain.
- Plucking: This is the tearing away of blocks of rocks which have become frozen on the sides or bottom of the glacier.
- Abrasion: This is the wearing away of rocks beneath a glacier by the scouring action of the rocks embedded in the glacier.
FEATURES OF GLACIER EROSION IN THE HIGHLAND AREAS
- Striations: These are scratches or marks left on rocks over which glacier passes. Rock fragments or mountains embedded in the glacier affect the striation of rocks.
- Corries or Cirque: A corrie is a deep and rounded hollow or depression with steep sides, formed through erosion by ice. They are arm – chair – shaped hollows, resulting from the plucking of rock materials down the slope. After the ice has moved, it later forms corrie lake.
- Arete: This occurs when two corries cut back opposite sides of the same mountain, resulting in a knife-edge ridge called Arete. Arete is therefore a wall – ridge – like structure separating two corries.
- Pyramidal peak: This occurs where three or more corries cut back on the same mountain. A pinnacle shaped like a pyramid, develops and is called pyramidal peak.
- Bergschrund: This is formed at or near the head of glacier. It is a deep and vertical crack separating the lower part of a glacier that has started to move down its valley and the upper part which is static. This takes place in summer when ice begins to move out of the corrie down the mountain valley. Small scaled cracks also develop where glacier negotiates a bend along its valley. Such small cracks are called crevasses.
- U-Shaped valley or Trough: This is a wide, flat floor with very steep side which has been eroded by glacier. All the sides and floors including all debris are washed away by glacier. This results in the formation of a U-shaped valley which forms the main valley.
- Hanging valley: This results when glacier action from tributaries erodes materials into the main or U-shaped valley. It can serve to generate electricity (H.E.P).
- Rock Basin and Rock Step: When two tributaries join a main valley, the additional weight of ice in the main valley cuts deeper into the valley floor at the point of convergence, thus forming a rock step. It can also be formed due to differences in resistance to frost action. Rock basin on the other hand is formed when glacier erodes and excavates the valley floor so deep to form rock basin.
- Moraines: Moraines are made up of pieces of rocks that are shattered by frost action, embedded in the glacier and brought down the valley. Moraines exist in lateral, medial ground and in terminal forms.
GLACIER EROSION IN LOWLAND
Erosional feature of glacier which occur in lowland areas include:
- Roche Moutonnee: these are resistant residual rock structures. The surface is striated by ice movement. Its upstream side is smooth due to abrasion while the downstream is rough due to plucking. The surface is also rough.
- Crag and Tail: Crag is a mass of hard rock which slopes on the upstream side that protects the softer leeward slope from erosion and later develops to form the tail.
FEATURES OF GLACIER DEPOSITION IN LOWLANDS
- Boulder Clay: Boulder clay is the ground moraine of the glaciers. It is generally consists of stones of various sizes and shapes in a mass of sand and clay.
- Erratics: Erratics are transported rock fragments which are composed of materials entirely different from the bedrock or rock fragments of the region in which they are deposited. They are deposited when the ice carrying them melts into water.
- Drumlins: These are swarms of oval, elongated whale-black hummocks composed mainly of boulder clay with their elongation in the direction of the ice flow. The shape of drumlins is better described as baskets of eggs.
- Eskers: Eskers are long, narrow and winding ridges of sand and gravel deposited by melt-water streams. They are porous and numerous.
- Terminal moraines: They are made up of boulders which are deposited at the edge of the ice sheet. They usually form large debris deposited at the edge of the ice sheet.
- Outwash plain: This refers to the large area beyond the terminal moraines in sand and gravel, washed down the mountainous zone and deposited in an extensive area. This wide area of sand and gravel is called the outwash plain.