Glacial Erosion Processes and Landforms

Glacial erosion is among the most powerful geomorphological processes on Earth. Glaciers reshape landscapes on a grand scale, carving deep valleys, steep-sided peaks, and dramatic mountain scenery. The landforms produced by glacial erosion dominate upland areas of the UK, the Alps, Scandinavia, and many other regions. Understanding the processes of erosion and the landforms they create is central to A-Level Geography.

---

Processes of Glacial Erosion

1. Plucking (Quarrying)

Plucking occurs when meltwater at the base of a glacier seeps into joints and cracks in the bedrock, refreezes, and bonds to the overlying ice. As the glacier moves forward, it pulls (plucks) blocks of rock away from the bedrock surface.

• Most effective where the bedrock is well-jointed or fractured
• Produces a rough, jagged surface on the downstream (lee) side of bedrock obstacles
• Creates the characteristic steep, rough lee slope of roches moutonnées
• Requires the glacier to be at or near the pressure melting point at its base (temperate glaciers)

2. Abrasion

Abrasion occurs when rock fragments embedded in the base of the glacier scrape across the bedrock surface, acting like sandpaper. The process was vividly described by Louis Agassiz (1840) in his pioneering work Études sur les glaciers, which established the theory of Ice Ages.

• Produces smooth, polished surfaces and striations (scratches) on the bedrock
• Striations indicate the direction of ice flow — invaluable evidence for reconstructing past glacial movement
• The effectiveness of abrasion depends on:
  • Ice thickness — greater pressure forces debris against the bedrock
  • Debris concentration — more embedded fragments increase the abrasive effect
  • Relative hardness — debris must be harder than the bedrock to scratch it
  • Ice velocity — faster movement increases the abrasive force

Exam Tip: Abrasion and plucking often work together. Plucking loosens blocks of rock, which are then incorporated into the glacier base and used as "tools" for abrasion. Always link the two processes in your answers.

3. Freeze-Thaw Weathering (Frost Shattering)

Although not strictly a glacial process (it is a periglacial weathering process), freeze-thaw weathering is critical in preparing rock for glacial erosion:

• Water enters cracks in exposed rock above and around the glacier
• When the temperature drops below 0°C, the water freezes and expands by approximately 9%
• Repeated freezing and thawing (most effective between −3°C and −8°C with frequent oscillation around 0°C) shatters the rock
• The shattered fragments fall onto the glacier surface as scree, contributing to the supraglacial debris load
• This process steepens valley sides and creates the jagged peaks and ridges characteristic of glaciated uplands

4. Meltwater Erosion

Meltwater flowing beneath, within, and in front of glaciers erodes through:

• Hydraulic action — the force of flowing water removes loose material
• Corrasion — sediment carried by meltwater abrades channel beds and sides
• Cavitation — air bubbles in fast-flowing turbulent water collapse, creating shock waves that erode rock surfaces
• Subglacial meltwater channels can erode deep gorges and potholes in the bedrock

---

Landforms of Glacial Erosion

Corries (Cirques / Cwms)

A corrie is an armchair-shaped hollow found on mountainsides, with a steep back wall and sides, an over-deepened floor, and a raised lip (threshold) at the front. Many contain small lakes called tarns (e.g., Red Tarn on Helvellyn, Lake District).

Formation

1. Snow accumulates in a pre-existing north- or north-east-facing hollow (in the Northern Hemisphere), where it is shaded from the sun
2. Nivation (freeze-thaw weathering beneath and around a snow patch) enlarges the hollow
3. As snow compresses into firn and then glacier ice, a small corrie glacier forms
4. Rotational movement of the ice deepens the floor through intense abrasion
5. Plucking steepens the back wall as meltwater refreezes in joints
6. Freeze-thaw weathering above the bergschrund (the crevasse between the glacier and the back wall) shatters the exposed rock, further steepening the back wall
7. Erosion is less effective at the front of the glacier, leaving a raised lip (threshold)
8. When the glacier melts, water is impounded behind the lip, forming a tarn

Case Study: Red Tarn, Helvellyn, Lake District — Red Tarn occupies a classic north-east-facing corrie at an altitude of 718 m. The back wall rises over 250 m to the summit of Helvellyn (950 m). The tarn is approximately 25 m deep, held back by a moraine-enhanced rock lip.

Arêtes

An arête is a narrow, knife-edged ridge formed when two corries erode back-to-back on either side of a ridge. As freeze-thaw weathering and plucking extend the back walls of adjacent corries toward each other, the ridge between them becomes increasingly narrow and sharp.

• Striding Edge (Helvellyn, Lake District) — one of England's most famous arêtes, connecting Helvellyn to Birkhouse Moor. The ridge is less than 1 m wide in places.
• Crib Goch (Snowdon, Wales) — a dramatic arête with near-vertical drops on either side.

Pyramidal Peaks (Horns)