Glacial Erosion Processes and Landforms

Glacial erosion is one of the most powerful landscape-shaping forces on Earth. This lesson examines the processes by which glaciers erode rock and the distinctive landforms they create. Understanding these processes and landforms is central to Edexcel A-Level Geography Enquiry Question 1 (EQ1) and is essential for fieldwork interpretation and exam case studies.

---

Processes of Glacial Erosion

Glacial erosion occurs through several interconnected processes. The rate and effectiveness of erosion depend on the glacier's thermal regime (warm-based glaciers erode far more effectively), ice thickness, velocity, bedrock lithology and the availability of debris tools embedded in the ice.

Abrasion

Abrasion is the wearing away of bedrock by debris embedded in the base and sides of a moving glacier. It is analogous to sandpaper grinding a surface.

• Rock fragments frozen into the base of the glacier (called clasts or tools) are dragged across the bedrock surface
• This produces striations (scratches) and grooves on the bedrock, aligned with the direction of ice flow
• Abrasion rates increase with: greater ice velocity, higher basal debris concentration, harder clasts relative to bedrock, and higher basal pressure
• Typical abrasion rates range from 0.1 to 10 mm per year, though they can be much higher in fast-flowing warm-based glaciers
• Abrasion produces fine-grained sediment called rock flour (glacial flour), which gives meltwater streams their characteristic milky, turquoise colour
• The clasts themselves become worn, developing flat, striated surfaces called facets

Exam Tip: Do not confuse abrasion with plucking. Abrasion smooths and scratches surfaces; plucking removes blocks. Both occur simultaneously in a warm-based glacier but produce different landform characteristics (e.g., the smooth stoss side vs the rough lee side of a roche moutonnée).

Plucking (Quarrying)

Plucking is the removal of blocks of bedrock from the glacier bed and valley sides. It is the primary mechanism for removing large volumes of rock.

The process involves:

1. Meltwater penetrates cracks and joints in the bedrock beneath or beside the glacier
2. The water freezes, bonding the rock to the base of the glacier
3. As the glacier moves forward, it pulls the attached block away from the bedrock
4. The rock is incorporated into the base of the glacier and becomes a tool for abrasion

Plucking is most effective where:

• The bedrock is well-jointed or fractured (e.g., granite, limestone)
• Regelation occurs — meltwater refreezing on the lee side of obstacles bonds ice to rock
• The glacier is warm-based with fluctuating pressure and meltwater availability
• The bed is irregular, creating pressure differences

Freeze-Thaw Weathering (Frost Shattering)

Freeze-thaw weathering is not strictly a glacial erosion process — it is a periglacial weathering process — but it plays a vital role in glacial landscape development:

• Water enters cracks in exposed rock above and beside the glacier
• Repeated freezing (water expands by approximately 9% on freezing) and thawing widens the cracks
• Rock fragments are loosened and fall onto the glacier surface as supraglacial debris
• This process shapes the valley walls above the glacier (creating steep, angular profiles) and contributes to arête and pyramidal peak formation
• It is most effective where there are frequent temperature fluctuations across 0°C — the diurnal freeze-thaw regime common in alpine environments

Rotational Sliding (Rotational Slumping)

In cirque glaciers, the ice moves with a rotating motion — pivoting about a point rather than flowing straight down. This rotational sliding concentrates erosion at the base of the cirque, deepening the floor and creating the characteristic over-deepened, armchair-shaped hollow with a raised lip (threshold) at the front.

Erosion Process	Main Effect	Key Landform Features
Abrasion	Smoothing and scratching of bedrock	Striations, polished surfaces, rock flour, smooth stoss sides
Plucking	Removal of bedrock blocks	Rough lee sides, stepped bedrock surfaces, angular debris
Freeze-thaw	Shattering of exposed rock	Angular debris, steep valley walls, arêtes, pyramidal peaks
Meltwater erosion	Hydraulic action and chemical weathering beneath the glacier	Subglacial channels, potholes, rock basins

---

Erosional Landforms

Corries (Cirques, Cwms)

A corrie (cirque in French, cwm in Welsh) is an armchair-shaped hollow with steep backwalls and sidewalls, a flat or over-deepened floor, and a raised lip (threshold) at the front, often containing a small lake (tarn).

Formation process:

1. Snow accumulates in a pre-existing north- or north-east-facing hollow (in the Northern Hemisphere), sheltered from direct sunlight and prevailing winds
2. Nivation (a combination of freeze-thaw weathering, meltwater erosion and chemical weathering beneath and around a snowpatch) deepens and enlarges the hollow
3. As snow accumulates and compresses into firn and ice, a small glacier forms
4. The glacier erodes the hollow through rotational sliding — abrasion deepens the floor while plucking steepens the backwall
5. Freeze-thaw weathering above the glacier (in the bergschrund zone — the crevasse between the glacier and the backwall) loosens rock from the headwall, which falls onto the glacier
6. The rotational motion concentrates erosion at the base, creating an over-deepened basin behind a lip of less-eroded rock
7. When the glacier retreats, the hollow may fill with water, forming a tarn (e.g., Red Tarn below Helvellyn, Lake District)

Case Study — Red Tarn, Helvellyn (Lake District):

• Classic north-east-facing corrie at approximately 720 m altitude
• Steep backwall rises approximately 250 m to the summit of Helvellyn (950 m)
• The corrie is flanked by the arêtes of Striding Edge and Swirral Edge
• Red Tarn occupies the over-deepened basin behind the moraine threshold
• The corrie orientation (NE-facing) maximised snow accumulation and minimised solar insolation

Arêtes

An arête is a narrow, knife-edged ridge formed when two corries on opposite or adjacent sides of a mountain erode backwards towards each other. As freeze-thaw weathering and plucking steepen the backwalls of both corries, the ridge between them becomes progressively narrower and sharper.