Glacial Erosion Landforms in Detail
Glacial erosion creates some of the most dramatic landscapes on Earth. AQA A-Level Geography requires detailed knowledge of how these landforms develop at multiple scales, from microscopic striations to entire mountain ranges. This lesson examines glacial erosion processes and the landforms they produce.
Glacial Erosion Processes
Two main processes are responsible for glacial erosion:
1. Abrasion
- Rock fragments embedded in the base and sides of the glacier are dragged across the bedrock
- Acts like sandpaper — scratching, scouring and polishing the rock surface
- Produces striations (scratches on the bedrock surface) that indicate the direction of ice movement
- The rate of abrasion depends on:
- Ice velocity — faster movement = more abrasion
- Ice thickness — greater weight = greater pressure on embedded rocks
- Debris concentration — more rock fragments = more abrasive power
- Debris hardness — harder fragments abrade more effectively
- Bedrock resistance — softer bedrock is abraded more quickly
2. Plucking (Quarrying)
- Meltwater at the base of the glacier seeps into joints and cracks in the bedrock
- The water refreezes, bonding the rock to the glacier
- As the glacier moves forward, it pulls away blocks of rock
- Most effective where:
- Bedrock is well-jointed (e.g., granite, limestone)
- Meltwater is present (warm-based glaciers)
- The glacier passes over the lee side of obstacles where pressure drops and refreezing occurs
- Produces a rough, angular rock surface on the downstream side of obstacles
Additional Processes
- Meltwater erosion — water flowing under pressure beneath the glacier erodes channels in the bedrock (subglacial streams)
- Freeze-thaw weathering — operates above the glacier on exposed rock faces (e.g., above the bergschrund in a corrie), shattering rock that falls onto the glacier surface
- Chemical weathering — meltwater can dissolve soluble rocks, although this is a minor process
Micro-Scale Landforms
Striations
- Scratches on bedrock surfaces, typically a few millimetres deep and up to several metres long
- Created by rock fragments embedded in the base of the glacier being dragged across the surface
- Direction of striations indicates the direction of former ice movement
- Useful for reconstructing past glacial flow patterns
Glacial Polish
- Very fine abrasion produces a smooth, polished rock surface
- Occurs where fine-grained sediment (silt, clay) is dragged across the bedrock under high pressure
- Creates a shiny, reflective surface on hard rocks like granite
Chatter Marks
- Crescent-shaped fractures on the bedrock surface
- Created by large boulders bouncing or rotating against the rock under the weight of the glacier
- The concave side faces the direction from which the ice came
Meso-Scale Landforms
Roches Moutonnées
A roche moutonnée is an asymmetric bedrock knoll shaped by glacial erosion.
Formation:
- Ice flows over a bedrock obstacle (a resistant rock outcrop)
- Upstream (stoss) side: The ice is compressed against the rock, increasing pressure and producing meltwater. Abrasion smooths and streamlines this surface, creating a gentle, rounded slope
- Downstream (lee) side: Pressure decreases as ice flows over the crest. Meltwater refreezes in joints (regelation), and the glacier plucks blocks of rock away, creating a steep, rough, angular surface
- The result: smooth upstream, rough downstream — indicating the direction of former ice flow
Example: Roches moutonnées are common in the Lake District and the Scottish Highlands
Crag and Tail
A crag and tail is a larger-scale feature formed when a glacier encounters a very resistant rock mass.
Formation:
- A resistant rock outcrop (the crag) deflects the glacier around it
- On the lee (downstream) side, weaker rock is protected from erosion by the crag
- This protected rock forms a tapering ridge — the tail
- The crag shows evidence of abrasion on its upstream face and plucking on its sides
Example: Edinburgh Castle Rock — the Castle sits on a volcanic plug (crag) with the Royal Mile extending eastward along the tail of softer sedimentary rock
Macro-Scale Landforms
Corries (Cirques)
A corrie (cirque in French, cwm in Welsh) is an armchair-shaped hollow on a mountainside, with a steep back wall and an overdeepened basin, often containing a small lake (tarn).