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. Glaciers form in places where more snow accumulates each year than that melts away. They are found chiefly in high attitudes as in the Arctic regions, or at high elevations as on the Himalayan mountains above the snow-line. The snow line is the lower limit of accumulating snow. Below the snowline, the snow melts in summer. The elevation of the snowline varies considerably. In polar regions it may be at sea level, whereas in areas near the equator, the snowline may occur at 6000 meters. In the Himalayas the snowline lies at altitudes varying between 4200 to 5700 meters.

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Most of the glaciers move at the rate of a few meters per day. They move partly by plastic flow and partly by shear movements. In the high gradient valleys a mountain glacier flows down the slope much like a stream of water under gravity. But in basin-shaped, flat or upland areas where the ice can not move under gravity, the glaciers move as a result of differential pressure within the ice mass. The first type of movement is called the “gravity flow” and the second “extrusion flow”. A mountain glacier may have gravity flow in one part of its course and extrusion flow in another, depending upon the irregularities present in the path.

In a moving glacier two zones can be identified: (i) zone of flow, and (ii) zone of fracture. The “zone of flow” is found in the deeper layers of ice. Here the weight of overlying ice is great, and the ice behaves plastically. However, the upper layers of a glacier have little pressure on them and therefore, the surface ice behaves as a brittle mass. It often develops crakes known as “crevasses”. This upper zone is called the “zone of fracture”.

Glacial Erosion

The glaciers cause erosion in three ways: (i) by plucking or quarrying, (ii) by abrasion, and (iii) by frost wedging.

  1. Plucking or Quarrying: While flowing over a jointed rock surface, the glacial ice adheres to blocks of jointed bedrock, pulls them out and carries them along.
  2. Abrasion: The moving ice grids and polishes the rock surface with the help of rock fragments which are held firmly within the body of the glaciers. The abrasion produces striations and grooves in the bedrock surface. A polished surface results when the glaciers performs abrasion by fine silt-sized sediments. The ground up rock produced by the grinding effect of the glaciers is called “rock flour”.
  3. Frost Wedging: Thawing and freezing of water in the cracks and joints of rocks break them by wedge action. In this manner rock fragments of all sizes are added into the glaciers.

Features of Glacial Erosion

Striations: Glaciers carry rock fragment firmly embedded in the ice.. They scratch, grind or groove the rock surface over which they move. These scratches and grooves left on bed rock 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 valleys with steep walls and flat floor are produced.

Hanging Valleys: Since the magnitude of the glacial erosion depends upon the thickness of the ice, main glaciers cut their valleys deeper than those of their tributaries. As a result, at the junctions where a tributary joins the main glaciers, the floor of the valley do not meet at the same level. The valley of the tributary stands at a higher elevation than that of the main valley. Such valleys are called “hanging valleys”. When the glaciers disappear, the hanging valley are occupied by streams which discharge into the main valley forming a waterfall.

Where a valley glacier terminate on land, the streams of melt-water flowing on and under the glacier meet downstream to form a single river. The glaciers that end at the sea coast, discharge huge cliffs of ice into the sea. Because ice is less dense than water, it floats. Such floating ice hills are called “icebergs”.

Cirques: The bowl-shaped hollows present at the glacier valley heads  in the mountains, are called “cirques”. They are formed mainly by 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”.

Fiords: The glaciers that descend from coastal mountains may cut their valleys below sea level. Such valleys produces “fiords”. Fiords are highly over-deepened narrow channels of glacial origin along which the sea encroaches inland. Fiords are found along many coasts including those of Norway, British Columbia, and Alaska.

Transport by Glaciers

Glaciers contain a huge amount of rock debris by plucking, abrasion, and frost wedging. This material is transported in three ways.

  1. Super Glacial Load: The debris that falls from the valley walls on the glacier’s surface is transported as a conveyor belt. Such debris is known as “Super Glacial Load”.
  2. Englacial Load: Sooner or later a part of the debris is engulfed into crevases. This material, which is enclosed within the ice is called “englacial load”.
  3. Subglacial Load: The debris present at the bottom of the glacier is called “subglacial load”. The subglacial load includes the material plucked from the rocky floor and a portion of the debris that reaches the base from above.

Glacial Deposits

The regions that were formerly covered by ice are characterized by the following features.

  1. The topography generally lacks an organised drainage network.
  2. There are hollows with no outlets some of which form lakes.
  3. In some places, there are isolated hills, in others long and winding ridges, none of which seem to have much relation to the underlying bedrock.
  4. In some places, the bedrock is deeply buried, sometimes 50 meters or more, below the glacial drift.
  5. The ground over the bedrock is mostly a heterogeneous mixture of sand, clay, pebbles and boulders. Many of the pebbles and boulder differ entirely in composition from the bedrock.

Glacial deposits are of two types: (i) those deposited directly by the glaciers are known as “till”, and (ii) those materials which are deposited by the glacial melt water are called “fluvioglacial deposits”. Deposits of till are a mixture of sand, clay, pebbles, and boulders. This material, in general, is heterogeneous and unassorted with no stratification. The pebbles and boulders are commonly striated and they differ markedly in composition from the bedrock. The glacial drift does not exhibit chemical weathering.

Glaciers may transport huge rock boulders , many thousands of tonnes in weight. When ice melts, they are left behind great distances away from their natural bedrock. Such boulders are called “erratci boulders”.

The “fluvioglacial deposits” are also called “out wash deposits”. These deposits are usually well sorted and stratified accumulations of silt, sand and gravel.

Depositional Landform

Moraines

Ridges or layers of till are called “moraines”. They are of four types: (i) ground moraines, (ii) lateral moraines, (iii) medial moraines, (iv) terminal moraines.

  1. Ground Moraines: A layer of till deposited beneath the moving ice on the ground is called the “ground moraines”. Ground moraines fill low spots and old stream channels, thereby creating a labelling effect.
  2. Lateral Moraine: 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 sides of the valley. When the glacier disappears, these materials are left as ridges along the sides of the valley. Such deposits are called “lateral moraines”.
  3. Medial Moraines: When two glaciers meet, a “medial moraines” is formed by the union of two lateral moraines.
  4. Terminal MOraines: At the terminus of a glacier where the ice starts melting, the rock debris is deposited in the form of a ridges, which extends across the valley. Such deposits are called “terminal moraines: or “end 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 kettle holes ranges from a few meters to a few kilometres. 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 mountaineers the uphill sides of the drumlins are steep and the downhill sides are gently sloping. They may be 20-30 meters high and a kilometer long. Drumlins are not found singly but they occur in clusters thereby forming drumlin fields. They are believed to have formed by a subglacial shaping of an accumulated till into streamlined forms.

Eskers

Eskers are long winding ridges of stratified drift found in the middle of ground moraines. They run for kilometres in a direction more or less parallel to the direction in which ice moves. Eskers are formed due to the deposition of gravel and sand by the englacial and subglacial streams. In many areas, they are mined for sand and gravel.

Local melting of ice of glaciers produces streams of water. If these streams flow beneath the glaciers, they are called “subglacial streams” and if these flow on or within the glaciers, they are cllaed “englacial streams”.

Kames

Kames are hillocks 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 along the margin of the glacier as alluvial cones.

Varves

Varves are thinly laminated deposits formed in glacial lakes. They consist of an alternation of light-coloured bands of silt and dark coloured bands of silt and dark-coloured bands of clays. The former gets deposited during the summer season while the later in winter. Thus each pair of varves corresponds to one year of deposition. The thickness may vary from a very small fraction of a centimeter to 0.75 centimeter.

Buried Valley

The buried valleys are the ancient deep valleys, 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. Such valleys create unexpected problems for the civil engineers.

Ice Age

The Pleistocene Epoch is called “Ice Age”. The ice age began at least 2.5 million years ago and had a duration perhaps of a million years. The most recent retreat of ice had taken place between 10,000 and 15,000 years ago. The areas, which were extensively buried by ice sheets included northern North America, northern Europe, and northwest Asia. In these areas, ice sheets do not exist today.

The Pleistocene epoch, however, was not a period of continuous glaciation. During this period, the continental glaciers alternately advanced interested. The time spans between glacial advances are called “interglacial periods”. During these periods warm climates returned to most of the formerly glaciated lands. Some of the interglacial periods lasted longer than the glacial zones. Four major periods of glacial advance and retreat have been identified in North America and up to six in Europe.

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