<P> 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 or rounded bottom (by contrast, valleys carved by rivers tend to be V - shaped in cross-section). 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 . </P> <P> 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 . </P> <P> 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 . </P> <P> The floors of these glacial valleys is where the most evidence can be found regarding glaciation cycles . For the most part, the valley floor is wide and flat, but there are various glacial features that signify periods of ice transgression and regression . The valley can have various steps, known as valley steps, and over-deepenings anywhere from ten to hundreds of meters deep . These then fill in with sediments to create plains or water to create lakes, sometimes referred to as "string - of - pearl" or ribbon lakes . Such water filled U-valley basins are also known as "fjord - lakes" or "valley - lakes" (Norwegian: fjordsjø or dalsjø). Gjende and Bandak lakes in Norway are examples of fjord - lakes . Some of these fjord - lakes are very deep for instance Mjøsa (453 meters) and Hornindalsvatnet (514 m). The longitudinal profile of a U-shaped, glaciated valley is often stepwise where flat basins are interrupted by thresholds . Rivers often dig a V - shaped valley or gorge through the threshold . </P>

Formation of a u shaped valley national 5