U-shaped valley

A glaciated valley in the Mount Hood Wilderness showing the characteristic U shape.

U-shaped valley in Leh valley, Ladakh, NW Indian Himalaya. The glacier visible at the head of the valley is the last remnants of the formerly much more extensive valley glacier which carved this valley.

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 and Canada. A classic glacial trough in Glacier National Park in Montana, USA in which the St. Mary River runs.

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 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.

U-shaped valley in Glacier National Park in Montana, United States.

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.

See also

• Glacial landforms
• V-shaped valley
• Hanging valley
• Side valley