U-shaped valley
From Wikipedia, the free encyclopedia
A U-shaped valley is one 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 or Rocky mountains but also in Scotland, Scandinavia or Canada. A classic glacial trough was formed by the St. Mary River in Glacier National Park in Montana, USA.
[edit] Formation
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 (glaciation) 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 they 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.
