Glaciers are powerful, slow-moving agents of change that sculpt the Earth’s surface over millennia. These massive, persistent bodies of dense ice form wherever the accumulation of snow exceeds its loss through melting or evaporation. While vast ice sheets cover Antarctica and Greenland, the most common type of glacier people envision is the one nestled high in a mountain range. This flowing mass of ice, confined by the surrounding topography, is known as the valley glacier.
Defining the Valley Glacier
A valley glacier is a body of ice that originates high in a mountain range and flows downward through a pre-existing stream valley. Also termed an alpine or mountain glacier, this ice mass is distinctly confined by the steep rock walls of the valley it occupies. This confinement forces the ice to follow the path of the original river or stream. Valley glaciers can vary greatly in size, ranging from small streams of ice only a few kilometers long to immense flows that extend for tens of kilometers down a mountain slope.
The existence of a valley glacier is highly dependent on both elevation and latitude, requiring consistent cold temperatures and sufficient annual snowfall. This distinguishes them fundamentally from continental glaciers, or ice sheets, which are massive, unconfined bodies of ice that cover entire landmasses and obscure the underlying landscape. The scale and setting of a valley glacier make it a localized but profound agent of erosion in mountainous areas worldwide.
How Valley Glaciers Form and Move
The formation of a valley glacier begins with the transformation of fallen snow through a process called glaciogenesis. Fresh snow, which is about 90% air, is buried under subsequent layers, causing its intricate crystal structure to collapse. This increasing pressure and repeated cycles of melting and refreezing compact the snow into a denser, granular material called firn. Over decades or even centuries, the weight of the overlying ice continues to compress the firn, forcing out remaining air pockets until the material recrystallizes into dense, blue-tinged glacial ice.
Once the ice mass achieves sufficient thickness, the force of gravity causes it to flow downhill. This movement is accomplished through two primary mechanisms: internal deformation and basal sliding. Internal deformation, or creep, occurs when the weight of the ice causes the individual ice crystals to slide past one another and permanently change shape. The ice crystals align and stretch, allowing the body of the glacier to flow under its own stress.
Basal sliding is a much faster movement mechanism that occurs when a thin layer of meltwater is present at the glacier’s base. The enormous pressure from the ice mass lowers the melting point of water, allowing a film of water to form at the ice-bedrock interface. This water acts as a lubricant, enabling the entire glacier to slide over the underlying bedrock with reduced friction. The balance between the accumulation zone high up—where snow is added—and the ablation zone down-valley—where ice is lost to melting—determines whether the glacier advances or retreats.
Distinctive Geological Features
The erosive power of a valley glacier drastically reshapes the landscape, leaving behind a suite of unique and identifiable landforms. The most recognizable feature is the U-shaped valley, or glacial trough, which is created as the slow-moving ice scrapes and plucks rock from the valley sides and floor. Glaciers transform the V-shaped cross-section carved by rivers into a broad, parabolic trough with steep, straightened walls and a wide, flat bottom. This scouring action, which involves the ice carrying embedded rock fragments that grind against the bedrock, can deepen valleys by hundreds of meters.
At the head of a valley glacier is the cirque, a steep-sided, amphitheater-like bowl where the ice first accumulated. When two cirques form back-to-back or side-by-side on a mountain, the narrow, jagged ridge left between them is called an arête. As the glacier flows, it also deposits the rock and sediment it has eroded, a material known as till, into distinct landforms called moraines. Lateral moraines form as ridges of debris along the sides of the ice, while a terminal moraine marks the furthest point the glacier ever reached down the valley.