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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.
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.
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.
The third type of valley is called a flat-floored valley and is the most common type in the world. These valleys, like V-shaped valleys, are formed by streams, but they are no longer in their youthful stage and are instead considered mature. With these streams, as the slope of a stream's channel becomes smooth, and begins to exit the steep V or U-shaped valley, the valley floor gets wider. Because the stream gradient is moderate or low, the river begins to erode the bank of its channel instead of valley walls. This eventually leads to a meandering stream across a valley floor.
Over time, the stream continues to meander and erode the valley's soil, widening it further. With flood events, the material that is eroded and carried in the stream is deposited which builds up the floodplain and the valley. During this process, the shape of the valley changes from a V or U shaped valley into one with a broad flat valley floor. An example of a flat-floored valley is the Nile River Valley.
Humans and Valleys
Since the beginning of human development, valleys have been an important place for people because of their presence close to rivers. Rivers enabled easier movement and also provided resources like water, good soils, and food such as fish. The valleys themselves were also helpful in that valley walls often blocked winds and other severe weather if the settlement patterns were positioned correctly. In areas with rugged terrain, valleys also provided a safe place for settlement and made invasions difficult.