Over time, rainfall has the effect of eroding the land to form valleys and other features. At mountain peaks, the rainwater flows quickly to form narrow gullies. Slowing down as it moves further downhill, the water forms a wide valley.
A valley is an extended depression in the Earth’s surface that is usually bounded by hills or mountains and is normally occupied by a river or stream. Since valleys are usually occupied by a river, they can also slope down to an outlet which can be another river, a lake or the ocean.
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. The shape of valleys varies but they are typically steep-sided canyons or broad plains, however, their form depends on what is eroding it, the slope of the land, the type of rock or soil and the amount of time the land has been eroded.
There are three common types of valleys which include V-shaped valleys, U-shaped valleys, and flat-floored valleys.
V-Shaped Valleys
A V-shaped valley is a narrow valley with steeply sloped sides that appear similar to the letter “V” from a cross-section. They are formed by strong streams, which over time have cut down into the rock through a process called downcutting. These valleys form in mountainous and/or highland areas with streams in their “youthful” stage. At this stage, streams flow rapidly down steep slopes.
An example of a V-shaped valley is the Grand Canyon in the Southwestern United States. After millions of years of erosion, the Colorado River cut through the rock of the Colorado Plateau and formed a steep-sided canyon V-shaped canyon known today as the Grand Canyon.
U-Shaped Valley
A U-shaped valley is a valley with a profile similar to the letter “U.” They are characterized by steep sides that curve in at the base of the valley wall. They also have broad, flat valley floors. U-shaped valleys are formed by glacial erosion as massive mountain glaciers moved slowly down mountain slopes during the last glaciation. U-shaped valleys are found in areas with a high elevation and in high latitudes, where the most glaciation has occurred. Large glaciers that have formed in high latitudes are called continental glaciers or ice sheets, while those forming in mountain ranges are called alpine or mountain glaciers.
Due to their large size and weight, glaciers are able to completely alter topography, but it is the alpine glaciers that formed most of the world’s U-shaped valleys. This is because they flowed down the 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.
Picture Credit : Google