Yosemite
A valley is a low area between hills, often with a river running through it. The Valley of Marvels in Babille, Ethiopia. In geology, a valley or dale is a depression that is longer than it is wide. The terms U-shaped and V-shaped are descriptive terms of geography to characterize the form of valleys. A valley carved by glaciers is normally U-shaped. The valley becomes visible upon the recession of the glacier that forms it. When the ice recedes or thaws, the valley remains, often littered with small boulders that were transported within the ice. Valleys are one of the most common landforms on the Earth and they are formed through erosion or the gradual wearing down of the land by wind and water. In river valleys for example, the river acts as an erosional agent by grinding down the rock or soil and creating a valley. One of the world's most famous U-shaped valleys is Yosemite Valley in California. It has a broad plain that now consists of the Merced River along with granite walls that were eroded by glaciers during the last glaciation (photo). A valley formed by flowing water, or river valley, is usually V-shaped. The exact shape will depend on the characteristics of the stream flowing through it. Rivers with steep gradients, as in mountain ranges, produce steep walls and a bottom. Shallower slopes may produce broader and gentler valleys. A valley carved by glaciers is normally U-shaped. The valley becomes visible upon the recession of the glacier that forms it. When the ice recedes or thaws, the valley remains, often littered with small boulders that were transported within the ice. A glacier is a large body of ice that formed on land by the compaction and recrystallization of snow, survives year to year, and shows some sign of movement downhill due to gravity. The three main types of valleys are the V-shaped valley, the flat-floored valley, and the U-shaped valley. A V-shaped valley is a narrow valley that has a profile suggesting the form of the letter V, characterized by steeply sloping sides. It results from a stream eroding downward, a process referred to as downcutting. U shaped Valley-Yosemite. 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. 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. This is because they flowed down pre-existing river or V-shaped valleys during the last glaciation and caused the bottom of the V to level out into a U shape as the ice eroded the valley walls, resulting in a wider, deeper valley. For this reason, U-shaped valleys are sometimes referred to as glacial troughs. One of the world's most famous U-shaped valleys is Yosemite Valley in California. It has a broad plain that now consists of the Merced River along with granite walls that were eroded by glaciers during the last glaciation (photo). A valley is a hollow or surface depression of the earth bounded by hills or mountains, a natural trough in the earth's surface, that slopes down to a stream, lake or the ocean, formed by water and/or ice erosion. Systems of valleys extend through plains, hills, and mountains.  Valleys are one of the most common landforms on the surface of the planet. They are carved by flowing water or flowing ice through the process of erosion, which is the gradual wearing away of Earth surfaces through the action of wind and water. A V-shaped valley is a narrow valley that has a profile suggesting the form of the letter V, characterized by steeply sloping sides. It results from a stream eroding downward, a process referred to as downcutting. Landforms of the upper valley-V-shaped valleys and waterfalls. Landforms in upland regions are dominantly created by erosion processes, where land is worn away. Generally, the volume and discharge of rivers in upland regions tends to be low, and the river uses much of it's energy in overcoming friction. 
https://en.wikipedia.org/wiki/Valley http://geography.about.com/od/physicalgeography/a/valleyformation.htm https://en.wikipedia.org/wiki/Valley http://www.scienceclarified.com/landforms/Ocean-Basins-to-Volcanoes/Valley.html https://en.wikipedia.org/wiki/U-shaped_valley http://geography.about.com/od/physicalgeography/a/valleyformation.htm http://www.edu.pe.ca/southernkings/valleyfl.htm http://www.scienceclarified.com/landforms/Ocean-Basins-to-Volcanoes/Valley.html http://coolgeography.co.uk/GCSE/Year%2010/PhysicalWorld/Upper%20valley/upper_valley.htm
what landform makes a u shaped valley