Glacial Landforms of Erosion (Corries & Aretes)

Formation of Corries

Corries, also called cirques, are armchair-shaped hollows found on mountainsides formed by glacial erosion. Their formation involves several key processes:

• Plucking: As the glacier moves, meltwater seeps into cracks in the rock beneath it. When this water refreezes, it expands and breaks pieces of rock away. The glacier then "plucks" these loosened rocks from the bedrock.
• Abrasion: Rocks and debris embedded in the base of the glacier scrape and grind against the bedrock as the ice moves, smoothing and deepening the hollow.
• Rotational movement: Ice within the corrie moves in a circular, rotational motion. This movement erodes the hollow more deeply and steepens the back wall.
• Overdeepening: The hollow becomes deeper than the surrounding landscape due to intense erosion, creating a basin-shaped depression.

These processes work together over thousands of years during ice ages to carve out the distinctive corrie shape.

For instance, in the Lake District, glaciers carved deep corries into the mountainsides by plucking rocks from the back wall and abrading the hollow floor as the ice rotated and moved.

Features of Corries

Corries have several distinctive physical features:

• Armchair-shaped hollow: The corrie is a steep-sided, bowl-shaped hollow with a rounded floor.
• Steep back wall: The back wall is often very steep or even vertical, formed by plucking and freeze-thaw weathering.
• Tarn formation: After the glacier melts, water can fill the overdeepened hollow, forming a small mountain lake called a tarn.
• Bergschrund: A bergschrund is a crevasse or gap that forms between the moving glacier ice and the stationary ice or rock above. It often appears near the back wall of the corrie and is important in the erosion process.

These features are commonly seen in UK upland areas that experienced glaciation, such as the Lake District and Scottish Highlands.

Formation of Aretes

Aretes are narrow, sharp ridges that form between two adjacent corries or glacial valleys. Their formation involves:

• Erosion between adjacent corries: When two corries form back-to-back on a mountain, the glaciers erode the rock between them from both sides.
• Narrow, sharp ridges: The erosion steepens the sides of the ridge, making it thin and knife-edged.
• Freeze-thaw weathering: Water enters cracks in the rock, freezes, and expands, breaking the rock apart and sharpening the ridge.
• Ice plucking on sides: Plucking removes rock from the sides of the ridge, further steepening and narrowing it.

Aretes are dramatic features that highlight the power of glacial erosion shaping mountain landscapes.

For example, in Snowdonia, aretes like Crib Goch show sharp ridges formed by glaciers eroding either side of the ridge during the last ice age.

Examples in the UK

The UK has many classic examples of corries and aretes formed during the last Ice Age, especially in upland areas:

• Lake District: Home to famous corries such as Red Tarn and Helvellyn's corries. These corries have steep back walls and often contain tarns.
• Snowdonia: Known for sharp aretes like Crib Goch and narrow ridges formed by glacial erosion between corries.
• Scottish Highlands: Extensive corries and aretes, such as those around Ben Nevis, show classic glacial erosion features.

These landforms are significant because they shape the rugged, dramatic landscapes popular for hiking and tourism. They also provide evidence of past glaciation in the UK.