Glacial erosion: valley landforms

The glacial trough, often more simply called a U-shaped valley, is an impressive landscape feature. These glaciated valleys can be hundreds of metres deep with vertical rock walls, down which waterfalls cascade from hanging valleys.On the top of the valley sides the land often flattens out to form a high-level bench, known as an 'alp' in the Alps of Switzerland. The width and flatness of the valley floor are in marked contrast to the steepness of the sides.

These valleys are drained by misfit streams which are dwarfed by the size and scale of the new glaciated valley. In some glacial troughs, lakes ill parts or all of the valley floor; these lakes are ribbon lakes, so-called because of their shape, which is long and thin.

In the lower parts of the valley, examples of landforms of glacial deposition, such as terminal moraines, are found.

The valley's long profile is characterized by its irregular shape providing many hollows for lake formation.

Everything about a glacial trough shows the power of ice to erode. The former V-shaped river valley is widened, deepened and straightened by the valley glacier into a U-shaped valley. Before the ice, river erosion was confined to the small part of the valley where the river flowed; the glacier, however, fills the whole valley. This means that ice is in contact with all the floor and with both valley sides, so erosion is no longer confined to the centre of the valley. The V-shaped river valley is changed into the U-shaped glacial valley because glacial erosion by abrasion and plucking occurs everywhere in the valley where the ice is in contact with rock. The river moved around obstacles in its path, and its winding course created interlocking spurs. The more powerful glacier cannot flow so freely around corners, and it pushes straight around corners, and it pushes straight forward, cutting off the edges of interlocking spurs to form truncated spurs and straight valley sides. The ice is thicker in the main valley because it is fed by all the glaciers from tributary valleys. In each tributary valley there was a smaller, less powerfull glacier than the main glacier. When only the rivers remained after the ice melted, those in tributary valleys were left hanging well above the level of the main valley floor The streams from these hanging valleys fall as waterfalls into the main valley.

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As a glacier flows down a valley it picks out weaknesses in rocks, eroding those rocks that are soft and well-jointed more rapidly than those that are hard and resistant. In those places where outcrops of hard and soft rocks alternate, the glacier erodes the soft rock more quickly and more deeply, by abrasion and plucking, forming a rock basin. The hard rock is left as a rock bar. After the ice melts, the rock basin is left as a hollow on the valley floor between two rock bars, and it is soon filled up by water to form a ribbon lake.