A valley is a low-lying, elongated depression in the Earth’s surface, typically found between higher landforms such as hills or mountains. These landforms are among the most common topographical features across the globe. Valleys often serve as drainage channels, containing a river or stream that flows down the gentle slope of the depression. Their exact appearance, cross-sectional shape, and scale are determined by the specific geological processes responsible for their creation. Water erosion, massive ice movements, and tectonic plate activity each sculpt valleys with distinct and recognizable characteristics.
Landscapes Defined by River Flow
The most widespread type of valley is carved by the long-term action of running water, a process known as fluvial erosion. These river valleys are commonly characterized by a distinct “V-shape” in their cross-section. This V-shape results from the river’s tendency to cut aggressively downward into the landscape, known as vertical erosion.
As the river deepens its channel, the valley sides are weathered and pulled down by gravity and mass movement. This combination of downward cutting and slope widening gives the valley its characteristic pointed bottom and steep sides. These formations are most prominent in youthful or mountainous regions where the river’s gradient is steep, providing high water velocity and significant erosive power.
River valleys are found in nearly all humid climates and mountainous drainage basins worldwide. The Grand Canyon is a prime example of a massive river system, the Colorado River, cutting down through layers of rock over millions of years. Over time, as a river reaches a lower gradient, the valley may mature, shifting from vertical erosion to lateral erosion, which results in a broader, flat-floored valley.
Depressions Sculpted by Glaciation
Valleys created by the movement of massive ice sheets, known as glaciers, possess a different profile than those formed by rivers. Glacial valleys are distinguished by a characteristic “U-shape” or trough, featuring a wide, flat floor and steep, almost vertical sides. This unique shape results from the immense erosive power of a glacier, which widens and deepens any pre-existing river valley.
The glacier’s erosive action involves two primary mechanisms: plucking and abrasion. Plucking occurs when meltwater enters cracks in the bedrock, freezes, and tears away large blocks of rock as the glacier moves. Abrasion involves rock fragments embedded in the ice scraping and grinding the valley floor and sides, smoothing and deepening the trough.
These U-shaped valleys are found in high-latitude or high-altitude regions that experienced glaciation during the Pleistocene Ice Ages, such as the Alps, the Rocky Mountains, and the Scandinavian Mountains. A fjord is a significant example of a glaciated valley, carved below sea level and subsequently flooded by the ocean after the glacier retreated. Fjords, like those in Norway and New Zealand, often feature deep water and towering, sheer valley walls.
Canyons Created by Tectonic Forces
A different category of valley formation bypasses surface erosion entirely and is created by the movement of the Earth’s tectonic plates. These features are known as rift valleys, which form not by cutting down, but by the crust pulling apart. Rift valleys occur at divergent plate boundaries, where extensional tectonic forces stretch and thin the lithosphere.
As the crust stretches, it fractures along large parallel cracks called normal faults, causing the central block of land to drop down relative to the blocks on either side. This sunken central block is known as a graben, forming the floor of the rift valley, which is bordered by steep, fault-created escarpments. The resulting valley is a long, linear depression that can be thousands of kilometers in length.
The most extensive continental example is the East African Rift Valley, a massive system where the African continent is slowly splitting into two separate plates. This rift stretches for thousands of miles, encompassing countries from Ethiopia to Mozambique. Its floor is characterized by volcanic activity and a series of deep, elongated rift lakes, such as Lake Tanganyika and Lake Baikal in Siberia.