Glacial Processes: Freeze-Thaw, Plucking, Corries, Arêtes & Drumlins
During the Pleistocene Ice Age (ending around 10,000 years ago), thick glaciers of moving ice carved out the dramatic upland landscapes of Northern and Western Britain, including the Lake District, Snowdonia, and the Scottish Highlands. Glaciers erode bedrock through two main processes (plucking and abrasion), assisted by freeze-thaw weathering above the ice, to sculpt corries, arêtes, pyramidal peaks, and steep-sided U-shaped glacial troughs. When glaciers melt and retreat, they deposit unsorted glacial till (boulder clay) as moraines, drumlins, and erratics, while meltwater streams deposit sorted outwash plains. Examiners regularly test step-by-step landform formation sequences, OS map contour recognition of glacial features, and land-use conflicts in glaciated upland areas like the Lake District.
- Glacial Weathering & Erosion Processes
- 1. Freeze-thaw weathering: Water in rock crevices freezes at $0^\circ\text{C}$ and expands by ~9%, wedging rock apart to supply angular scree to the glacier. 2. Plucking: Meltwater under the glacier freezes onto cracked bedrock; as the glacier advances, it rips out chunks of rock, leaving a jagged backwall. 3. Abrasion: Rocks embedded in the base of the glacier grind against the valley floor like sandpaper, deepening the hollow/valley and leaving scratch marks called striations. 4. Rotational slip: Circular movement of ice inside a hollow.
- Erosional Landforms: Corries, Arêtes & Pyramidal Peaks
- 1. Corrie (Cwm/Cirque): Armchair-shaped hollow on a mountainside with a steep backwall, deepened basin (via rotational slip, plucking, and abrasion), and a raised rock lip at the front, often holding a tarn lake (e.g., Red Tarn). 2. Arête: A narrow, knife-edged ridge formed when two adjacent corries erode backwards (back-to-back) via plucking and freeze-thaw (e.g., Striding Edge, Helvellyn). 3. Pyramidal Peak: A sharp, pointed mountain peak formed when three or more corries erode backwards into the same mountain (e.g., Mount Snowdon, Matterhorn).
- Glacial Troughs (U-Shaped Valleys), Hanging Valleys & Ribbon Lakes
- Glaciers flow down pre-existing V-shaped river valleys; using immense weight, plucking, and abrasion, the glacier bulldozes interlocking spurs (creating truncated spurs) and widens and deepens the valley into a steep-sided, flat-bottomed U-shaped glacial trough. Smaller tributary glaciers erode less deeply than the main glacier, leaving Hanging Valleys high on the valley sides (marked by waterfalls). Where the glacier erodes softer bedrock more deeply along the trough floor, a long, narrow Ribbon Lake (e.g., Lake Windermere) forms after the ice melts.
Glacial Processes: Freeze-Thaw, Plucking, Corries, Arêtes & Drumlins: Flood Hydrographs, Plate Margins & Global Systems
Test how land use changes storm hydrograph lag times, compare plate boundaries, and inspect global atmospheric cells and the Demographic Transition Model.
Key Concepts & Exam Definitions (6)
- Glacial Weathering & Erosion Processes
- 1. Freeze-thaw weathering: Water in rock crevices freezes at $0^\circ\text{C}$ and expands by ~9%, wedging rock apart to supply angular scree to the glacier. 2. Plucking: Meltwater under the glacier freezes onto cracked bedrock; as the glacier advances, it rips out chunks of rock, leaving a jagged backwall. 3. Abrasion: Rocks embedded in the base of the glacier grind against the valley floor like sandpaper, deepening the hollow/valley and leaving scratch marks called striations. 4. Rotational slip: Circular movement of ice inside a hollow.
- Erosional Landforms: Corries, Arêtes & Pyramidal Peaks
- 1. Corrie (Cwm/Cirque): Armchair-shaped hollow on a mountainside with a steep backwall, deepened basin (via rotational slip, plucking, and abrasion), and a raised rock lip at the front, often holding a tarn lake (e.g., Red Tarn). 2. Arête: A narrow, knife-edged ridge formed when two adjacent corries erode backwards (back-to-back) via plucking and freeze-thaw (e.g., Striding Edge, Helvellyn). 3. Pyramidal Peak: A sharp, pointed mountain peak formed when three or more corries erode backwards into the same mountain (e.g., Mount Snowdon, Matterhorn).
- Glacial Troughs (U-Shaped Valleys), Hanging Valleys & Ribbon Lakes
- Glaciers flow down pre-existing V-shaped river valleys; using immense weight, plucking, and abrasion, the glacier bulldozes interlocking spurs (creating truncated spurs) and widens and deepens the valley into a steep-sided, flat-bottomed U-shaped glacial trough. Smaller tributary glaciers erode less deeply than the main glacier, leaving Hanging Valleys high on the valley sides (marked by waterfalls). Where the glacier erodes softer bedrock more deeply along the trough floor, a long, narrow Ribbon Lake (e.g., Lake Windermere) forms after the ice melts.
- Glacial Transportation & Moraines
- Glaciers transport eroded rock debris (supraglacial on top, englacial inside, subglacial underneath) without sorting it. When the ice melts, it deposits unsorted glacial till as Moraines: 1. Lateral moraine (ridges of debris along the edges of the glacier from valley-side freeze-thaw). 2. Medial moraine (a ridge down the middle formed when two glaciers join and their lateral moraines merge). 3. Ground moraine (spread across the valley floor). 4. Terminal moraine (a high crescent ridge marking the furthest point reached by the glacier's snout). 5. Recessional moraine (formed during temporary pauses as the glacier retreats).
- Drumlins & Erratics (Depositional Landforms)
- 1. Drumlins: Smooth, elongated, egg-shaped hills of unsorted glacial till (up to 1 km long and 50 m high, often found in swarms called 'basket of eggs' topography) moulded underneath a moving glacier. They have a steep, blunt upstream ('stoss') end and a gently sloping, tapered downstream ('lee') tail, which shows the direction of ice movement. 2. Erratics: Large boulders transported miles by a glacier and deposited on top of a completely different rock type when the ice melted (e.g., Norber Erratics in Yorkshire).
- Economic Opportunities & Conflicts in Glaciated Uplands
- Case study: The Lake District (or Snowdonia). Economic uses: 1. Tourism (over 18 million visitors/year to the Lake District for hiking, watersports on ribbon lakes, sightseeing). 2. Hill sheep farming on steep upland slopes and cattle/crops on fertile valley floors. 3. Forestry (commercial conifer plantations). 4. Quarrying (slate and granite for construction) and HEP/reservoirs (Thirlmere supplying water to Manchester). Conflicts arise between tourists (traffic congestion on narrow roads, footpath erosion, second homes pricing out locals, noise from speedboats on Windermere) and farmers (gates left open, dogs worrying sheep), managed by Park Authorities via speed limits (10 mph on Windermere), Park-and-Ride buses, and stone footpath pitching.
Quick-Fire Flashcards (6)
Click any card to check the answer, then mark it as Known to track your progress.
Compare how plucking and abrasion erode rock beneath a glacier.
What is the relationship between a corrie, an arête, and a pyramidal peak?
Explain how truncated spurs and hanging valleys are formed in a glacial trough.
Distinguish between Lateral, Medial, and Terminal moraine.
Describe the shape of a drumlin and explain how it shows the direction in which the glacier moved.
Give two conflicts between tourists and local residents/farmers in the Lake District, and one management solution for each.
Exam-Style Questions & Mark Schemes (4)
Write or think through your answer first, then click Check Mark Scheme to see the exact points examiners award marks for.
- Q1
Explain the formation of a corrie and a tarn lake, referring to specific weathering and erosion processes in your answer. [4 marks]
Click “Check Mark Scheme” to compare your answer, 1. Snow accumulates in a sheltered, north-or-east-facing hollow high on a mountainside and compacts over many winters into dense glacial ice [1 mark]. 2. Freeze-thaw weathering shatters rock on the upper slopes above the ice, while meltwater freezes onto the backwall and rips rock out as the ice moves under gravity in a circular motion (rotational slip and plucking), making the backwall very steep [1 mark]. 3. Rock debris frozen into the base of the rotating ice scrapes and grinds the floor of the hollow (abrasion), deepening the armchair basin while eroding less at the front edge where the ice is thinner (leaving a raised rock lip) [1 mark]. 4. When the climate warms and the ice melts, meltwater and rainwater are trapped in the deep hollow behind the rock lip (and terminal moraine), forming a circular corrie lake called a tarn [1 mark].
- Q2
Explain how a V-shaped river valley is transformed into a U-shaped glacial trough with a ribbon lake. [4 marks]
Click “Check Mark Scheme” to compare your answer, 1. During an ice age, a thick, heavy glacier fills a pre-existing V-shaped river valley and moves slowly downhill under gravity [1 mark]. 2. Through plucking and abrasion, the glacier bulldozes away the interlocking spurs on the valley sides (creating steep truncated spurs) and widens and deepens the valley into a steep-sided, flat-bottomed U-shaped glacial trough [1 mark]. 3. As the glacier moves along the valley floor, it encounters bands of softer or more jointed bedrock and erodes this softer rock more deeply by abrasion and plucking, carving out an over-deepened rock basin [1 mark]. 4. When the glacier melts at the end of the ice age, water fills this long, narrow over-deepened basin on the flat trough floor (or is dammed behind a terminal moraine) to form a ribbon lake [1 mark].
- Q3
Compare the characteristics and formation of a terminal moraine and a drumlin. [4 marks]
Click “Check Mark Scheme” to compare your answer, 1. Shared material: Both terminal moraines and drumlins are depositional landforms composed of unsorted, angular glacial till (boulder clay) containing a mixture of clay, pebbles, and boulders [1 mark]. 2. Terminal moraine formation & shape: A terminal moraine is deposited at the stationary snout of a melting glacier (like a conveyor belt dumping debris), forming a high, crescent-shaped ridge running across the valley that marks the furthest extent of the ice [1 mark]. 3. Drumlin formation & shape: A drumlin is formed underneath a moving glacier when the ice becomes overloaded with sediment and moulds the till into a streamlined, elongated egg-shaped hill aligned parallel to the direction of ice flow [1 mark]. 4. Asymmetry: Unlike a moraine ridge, a drumlin has a distinct steep blunt upstream (stoss) end and a gently tapering downstream (lee) end that indicates the direction of glacier movement [1 mark].
- Q4
Using a named glaciated upland area in the UK, evaluate the effectiveness of strategies used to manage conflicts between different land uses (such as tourism, farming, and conservation). [6 marks]
Click “Check Mark Scheme” to compare your answer, 1. Named UK case study: The Lake District National Park in Cumbria (receiving over 18 million visitors per year, with 89% arriving by car) [1 mark]. 2. Conflict 1 (Traffic & Housing): Narrow winding roads around Keswick and Ambleside suffer severe summer gridlock and verge parking (blocking farm machinery), while over 20% of properties are holiday lets/second homes, inflating house prices beyond local workers' incomes [1 mark]. 3. Management of Traffic: The 'GoLakes' Travel programme introduced integrated Park-and-Ride bus services, boat shuttle tickets, and cycle paths; this successfully reduced car journeys in honey-pot hubs, though the vast majority of day-trippers still drive [1 mark]. 4. Conflict 2 (Footpath Erosion & Farming): Millions of hikers cause severe gullying and soil erosion on popular peaks like Helvellyn and Catbells, and walkers leave farm gates open or let dogs chase pregnant ewes [1 mark]. 5. Management of Erosion: The 'Fix the Fells' partnership uses helicopters to fly in local stone flags for 'stone pitching', digging drainage water-bars and reseeding bare peat; this is highly effective at halting erosion and blends into the surroundings, though it is expensive (~£500 per metre) [1 mark]. 6. Conflict 3 & Management (Lake Windermere Zoning): Fast powerboats and water-skiers caused noise pollution, wake bank erosion, and safety conflicts with swimmers, anglers, and wildlife on Windermere; enforcing a strict 10 mph speed limit across the lake successfully restored tranquillity and environmental conservation, despite reducing some powerboat business revenue [1 mark].
Common Exam Questions (4)
What is the difference between freeze-thaw weathering, plucking, and abrasion?
1. Freeze-thaw weathering (frost shattering) is a weathering process (breaking rock in situ): water enters cracks in rock, freezes at night, and expands by ~9%, exerting pressure that widens the crack until sharp rock fragments (scree) break off. 2. Plucking is an erosion process: meltwater beneath or behind the glacier freezes onto bedrock; as the glacier moves forward downhill, it rips (plucks) jagged blocks of rock out of the backwall. 3. Abrasion is an erosion process: angular rocks frozen into the base and sides of the heavy glacier scrape and grind against the bedrock beneath like giant sandpaper, leaving smooth rock with scratch marks (striations).
How do you write a full-mark 4-mark explanation for the formation of a Corrie (Cwm / Cirque)?
Follow this 4-step sequence: 1. Snow accumulates in a north-or-east-facing hollow high on a mountainside, where it is sheltered from the sun and compresses into glacial ice (firn/névé). 2. The ice rotates in the hollow under gravity (rotational slip); freeze-thaw weathering and plucking steepen the backwall, while abrasion deepens the hollow into an armchair-shaped basin. 3. At the front edge of the hollow, the ice is thinner and moves over a rock lip, eroding less deeply. 4. When the glacier melts after the Ice Age, water is trapped behind the rock lip (and moraine dam) to form a circular corrie lake (tarn, e.g., Red Tarn on Helvellyn).
What is the difference between glacial till (boulder clay) and fluvioglacial outwash deposits?
Glacial till (deposited directly by melting ice to form moraines and drumlins) is completely unsorted and angular: huge boulders are mixed randomly with fine clay because ice carries all sizes together and drops them simultaneously when it melts. Fluvioglacial outwash (deposited by meltwater streams in front of the glacier) is sorted and rounded: fast meltwater drops heavy pebbles first near the snout and carries fine sand/silt further downstream.
How do you identify a Corrie, Arête, U-shaped valley, and Drumlin on an Ordnance Survey (OS) contour map?
1. Corrie: Tightly packed, horseshoe-shaped (C-shaped) contour lines around three sides of a small blue circular lake (tarn), often with black cliff/outcrop symbols on the backwall. 2. Arête: Very tightly packed parallel contour lines forming a narrow ridge between two back-to-back corries (e.g., Striding Edge). 3. U-shaped glacial trough: A wide, flat valley floor (few or no contours in the middle, often containing a long ribbon lake or a tiny 'misfit stream') bordered by very steep parallel contour lines on both valley sides. 4. Drumlin: Oval, egg-shaped concentric contour lines (~30-50 m high) with a steeper blunt stoss end and gentler tapered lee end.
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