Glaciers are persistent bodies of dense ice that constantly move under their own weight. These immense, slow-moving rivers of ice cover about ten percent of the Earth’s land surface today. As they flow, glaciers act as powerful geological sculptors, fundamentally reshaping the bedrock and moving vast quantities of sediment. The resulting geological features, known collectively as glacial landforms, provide unmistakable evidence of the ice’s passage and demonstrate the dramatic impact ice has on the planet’s topography.
The Mechanics of Glacial Landform Creation
The transformation of the landscape occurs through two primary actions: erosion (the removal of material) and deposition (the subsequent placement of that material). Glacial erosion involves grinding and tearing the underlying rock. The first mechanism, glacial abrasion, acts like sandpaper; rock fragments embedded in the ice scrape against the bedrock as the glacier moves forward, wearing it down. This grinding polishes the rock surface and leaves behind parallel scratches or grooves, known as striations, which indicate the direction of ice flow.
The second erosive process is glacial quarrying, or plucking, which is effective where the bedrock is fractured. Meltwater penetrates cracks in the rock, refreezes, and exerts tremendous pressure, leveraging loose blocks. As the ice moves, it pulls these loosened blocks out of the ground, incorporating them into the glacier’s base and adding to its abrasive power. Quarrying primarily removes large, angular chunks of rock, contrasting with the fine sediment created by abrasion.
The material picked up through erosion is released when the ice melts, leading to deposition. Sediment deposited directly by the ice is called glacial till, which is unsorted and unstratified. Till contains a chaotic mixture of everything from fine clay particles to massive boulders, jumbled together without any layering.
Meltwater flowing away from the glacier also creates glaciofluvial deposits. Unlike till, this material is sorted by size. The running water carries finer sand and silt farther away, dropping heavier gravel and pebbles closer to the ice front. This action creates distinct landforms composed of layered, or stratified, sediment.
Erosional Features That Carve the Landscape
The powerful erosive action of glaciers carves out highly recognizable features, particularly in mountainous environments. One distinctive feature is the cirque, an amphitheater-like basin found at the head of a glacial valley. These bowl-shaped depressions are formed by rotational slip, where the ice rotates, deepening the basin. This action creates a steep, semicircular headwall through the combined forces of plucking and abrasion.
When two cirques form back-to-back or side-by-side, the narrow ridge of rock separating them is eroded into a sharp, knife-edge crest called an arête. These jagged ridges result from the headwalls of adjacent cirques migrating backward toward each other. If three or more cirques form around a central mountain peak, their backward erosion isolates the summit. This carves the peak into a distinctive, pyramid-shaped feature known as a glacial horn.
In contrast to sharp mountain features, valleys that once held glaciers exhibit a smooth, sweeping U-shape. Glacial flow widens and deepens pre-existing river valleys, which were typically V-shaped, by aggressively eroding the floor and lower sides. The massive volume and weight of the ice straightens the valley. This creates the characteristic glacial trough with steep side walls and a broad, flat bottom.
U-shaped valleys often feature hanging valleys, which are tributary valleys stranded high above the main valley floor. The smaller tributary glaciers did not possess the erosive power to cut down to the same depth as the main glacier. When the ice melts, the smaller valley appears to “hang” above the larger one, often marked by a waterfall plunging from the junction.
Depositional Features Left Behind
When a glacier retreats, it leaves behind a variety of depositional landforms composed of till and sorted sediment. The most common features are moraines, which are long ridges or mounds of unsorted glacial till. Lateral moraines form along the sides of the glacier where debris has accumulated. A terminal moraine marks the farthest point of the glacier’s advance.
Moraines that form in the middle of a glacier are called medial moraines; they occur when two valley glaciers merge and their adjacent lateral moraines combine. As the ice melts, these debris ridges are left behind, providing a clear record of the glacier’s former extent and flow paths. An irregular blanket of till spread across the land beneath the melting ice is referred to as ground moraine.
Another depositional feature is the drumlin, an elongated, streamlined hill composed of till. These smooth, oval-shaped mounds typically occur in clusters and are oriented parallel to the direction of ice movement. Drumlins often have a steeper slope on the side facing the glacier’s advance. They have a gentler slope on the lee side, indicating the flow of the ice that shaped them.
Meltwater deposits also create features such as eskers, which are long, winding ridges composed of stratified sand and gravel. Eskers form when sediment fills tunnels carved by streams flowing beneath the glacier. When the ice melts, the tunnel deposits collapse and remain as a sinuous raised embankment. Meltwater streams flowing away from the glacier’s terminus deposit sorted sediment across a broad, flat expanse known as an outwash plain, characterized by layers of sand and gravel.