Due to the erosion performed by glacier, the valley gets wider and resembels the english alphabet U. That's why it is called U-shapped valley.
A U-shaped valley is a geological formation characterized by high and steep sides and a rounded or flat valley bottom. As a result, U-shaped valleys have a distinct U-shaped outline. U-shaped valleys can measure thousands of feet in depth and stretch for several miles in length. These valleys are unique in that they have much wider bases than typical valleys, which tend to have narrow bottoms that form a V-shape. U-shaped valleys are found all over the world, but are particularly common throughout mountain ranges. This particular formation may also be referred to as a trough valley or a glacial trough.
How Is a U-Shaped Valley Formed?
U-shaped valleys are the result of glacial erosion, which typically occurs in pre-existing V-shaped valleys. V-shaped valleys, in contrast, are the result of rivers carving their course through the land. A glacier can take anywhere from 10,000 to 100,000 years to flatten out the bottom of a V-shaped valley and create a U-shaped valley. This erosion process occurs during periods of low temperatures, which are the result in the formation of glaciers along the mountain top. After formed, these glaciers begin to move, sliding slowly down the side of the mountains and into the valley below. Because the V-shaped valley constrains the movement of the glacier, its force is concentrated in the floor. This downward concentration of strength allows the glacier to dig into the ground, creating the flat-bottomed valley that is characteristic of U-shaped valleys. As the floor of the valley widens, the sides surrounding it are also eroded, leading to the high and steep sides seen today. Additionally, U-shaped valleys tend to be straighter than V-shaped valleys because of the non-bending movement of glaciers.