Glacial trough

U-shaped valleys, trough valleys or glacial troughs, are formed by the process of glaciation. They are characteristic of mountain glaciation in particular. They have 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, called glacial till or glacial erratic.

Examples of U-valleys are found in mountainous regions like the Alps, Himalaya, Rocky mountains, Scottish Highlands, Scandinavia, New Zealand and Canada. A classic glacial trough is in Glacier National Park in Montana, USA in which the St. Mary River runs. Another famous U-shaped valley is the Nant Ffrancon Valley in Snowdonia, Wales.

Formation of a U-shaped valley happens over geologic time, meaning not during a human’s lifespan. It can take anywhere between 10,000 and 100,000 years for a V-shaped valley to be carved into a U-shaped valley. These valleys can be several thousand feet deep and tens of miles long. Glaciers will spread out evenly in open areas, but tend to carve deep into the ground when confined to a valley. Ice thickness is a major contributing factor to valley depth and carving rates. As a glacier moves downhill through a valley, usually with a stream running through it, the shape of the valley is transformed. As the ice melts and retreats, the valley is left with very steep sides and a wide, flat floor. This parabolic shape is caused by glacial erosion striving to decrease friction as much as possible. There are two main variations of this U-shape. The first is called the Rocky Mountain model and it is attributed to alpine glacial valleys, showing an overall deepening effect on the valley. The second variation is referred to as the Patagonia-Antarctica model, attributed to continental ice sheets and displaying an overall widening effect on its surroundings.

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