A glacier is a thick mass of ice which moves over the ground under the influence of gravity. It originates on land from the compaction and recrystallization of snow.
Features of glacial erosion
- Striations:-glacier carry rock fragments firmly embedded in the ice. They scratch grind or groove the rock surface over which they move. These scratches and grooves left on bedrock and boulders are called striations. The striations indicate the direction of ice movement.
- U-shaped valley:-glaciers occupy valleys and flow downhill. As they erode their valleys both laterally and vertically, u-shaped with steep walls and flat floor are produced.
- Hanging valley:-since the magnitude of the glacial erosion depends upon the thickness of the ice main glacier cut their valley deeper than those of their tributaries. As a result at the junction where a tributary joints the main glacier, the floors of their valleys do not meet at the same level. The valley of the tributary stands at a higher elevation than that of the main valley. Such valley are called hanging valleys.
- Cirques:-the bowl –shaped hollows present at the glacier valley heads in the mountains are called cirques. They are formed mainly be the quarrying and frost-wedging action of ice. In cirques a little gap is generally left between the head of the glaciated valley and the mass of the glacier ice. This gap is known as the bergschrund.
- Serrate ridges:- as the adjacent cirques along the opposite side of a mountain are enlarged the space between them becomes narrow. As a result sharp divides are formed. Such divides which have jagged serrated and linear crest are called serrate ridges. When three or more cirques surround a mountain summit a pyramid-like peak is formed such a peak is called horn.
- Roche moutonnees:- these are small mounds of resistant bedrock which have atypically asymmetrical appearance. The side facing the direction of ice advance is gentle and smooth, while the leeward side is steep and rough. This form results from the plucking action on the leeward side and abrasion on the opposite side.
- Fiords:-the glaciers that descend from coastal mountains may cut their valley below sea level. Such valleys produce fiords.
Features of glacier deposition
- Moraines:- ridges or layers of till are called moraines, they are of four types
- Ground moraines:– a layer of till deposited beneath the moving ice on the ground is called the ground moraine.
- Lateral moraines:-the material that falls from the valley walls accumulates on the sides of a glacier. When the glacier disappears these materials are left as ridges along the side of the valley. Such deposits are called lateral moraines.
- Medial moraines:-when two glaciers meet a medial moraine is formed by the union of two lateral moraines.
- Terminal moraines:-at the terminus of a glacier where the ice starts melting the rock debris is deposited in the form of a ridge which extends across the valley. Such deposits are called terminal moraines.
- Outwash plains:-in front of the end moraines streams of meltwater deposit sediment producing stratified deposits of sand, silt and gravel. Such deposits constitute outwash plains.
- Kettle holes:-these are basin like depressions found in areas of both till and outwash plains. The diameter of kettle holes ranges from a few meters to a few kilometers. They commonly contain water. These depressions are created when the masses of buried ice melt.
- Drumlins:-drumlins are small smooth elliptical hills of till that lie parallel to the direction of ice movement. Unlike roche mountonnees the uphill sides of the drumlins are steep and the downhill sides are gently sloping.
- Eskers:- eskers are long winding ridges of stratified drift found in the middle of ground moraines.
- Kames:– karmes are hillrock of stratified drift which are formed at the edge of the retreating ice by glacial streams. These streams fall from a height and deposit sand and gravel along the margin of the glacier as alluvial cones.
- Varves:- varves are thinly laminated deposits formed in glacial lakes. They consist of alternation of light coloured bands of silt dark coloured bands of clays. The former gets deposited during the summer season while the later in winter.
- Buried valley:-buried valleys are the ancient deep valley which are excavated in the bedrock by glacial erosion and are filled back subsequently with glacial drift. The present day surface topography gives no clue to their existence. The rivers which are flowing in these areas may have no relation to the buried valleys.