The process of glacial erosion in upland areas

Glacial erosion is a powerful geological process that shapes landscapes through the movement of ice over time. As glaciers form and move, they carve out distinct features in upland regions, dramatically altering the terrain through various mechanisms.

Mechanisms of glacial erosion

• Plucking - Ice freezes onto rock surfaces, and as the glacier moves, it pulls away fragments of rock, contributing to the steepening of valley walls.
• Abrasion - Rocks and debris embedded in the base of the glacier grind against the underlying bedrock, wearing it down and smoothing surfaces.
• Freeze-thaw action - Water seeps into cracks in rocks, freezes, and expands, causing the rock to fracture and break apart, especially on exposed surfaces above the ice.

Formation of corries and associated features

Corrie formation is a key outcome of glacial erosion in mountainous areas. These features begin as small hollows on mountainsides and are transformed by ice movement into distinctive landforms, often accompanied by related structures.

Stages of corrie development

1. Ice accumulates in a hollow on a mountainside, initiating erosion through plucking and freeze-thaw action, which steepens the back and side walls.
2. The glacier moves in a rotational manner, deepening the hollow into a bowl-shaped depression known as a corrie, with a pronounced lip at the lower end.
3. As the corrie fills with ice, the glacier overflows the lip, moving down the valley, and a large crevasse often forms where the ice pulls away from the back wall.
4. After the ice melts, the corrie remains as a steep-sided, armchair-shaped hollow, frequently containing a small lake called a tarn, such as those seen in the Lake District.

Related features from corrie erosion

• Arêtes - When two corries form side by side, the glacial erosion between them narrows and sharpens the dividing ridge into a knife-edged feature known as an arête, for example, the ridges near Helvellyn in the Lake District.
• Tarns - These are lakes that form in the base of corries after the ice melts, resulting from the over-deepening of the hollow by glacial action.

Transformation of valley profiles by glaciers

As glaciers move down from upland areas, they significantly alter the shape and structure of pre-existing valleys. This transformation results in unique valley profiles and associated features that are characteristic of glaciated landscapes.

Changes to valley shape and structure

• From V-shape to U-shape - Glaciers erode valley sides with immense force, changing the typical V-shaped river valley into a wider, deeper U-shaped valley, often referred to as a glacial trough, as seen in areas like Snowdonia.
• Straighter valley paths - Unlike rivers, glaciers do not meander around obstacles; instead, they cut straight through protruding spurs, creating a more linear valley path.
• Truncated spurs - The spurs of land that once extended into the valley are eroded by the glacier, leaving steep, abrupt edges on the valley sides, a feature known as truncated spurs, evident in glaciated valleys like Nant Ffrancon.

Formation of hanging valleys

• Differential erosion - Tributary valleys, which feed into the main valley, experience less intense glacial erosion compared to the main glacier path. As a result, they are left at a higher elevation.
• Waterfall creation - Streams from these tributary valleys, now termed hanging valleys, enter the main valley as waterfalls due to the significant height difference created by the deeper erosion of the main glacial trough.

Key landscape features resulting from glacial erosion

Glacial erosion leaves behind a variety of distinctive landforms that define glaciated landscapes. These features are the result of the powerful erosive actions of ice over long periods and are often visible in regions that were once covered by glaciers.

Summary of major glacial landforms