Glacial Erosion (Abrasion & Plucking)

Glaciers are large masses of ice that move slowly over land. As they move, they erode the landscape beneath them through processes called abrasion and plucking.

Glacial Erosion Processes

Abrasion is the process where rocks and debris embedded in the base and sides of a glacier scrape against the bedrock beneath as the glacier moves. This acts like sandpaper, grinding and smoothing the rock surface below.

The glacier carries rock fragments that have been frozen into its base. As the glacier slides over the valley floor, these fragments scrape the bedrock, causing erosion. Meltwater lubricates the glacier's movement, making abrasion more effective.

Plucking occurs when meltwater from the glacier freezes into cracks in the bedrock. As the glacier moves, it pulls or 'plucks' chunks of rock away from the valley floor or sides.

Meltwater plays a key role in both processes. It seeps into cracks in the rock, freezes, and then helps loosen rock fragments. This water also lubricates the glacier’s movement, aiding abrasion.

Abrasion Effects

Abrasion leaves behind clear marks on the landscape:

• Scratches and striations: These are long, straight grooves scratched into the bedrock, showing the direction the glacier moved.
• Rock flour: Fine powder created by the grinding action of abrasion. This rock flour can be carried away by meltwater, making glacial streams cloudy.
• Smoothing: The valley floor and sides become polished and smoothed due to the constant scraping by the glacier.

For example, in the Lake District, polished rock surfaces with striations can be seen, indicating past glacial movement.

For instance, if a glacier moves over a rock surface and scratches it with embedded stones, the resulting striations show the glacier’s direction and help geographers understand past ice flow.

Plucking Effects

Plucking causes:

• Removal of large rock fragments: These are pulled away from the bedrock, leaving behind rough, broken surfaces.
• Steepening of valley sides: As rock is plucked from valley walls, the sides become steeper and more rugged.
• Jagged landscapes: The uneven removal of rock creates sharp, jagged features in glaciated valleys.

This process contributes to the dramatic, rugged scenery seen in many UK upland areas affected by glaciers.

Impact on UK Landscapes

Glacial erosion by abrasion and plucking has shaped many of the UK's upland landscapes:

• U-shaped valleys: Glaciers widen and deepen river valleys, transforming them from V-shaped to U-shaped with steep sides and flat floors.
• Corries (cirques): Abrasion and plucking at the back of a glacier hollow create bowl-shaped hollows called corries, often with a steep back wall.
• Arêtes: Narrow, knife-edge ridges formed when two corries erode back towards each other by plucking and abrasion.

Examples include:

• The Lake District: Features many U-shaped valleys such as Borrowdale and corries like Red Tarn.
• The Scottish Highlands: Known for sharp arêtes and deep corries formed by glacial erosion.
• Snowdonia in Wales: Displays classic glacial landforms shaped by abrasion and plucking.

For example, the steep valley sides of Glen Coe in Scotland were formed by plucking, while the smooth valley floor was shaped by abrasion.

For instance, when a glacier moves downhill, abrasion smooths and widens the valley floor while plucking steepens the valley sides by removing chunks of rock.