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Spec mapping (AQA 7037): Paper 1 (Physical), §3.1.4 Glacial systems and landscapes — the processes of glacial erosion (abrasion and plucking) and the erosional landforms they produce at micro (striations), meso (roches moutonnées) and macro (corries, arêtes, pyramidal peaks, troughs, hanging valleys, truncated spurs) scales, developed as formation sequences. This depth lesson assumes the basic landforms are known and concentrates on the mechanics of erosion and the sequence by which each landform develops, framed by the thermal-regime and flow theory of the previous lesson (erosion requires a warm base). It links to §3.1.1 systems thinking and to the deposition of lesson 8 (eroded debris becomes till and moraine). Assessment objectives: AO1 (process mechanics and landform sequences), AO2 (applying processes to a named glaciated upland), AO3 (manipulating long-profile, corrie-orientation and striation data).
The intellectual move at depth is to insist that every glacial erosional landform is the product of just two processes — abrasion and plucking — applied to a particular structure by ice whose flow is set by its thermal regime. Master the two processes and the geometry of where each dominates, and every landform from a striation to a pyramidal peak becomes explicable as the same machinery operating at a different scale and site.
Glacial erosion is overwhelmingly the work of abrasion and plucking, both of which require basal meltwater and therefore a warm-based glacier (lesson 6). This precondition is the reason cold-based ice preserves rather than carves.
Rock debris held in the basal ice is dragged across bedrock, grinding it like sandpaper — scratching, scouring and polishing — and producing striations that record ice-flow direction and the very fine rock flour that turns meltwater milky.
The rate of abrasion depends on a clear set of controls (the Boulton abrasion model):
Basal meltwater penetrates joints and fractures, refreezes (regelation) to bond blocks to the ice, and the moving glacier pulls the blocks away, leaving a rough, angular surface. Plucking is most effective where:
Plucking supplies the tools that abrasion then uses — the two processes are coupled: plucking loosens and entrains angular blocks; abrasion grinds them (and the bed) down. This coupling is why glacial erosion is so effective on jointed crystalline uplands (the granites and slates of Snowdonia, the Lake District, the Cairngorms).
The relative roles can be judged from the debris itself. Abrasion products are fine — silt-grade rock flour and striated, faceted pebbles polished on their dragged faces. Plucking products are coarse and angular — joint-bounded blocks with sharp, fresh faces. So the size and shape distribution of basal and englacial debris encodes which process dominated, and the till deposited downstream (lesson 8) inherits that signature: a till rich in faceted, striated clasts in a fine matrix records vigorous abrasion, while one full of large angular blocks records plucking of well-jointed bedrock. This forensic link between process at the bed and deposit at the snout is exactly the kind of joined-up reasoning that connects this lesson to the next and lifts an answer into the top band.
The balance between abrasion and plucking, and hence the style of erosion, depends on the bedrock. Massive, sparsely-jointed rock (some granites) resists plucking (few blocks to pull out) and is eroded mainly by abrasion, producing smoothed, streamlined, whaleback forms. Densely-jointed rock (well-bedded sandstones, cleaved slates, jointed granite) is readily plucked, producing rough, stepped, quarried surfaces. So two adjacent rock types under the same glacier can develop quite different micro-relief — smoothed where massive, plucked where jointed — and reading that contrast back to the jointing (rather than to differences in the ice) is a sharp AO2 skill. This also explains why glacial erosion so strongly exploits geological structure: faults, joint swarms and weak beds are preferentially excavated, so the resulting landscape (over-deepened basins along fault zones, plucked lee-faces on jointed outcrops) is as much a map of the geology as of the ice — the glacial expression of the structural control that also governs the coast.
Why micro-features matter. These centimetre-scale marks are the most precise recorders of ice-flow direction available, and because a bed can carry cross-cutting striation sets, they reveal changing flow directions through a glaciation (e.g. as an ice cap thickened and ice was no longer steered by topography, or as flow switched between an early valley-glacier and a later ice-sheet phase). A field geomorphologist maps striation azimuths across an upland to reconstruct the evolving ice-flow geometry — a direct AO3 use of micro-landform orientation data. The fine rock flour produced alongside striations is itself diagnostic: its presence in lake-floor and outwash sediments fingerprints active glacial abrasion upstream.
A roche moutonnée is an asymmetric bedrock knoll shaped by glacial erosion.
Formation:
Example: Roches moutonnées are common in the Lake District and the Scottish Highlands
A crag and tail is a larger-scale feature formed when a glacier encounters a very resistant rock mass.
Formation:
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. (Note the tail lies downstream of the crag — the opposite arrangement to a roche moutonnée's plucked lee — because here the resistant crag shelters softer rock behind it, rather than the ice plucking the lee of the resistant bump itself; distinguishing the two is a common exam discriminator.)
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).
Formation sequence:
flowchart TD
SNOW[Snow lodges in NE-facing hollow<br/>shaded, leeward, snow persists] --> NIV[Nivation enlarges hollow<br/>freeze-thaw + meltwater removal]
NIV --> ICE[Firn then ice forms;<br/>thickens past ~30 m]
ICE --> ROT[Rotational flow:<br/>max erosion at backwall + floor]
ROT --> PLUCK[Plucking steepens backwall<br/>via bergschrund freeze-thaw]
ROT --> ABR[Abrasion over-deepens floor]
PLUCK --> BASIN[Armchair basin]
ABR --> BASIN
BASIN --> LIP[Thin ice at front = rock lip,<br/>least erosion]
LIP --> TARN[Deglaciation: tarn dammed<br/>behind rock lip]
Characteristics:
Example: Red Tarn and Helvellyn, Lake District — a classic corrie with a tarn at approximately 718 m altitude. In Snowdonia, Cwm Idwal is the archetypal Welsh corrie (cwm), holding Llyn Idwal behind its rock lip beneath the plucked backwall of the Glyderau.
Corrie orientation as evidence. The strong north-east preference of Northern-Hemisphere corries is itself a dataset. North- and east-facing hollows are shaded (less insolation, so snow survives the summer) and lie leeward of the prevailing south-westerlies (so wind-drifted snow accumulates there). A rose diagram of corrie aspects in an upland therefore reconstructs both the palaeo-radiation balance and the palaeo-wind direction at the time of glaciation — a classic AO3 exercise in which you read former climate from landform geometry. Measuring corrie altitude across a range similarly maps the former regional ELA. The corrie is thus not just a landform but a palaeoclimate proxy.
The nivation-to-corrie threshold and feedback. A corrie does not begin as a glacier; it begins as a snowpatch enlarging a hollow by nivation (lesson 9). The transition to true corrie glaciation is a threshold: only once snow accumulation lets firn compress to ice thick enough (~30 m) to flow does rotational sliding, plucking and abrasion begin. Thereafter a powerful positive feedback operates — the deeper the hollow is eroded, the more snow it traps and shelters, the larger the ice body, the more vigorous the erosion — so a small initial advantage of aspect or shelter is amplified into a deep corrie. This is why corries are self-focusing and why they cluster on favoured aspects: the landform helps create the very conditions that enlarge it. Recognising the nivation→glaciation threshold and the snow-trapping feedback links the periglacial and glacial parts of the course and explains the selectivity of where corries form.
An arête is a narrow, knife-edged ridge formed between two corries that have eroded back-to-back on opposite sides of a mountain.
Formation:
Example: Striding Edge on Helvellyn, Lake District — one of England's most famous arêtes, connecting Helvellyn to Nethermost Pike
A pyramidal peak forms when three or more corries erode back into a mountain from different sides.
Formation:
Example: The Matterhorn, Swiss/Italian Alps — a classic pyramidal peak with four arêtes radiating from the summit; Snowdon (Yr Wyddfa) is a UK example, its summit ringed by the corries (Cwm Glaslyn, Cwm Dyli, Cwm Clogwyn) whose backwall retreat sharpened the arêtes of Crib Goch and Y Lliwedd.
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