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Spec mapping (AQA 7037): Paper 1, §3.1.4 Glacial Systems and Landscapes — geomorphological processes of glacial erosion; origin and development of landforms of glacial erosion (corries, arêtes, pyramidal peaks, glacial troughs, roches moutonnées, hanging valleys, truncated spurs). Erosion converts the energy and movement of the glacial system (this option's systems framework, §3.1.4, linked to §3.1.1) into landscape. The role of structure and lithology is a recurring synoptic theme shared with coastal and fluvial landform study. The assessment objectives are AO1 (process mechanisms and landform sequences), AO2 (applying process to explain why specific landforms develop in specific places, including the role of geology) and AO3 (interpreting maps, long-profiles and field evidence such as striations).
Glacial erosion is among the most powerful geomorphological processes on Earth, capable of excavating rock basins hundreds of metres deep and sculpting the most dramatic mountain scenery on the planet. The landforms it produces dominate the uplands of the Lake District, Snowdonia, the Scottish Highlands, the Alps and Scandinavia. The essential principle, carried forward from the movement lesson, is that effective erosion requires a warm-based glacier sliding on basal meltwater — cold-based ice frozen to its bed largely protects the rock beneath it.
A helpful framing is to think of erosional landforms at three scales: micro-scale features cut into bedrock (striations, polish, chatter marks); meso-scale landforms (corries, roches moutonnées, hanging valleys); and macro-scale landscapes (glacial troughs, fjords, knock-and-lochan terrain, whole glaciated uplands). All are produced by the same fundamental processes — plucking and abrasion — but operating at different scales and over different lengths of time. Keeping this scale hierarchy in mind helps structure answers and shows the examiner you understand that a landscape is an assemblage of landforms produced by a coherent set of processes.
Plucking occurs where basal meltwater seeps into joints and fractures in the bedrock and refreezes, bonding rock to the overlying ice; as the glacier moves on, it tears (plucks) blocks away from the bed. It is the dominant process on the down-glacier (lee) side of bedrock obstacles, where reduced pressure encourages refreezing and where blocks loosened by frost action and dilatation are most easily entrained. Because plucking depends so heavily on the rock already being fractured, geology — the density and orientation of joints, faults and bedding planes — exerts strong control over how effective it is: a massive, unjointed granite resists plucking, whereas a well-bedded, jointed slate or sandstone is quarried readily. The plucked blocks then become the tools for abrasion, so the two processes are mechanically coupled in a continuous cycle of loosening, entrainment and grinding.
Abrasion is the scouring of the bed by rock debris held in the base of the ice, which acts like the grit on sandpaper. The process was first attributed to ice (rather than a biblical flood) by Louis Agassiz (1840) in Études sur les glaciers.
Exam Tip: Abrasion and plucking work together: plucking supplies the angular blocks that abrasion then drags across the bed as tools. The cleanest answers explain this coupling and tie both to the requirement for basal meltwater (warm-based ice).
Strictly a weathering process rather than erosion, frost shattering is nonetheless essential in preparing rock for glacial attack, especially above the ice surface:
This is a crucial conceptual point: the most dramatic elements of glaciated mountain scenery — the knife-edge arêtes, the soaring horns, the near-vertical corrie back walls — are produced largely above the ice surface by frost shattering, not by the ice itself. The glacier deepens the floor and undercuts the base of the slopes; frost action does the rest, steepening and sharpening everything that projects above the ice. The two processes are complementary: glacial over-deepening lowers the valley floor and removes lateral support, which steepens the slopes and accelerates frost shattering and rockfall on them. Recognising this division of labour between glacial erosion (below) and periglacial weathering (above) is exactly the integrated understanding that distinguishes strong answers on mountain landscapes.
Pressurised subglacial and proglacial meltwater erodes by:
Meltwater erosion is easy to overlook but can be highly effective precisely because subglacial water is pressurised and laden with abrasive rock flour. It is increasingly thought to be responsible for many of the smooth, sculpted p-forms on glaciated bedrock and for the deep, narrow subglacial gorges that cut across the regional slope — features a glacier's ice alone could not produce. The best evaluative answers therefore credit meltwater alongside ice as a genuine agent of glacial erosion, rather than treating erosion as the work of ice in isolation.
Glacial erosion is neither uniform nor constant, and the strongest answers can explain where and how fast it acts. Measured rates for fast-flowing temperate glaciers are often of the order of a few millimetres per year, but they range over several orders of magnitude — from negligible under cold-based ice to perhaps 10–20 mm/yr or more under fast, debris-rich, wet temperate ice. The controls follow directly from the process mechanisms:
| Control | Effect on erosion rate |
|---|---|
| Thermal regime | Warm-based (sliding) ice erodes; cold-based ice frozen to the bed barely erodes |
| Ice (sliding) velocity | Faster sliding drives more abrasion and plucking — erosion roughly scales with sliding speed |
| Basal debris supply | More tools at the bed = more abrasion; "clean" ice abrades little |
| Geology | Well-jointed, weak rock is plucked rapidly; massive, unjointed rock resists |
| Meltwater | Flushes rock flour, keeps fresh debris in contact, and erodes directly |
This explains the spatial pattern of erosion within a single glacier: it is concentrated where ice is thick, fast and warm-based — under the trunk of a valley glacier, on steepened "rock steps" under extending flow, and in over-deepened basins — and weak where ice is thin, slow or cold, such as near the snout or on interfluves. It also explains selective linear erosion: in some upland ice fields (classically parts of the NW Scottish Highlands), fast ice in the valleys deeply eroded troughs while the intervening plateau surfaces, protected beneath slow, cold ice, survived almost untouched — producing a landscape of deep troughs separating barely-modified summits.
A corrie is an armchair-shaped hollow with a steep back wall, over-deepened floor and a raised front lip (threshold), often holding a tarn.
graph TD
A["Snow accumulates in a shaded,<br/>NE-facing hollow"] --> B["Nivation enlarges the hollow;<br/>firn → corrie glacier forms"]
B --> C["Rotational sliding deepens the floor<br/>by abrasion (over-deepening)"]
C --> D["Plucking + freeze-thaw above the<br/>bergschrund steepen the back wall"]
D --> E["Less erosion at the front leaves a raised LIP"]
E --> F["Deglaciation: meltwater impounded<br/>behind the lip → TARN"]
Formation in sequence: (1) snow accumulates in a pre-existing, sheltered hollow — in the Northern Hemisphere preferentially north- to north-east-facing, where it is shaded from the sun and loaded by wind-drifted snow off the prevailing south-westerlies; (2) nivation enlarges the hollow; (3) firn and ice build a small corrie glacier; (4) rotational sliding concentrates abrasion at the base, over-deepening the floor; (5) plucking steepens the back wall, exploiting joints opened by dilatation; (6) frost shattering above the bergschrund (the crevasse between glacier and back wall) keeps feeding debris and steepening the wall; (7) erosion weakens at the front, leaving a lip; (8) on deglaciation, water ponds behind the lip as a tarn.
Case Study — Red Tarn, Helvellyn (Lake District): a textbook north-east-facing corrie at ~718 m, its back wall rising over 250 m to the Helvellyn summit (950 m). The tarn is roughly 25 m deep, held behind a rock-and-moraine lip. Its aspect is no accident — Ian Evans's analysis of Lake District corries shows a pronounced north-east bias consistent with shade and wind-drifted snow.
The aspect control deserves unpacking because it is so frequently examined. In the Northern Hemisphere, north- and east-facing hollows receive the least direct insolation, so summer ablation is minimised and snow persists; simultaneously, the prevailing south-westerly winds scour snow from windward slopes and dump it as drift on the sheltered lee (north-east) side, boosting accumulation. The two effects reinforce each other, so glaciers nucleate preferentially in NE-facing hollows — which is why corrie orientation is read as evidence of past wind and radiation regimes. Once a corrie glacier exists, the bergschrund plays a special role: this deep crevasse at the head opens a route for meltwater and air to reach the back wall, intensifying freeze–thaw and frost wedging there, and helping explain why back walls become so steep. Where snow accumulates in a hollow but never quite thickens enough to form flowing ice, nivation alone enlarges the hollow — the embryonic stage that, given a colder climate, becomes a true corrie. Corries are thus a sensitive threshold landform, poised between periglacial nivation hollow and full glacial cirque.
An arête is a narrow, knife-edged ridge formed where two corries erode back-to-back (or side-by-side), their steepening back walls converging until only a sharp crest remains. The processes are the same ones that steepen a single corrie wall — plucking exploiting jointed rock and frost shattering above the ice — but now operating on two opposing walls that migrate toward each other. As the intervening ridge is progressively narrowed and sharpened, frost-wedged blocks tumble away on both sides, maintaining the knife-edge. Continued frost action and rockfall keep arêtes unstable and gradually lower them, which is why they are often serrated with pinnacles (gendarmes).
Where three or more corries gnaw back toward a common summit, the residual peak is a steep-sided pyramidal peak (horn) with radiating arêtes.
The horn is therefore the end-member of corrie erosion: arêtes are the product of two back-to-back corries; a pyramidal peak is the product of three or more. This neat hierarchy (corrie → arête → horn) is a favourite for exam questions asking you to link landforms genetically rather than describe them in isolation.
A glacial trough is a wide, steep-sided, flat-floored valley produced when a glacier occupies and reshapes a pre-existing V-shaped river valley.
graph TD
A["Pre-glacial V-shaped river valley<br/>with interlocking spurs"] --> B["Valley glacier fills the valley"]
B --> C["Abrasion deepens the floor;<br/>plucking + abrasion steepen the walls"]
C --> D["Valley widened, deepened & STRAIGHTENED;<br/>interlocking spurs sliced to TRUNCATED SPURS"]
D --> E["Extending flow over rock steps →<br/>over-deepened basins"]
E --> F["Deglaciation: broad U-profile, flat floor,<br/>misfit stream, ribbon lake, hanging valleys"]
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