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A U-shaped valley or glacial trough is formed by the process of glaciation. It has a characteristic U shape, with steep, straight sides and a flat bottom. Glaciated valleys are formed when a glacier travels across and down a slope, carving the valley by the action of scouring. When the ice recedes or thaws, the valley remains, often littered with small boulders that were transported within the ice.
Examples of U-valleys are found in mountainous region like the Alps, Himalaya, Rocky mountains, Scottish Highlands, Scandinavia, New Zealand and Canada. A classic glacial trough in Glacier National Park in Montana, USA in which the St. Mary River runs.
As a glacier moves downhill through a valley, the shape of the valley is transformed. A V-shaped valley is transformed into a U-shaped valley through the glacial erosion processes of plucking and abrasion. This results in large rocky material (glacial till) being carried in the glacier. A material called boulder clay is deposited on the floor of the valley. As the ice melts and retreats, the valley is left with very steep sides and a wide, flat floor. A river or stream may flow through the valley due to melt-water from the glacier.
This replaces the original stream or river and is known as a misfit stream. If the material which pushed in front of the glacier is left, this material is called a terminal moraine. The valley dammed by the moraine may then flood creating a lake which may twist and turn, which is termed a ribbon lake.