A valley is an elongated, low-lying area situated between higher landforms such as hills, mountains, or plateaus. These depressions are found across the globe and serve as primary conduits for water flow, often containing a river or stream. Valley forms are diverse, ranging from deep, narrow canyons to broad, flat plains. Their shape provides a clear record of the forces that created them, whether through erosion by water or ice, or by movement within the Earth’s crust.
Defining Features of a Valley
Every valley consists of three primary structural components: the valley floor, the valley walls, and a drainage system. The valley floor is the flat or gently sloping bottom portion, often covered in sediments deposited by the flowing water. This floor is the lowest point in the cross-section, providing a clear elevation difference from the surrounding terrain.
The valley walls, also known as the slopes, rise up from the floor to meet the higher ground on either side. The steepness and curvature of these walls result directly from the formation process, providing the distinctive V-shape, U-shape, or steep sides seen in tectonic valleys. The drainage system is typically a river or stream that flows along the length of the valley, often acting as the main agent of erosion.
Formation by Fluvial Erosion
The most common method of valley creation involves fluvial erosion, the action of running water, resulting in the characteristic V-shaped valley. This process begins when a stream or river cuts vertically downward into the landscape, a process known as downcutting. Downcutting is powered by the force of the water and the abrasive action of sediment, such as rocks and pebbles, carried in the current.
As the river erodes deeper, the valley walls are simultaneously steepened by weathering and mass movement, causing material to fall into the river channel for transport. This combination of vertical erosion and side slope collapse creates the classic V-shaped cross-section. Where the gradient is steep and the flow is fast, the river lacks the power to erode sideways, resulting in a narrow floor and steep, interlocking spurs of rock. The Grand Canyon, carved by the Colorado River, is a dramatic example of a deep, V-shaped river valley.
Formation by Glacial Action
A distinctly different valley type is formed by the power of moving ice, resulting in a U-shaped valley, also known as a glacial trough. These valleys begin when a large glacier flows down an existing V-shaped river valley, widening and deepening it significantly. Unlike rivers that focus erosion downward, glaciers erode both vertically and laterally, scouring the entire cross-section.
The glacier uses two main processes to reshape the land: abrasion and plucking. Abrasion involves rock fragments embedded in the ice grinding away the bedrock on the floor and sides. Plucking occurs when the glacier freezes onto chunks of bedrock, pulling them away as the ice moves. This pervasive erosion transforms the narrow V-shape into a broad, flat floor and steep, smooth sides, creating the parabolic U-shape. When the ice melts, it leaves behind a flat-bottomed valley, often with a small “misfit” stream flowing through it.
Formation by Earth Movement
Some of the largest valleys on Earth are created not by erosion, but by the movement of the Earth’s crust, known as tectonic activity. Rift valleys are linear, elongated lowlands that form where the Earth’s lithosphere is pulled apart by extensional forces at a divergent plate boundary. This process is called rifting, and it causes the crust to stretch and thin.
As the crust stretches, it fractures along large, parallel normal faults. The central block of crust, known as a graben, subsequently drops down relative to the elevated blocks on either side. This subsidence creates a valley with a relatively flat floor and steep, often stepped, sides. The East African Rift System is the most extensive example of this process, where the African plate is slowly splitting apart.