LANDFORMS MADE BY GLACIER ACTION

The level above which there is perpetual snow cover is called the snowline. The snowline varies with altitude and latitude. In the polar region it is at sea-level; in East Africa it is at 5000 m; in the northern hemisphere it is lower on the shady north-facing side of a mountain than the south­facing side. When the accumulation of snow in a region increases year by year, it gradually turns into ice by its own weight.

Masses of ice that cover large areas of a continent are called ice s~ets, and those which occupy mountain valleys are called valley glaciers. Today ice sheets occur in Antarctica and Greenland, while valley glaciers are found in the Himalayas, Andes, Alps and Rockies. The period when the high latitudes were buried under ice sheets is known as the Ice Age. With the melting of the ice at the end of the Ice Age, enormous quantities of water were set free. Some of these formed lakes, examples being the Great Lakes of America...and the lakes of Finland.

A glacier is defined as a mass of ice that moves under the influence of gravity along a confined course away from its source area. However, the movement is not of the glacier as a whole. Throughout the glacier bits of ice are melting, tric,kling down-valley and then turning back into ice the whole time. This means that within the glacier there is a gradual down-valley movement.

Glacial erosion consists of two processes: (i) plucking or the tearing away of blocks of rock which have become frozen into the base and sides of a glacier, and (ii) abrasion or the wearing away of rocks beneath a glacier by the scouring action of the rocks embedded in the glacier.

The erosional features produced by glaciers include the cirque. A cirque or corrie originates as a small hollow where snow accumulates. The snow becomes compacted to glacial- ice, forming a cirque glacier, and eventually flows downslope under the influence of gravity. The characteristic shape of the cirque is a result. of the freeze-thaw erosion on the headwall and the rotational slip of ice withi.,n the concave floor of the hollow, which is widened and deepened by plucking. Many cirques contain small circular lakes called tams. Sometimes corries develop on adjacent slopes and only a knife-edge ridge, called an arete, separates them.

If a glacier extending down a valley enters a part of the valley which is wider than the rest, the glacier ice spreads out to fill the valley; this causes the upper layers of the ice to crack along lines parallel to the valley sides. These cracks are very deep and are called crevasses. As the amount of ice in a valley increases, the power to erode by a valley glacier also increases. This results in the glacier deepening, straightening and widening a river valley.

The overdeepening of the valley gives it a characteristic U shape. Hanging valleys are another common feature ii1. areas that have been glaciated. These are tributary valleys that lie above the main valley and are separated from it by steep slopes down which streams may flow as a waterfall or a series of rapids. (Hanging valleys may also form during the retreat of a coastline under rapid erosion.)

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Certain features are produced by glacial deposits. A valley glacier carries a large amount of rock waste called moraine. The moraine forming along the sides of a glacier is called lateral moraine; that along the front of a glacier is called terminal moraine; that at the bottom of a glacier is the ground moraine. When two glaciers join together, their inner lateral moraines coalesce to give a medial moraine. Terminal moraine material is carried down-valley by the melt waters issuing from the glacier's snout (front) and is deposited as a layer called an outwash plain. One of the most conspicuous features of lowlands which have been glaciated by ice sheets is the widespread morainic deposits. Because of the numerous boulders in the clay these are called boulder clay deposits.

The deposits are sometimes several hundred metres thick and their surface is marked by long rounded hills, called drumlins. Large blocks of rock of a material, quite different to that of the rocks of the region, often occur in areas which lay under ice sheets. These blocks are known as erratics. Rivers and streams occur inside most glaciers and these are heavily loaded with rock debris. As an ice front retreats the rivers build up ridge-like deposits called eskers. They develop on top of the boulder clay deposits. Roche moutonnees are another feature produced by glacial deposition.