Glaciers are massive, slow-moving rivers of ice that act as powerful agents of geological transformation. Over thousands of years, these bodies of ice exert tremendous force, fundamentally reshaping mountain ranges and carving out distinct topographic features. The sheer weight and relentless flow of a glacier allow it to erode, transport, and deposit vast quantities of material. This process creates the deep valleys and sharp peaks that define many dramatic mountain profiles.
The Engine of Change: Glacial Erosion Processes
Glacial ice erodes solid bedrock through two primary mechanical processes: plucking and abrasion. Plucking, also known as quarrying, occurs when meltwater infiltrates small cracks and joints within the rock underneath the glacier. The immense pressure of the overlying ice causes the water to freeze and expand, widening these fractures and loosening chunks of bedrock. As the glacier moves, it pulls these fractured pieces away from the mountain face, incorporating them into its base.
The second process, abrasion, involves the constant grinding action of the rock fragments that have become frozen into the bottom layer of the glacier. These embedded pieces act like coarse sandpaper, scouring the underlying bedrock as the ice flows downhill. This action smooths and polishes the rock surfaces, leaving behind long, parallel scratches known as glacial striations. Abrasion intensity relates to the amount of debris carried, the speed of the ice, and the hardness of the rock. A byproduct of this grinding is fine-grained sediment called rock flour, which often gives glacial meltwater streams a milky or turquoise appearance.
Landforms of Destruction: Shaping the Mountain Profile
Plucking and abrasion create dramatic landforms characteristic of glaciated mountain environments. At the head of a glacier, where rotational movement is concentrated, a deep, armchair-shaped hollow called a cirque is carved into the mountainside. Repeated freeze-thaw cycles and plucking steepen the back wall of the cirque, deepening the bowl-like depression.
As multiple cirques form on different sides of a single mountain peak, their back walls erode toward each other, resulting in the formation of an arête. This is a sharp, knife-edge ridge that separates two adjacent glacial valleys or cirques. When three or more cirques erode a central mass from multiple directions, the remaining rock forms a steep, pyramid-shaped peak known as a horn. The Matterhorn in the Swiss Alps is a famous example of this convergent erosion.
Further down the mountain, glaciers transform the narrow, V-shaped valleys initially carved by rivers into broad, distinctive U-shaped valleys, also called glacial troughs. The flowing ice straightens the valley course while abrasion deepens the floor and plucking steepens the side walls. Smaller tributary glaciers, which feed into the main trunk glacier, do not cut down as deeply due to less volume and weight, creating a feature called a hanging valley. When the ice melts, these hanging valleys are often marked by waterfalls plunging down to the floor of the main glacial trough.
Landforms of Construction: Depositing Sediments
While erosion is responsible for the mountain’s dramatic shape, the deposition of the transported material builds new features in the lower reaches of the landscape. The unsorted mixture of rock fragments, ranging from fine clay to large boulders, deposited directly by the ice is collectively termed glacial till or glacial drift. This till forms various types of moraines, which are linear ridges or mounds of sediment.
Lateral moraines form as debris falls from the valley walls onto the glacier’s edges, creating ridges of till along the sides of the valley after the ice melts. When two tributary glaciers merge, their adjacent lateral moraines join to form a medial moraine, appearing as a dark stripe of debris running down the center of the combined ice flow. The furthest extent of the glacier’s advance is marked by a terminal moraine, a crescent-shaped ridge of sediment deposited at the glacier’s terminus. Glacial erratics are large boulders composed of foreign rock types, transported a great distance by the moving ice before being dropped as the glacier retreated.