Glacial Systems and Landscapes
Glacial Systems and Landscapes
Introduction
Glacial systems are powerful agents of landscape transformation, reshaping valleys, transporting vast quantities of debris, and leaving distinctive landforms that persist long after the ice has retreated. This topic examines the processes operating beneath and within glaciers, the landforms they create, and the evidence they leave for past climate change. Approximately 10% of the Earth’s land surface is currently covered by glaciers and ice sheets, but during the last glacial maximum (approximately 22,000 years ago), ice covered roughly 30% of the planet.
Key Concepts and Definitions
| Term | Definition |
|---|---|
| Glacier | A persistent body of dense ice that moves under its own weight, formed where snow accumulation exceeds ablation over many years |
| Accumulation | All processes that add mass to a glacier (snowfall, refreezing of meltwater, avalanches) |
| Ablation | All processes that remove mass from a glacier (melting, sublimation, calving) |
| Equilibrium line | The line on a glacier where accumulation equals ablation; separates the zone of accumulation from the zone of ablation |
| Firn | Partially compacted granular snow, transitional between snow and glacial ice |
| Basal sliding | Movement of a glacier caused by the glacier sliding over lubricated bedrock, facilitated by meltwater at the base |
| Internal deformation | Movement within the glacier ice caused by the rearrangement of ice crystals under pressure (creep) |
| Extensional flow | Stretching of ice where a glacier accelerates (e.g., over a steepening bed), creating crevasses |
| Compressional flow | Compression of ice where a glacier decelerates (e.g., at the base of a slope), thickening the ice |
| Moraine | Unconsolidated debris (till) transported and deposited by a glacier |
| Till | Unsorted, unstratified glacial sediment deposited directly by ice |
| Erratic | A rock fragment transported by a glacier and deposited in an area of different lithology |
| Drumlin | An elongated hill of till shaped by glacial flow, with a steep stoss end and gentle lee end |
| Periglacial | Cold, non-glacial environments where freeze-thaw processes dominate, in most cases in regions of permafrost |
| Permafrost | Permanently frozen ground where temperatures remain below 0°C for at least two consecutive years |
| Active layer | The surface layer of ground above permafrost that thaws in summer and refreezes in winter |
Glacial Processes
Glacier Formation and Mass Balance
A glacier forms when accumulation exceeds ablation over successive years:
- Snow falls and accumulates in a hollow or on high ground (the accumulation zone)
- Repeated melting and refreezing converts snow to firn (granular, partially compacted ice)
- Over decades, the weight of overlying snow compresses firn into dense glacial ice (density > 0.9 g/cm³)
- When the ice reaches a critical thickness (in most cases > 50 m), it begins to flow under its own weight
The glacier mass balance describes the net change in glacier mass:
- Positive mass balance: accumulation > ablation → glacier advances
- Negative mass balance: ablation > accumulation → glacier retreats
- Equilibrium: accumulation = ablation → glacier is stationary (terminus position stable)
Mechanisms of Glacial Movement
Glaciers move through three primary mechanisms:
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Basal sliding (warm-based glaciers only): A thin film of meltwater at the base of the glacier reduces friction between ice and bedrock, allowing the glacier to slide. This can account for 50–90% of total movement in temperate glaciers. Rates can reach 10–200 m per year.
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Internal deformation (creep): Ice behaves as a viscous fluid under pressure. Ice crystals realign and slide past each other. Movement is fastest at the surface (where friction is lowest) and slowest at the base (where friction with bedrock is highest). This operates in all glaciers, including cold-based glaciers frozen to their bed.
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Bed deformation: In areas with soft, unconsolidated sediment beneath the glacier, the weight of the ice can deform the bed material itself, contributing to forward movement.
Factors affecting the rate of movement:
- Temperature: Warmer ice deforms more readily; meltwater facilitates basal sliding
- Ice thickness: Greater thickness increases pressure and therefore deformation rate
- Gradient: Steeper slopes increase gravitational driving force
- Bedrock lithology: Hard, smooth bedrock allows faster sliding; rough, uneven bedrock creates friction
- Debris content: Debris-rich basal ice is more abrasive but may also increase friction
Glacial Erosion
Glaciers erode through three main processes:
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Plucking (quarrying): Meltwater freezes around joints and fractures in the bedrock. As the glacier moves forward, it literally plucks blocks of rock from the surface. This requires the glacier to be warm-based (at pressure melting point). Plucked rock surfaces are characteristically jagged and angular.
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Abrasion: Rock debris embedded in the base of the glacier is dragged across the bedrock surface, scoring and polishing it. This produces smooth, striated (scratched) surfaces. The effectiveness of abrasion depends on the debris load, ice velocity, and ice thickness (pressure).
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Freeze-thaw weathering: Although not strictly a glacial process, freeze-thaw operates on exposed rock faces above and around glaciers, loosening debris that falls onto the ice surface (supraglacial debris) or is incorporated at the glacier margins.
Glacial Transportation and Deposition
Glaciers transport enormous volumes of debris:
- Supraglacial: On the surface (from rockfall, freeze-thaw)
- Englacial: Within the ice (debris that has fallen into crevasses or been incorporated by folding)
- Subglacial: At the base (material eroded by plucking and abrasion, plus pre-existing sediment)
- Meltwater transport: Glacial meltwater transports fine sediment (glacial flour) downstream, often creating braided river channels
Deposition occurs when the glacier loses kinetic energy — in most cases when it retreats, thins, or encounters a flattening of gradient. Glacial deposits (till) are characteristically:
- Unsorted: Mixed particle sizes from clay to boulders
- Unstratified: No layering or bedding
- Angular to sub-angular: Particles have not been rounded by water transport
Glacial Landforms
Erosional Landforms
| Landform | Description and Formation |
|---|---|
| Corrie (cirque/cwm) | A deep, steep-sided, armchair-shaped hollow on a mountainside, formed by glacial erosion in a pre-existing hollow. Rotational movement of the ice deepens the base, creating an overdeepened basin that often fills with a tarn (mountain lake) after glaciation. The backwall is steepened by freeze-thaw weathering and plucking. Example: Red Tarn, Lake District. |
| Arête | A narrow, knife-edge ridge separating two adjacent corries, formed as both corries erode backwards towards each other. Example: Striding Edge, Lake District; Aiguille du Midi, Alps. |
| Pyramidal peak (horn) | A pointed mountain peak formed where three or more corries erode back towards a central point from different sides. Example: the Matterhorn, Switzerland; Mount Snowdon, Wales. |
| U-shaped valley (glacial trough) | A steep-sided, flat-bottomed valley formed by a valley glacier deepening and widening a pre-existing V-shaped river valley. The glacier erodes the valley floor through abrasion and plucking, and steepens the valley sides through freeze-thaw weathering and mass movement. Example: Lauterbrunnen, Switzerland; Nant Ffrancon, Snowdonia. |
| Hanging valley | A tributary valley that enters the main U-valley at a considerable height above the main valley floor. The smaller tributary glacier was less powerful and eroded less deeply. When ice retreats, the tributary valley is left “hanging,” often with a waterfall. Example: Stickle Ghyll, Lake District. |
| Truncated spur | A ridge of rock that has been cut off abruptly by a valley glacier eroding the valley side, creating steep cliff faces where the spur formerly projected into the valley. |
| Roche moutonnée | An asymmetric rock outcrop shaped by glacial erosion: the stoss (up-ice) side is smooth and gently sloping (abraded), while the lee (down-ice) side is steep, rough, and angular (plucked). Indicates the direction of ice flow. |
Depositional Landforms
| Landform | Description and Formation |
|---|---|
| Lateral moraine | A ridge of till deposited along the sides of a glacier, formed from debris that fell onto the glacier margins from the valley walls. |
| Medial moraine | A ridge of till in the centre of a glacier, formed where two lateral moraines merge at the confluence of two glaciers. |
| Terminal moraine | A ridge of till deposited at the furthest point reached by the glacier snout. Marks the maximum extent of the ice advance. |
| Recessional moraine | Ridges of till deposited during temporary pauses in glacier retreat, running across the valley floor. |
| Push moraine | A moraine formed when a glacier advances and pushes existing sediment into a ridge, often showing evidence of folding and faulting. |
| Drumlin | An elongated, egg-shaped hill of till, with a steep stoss end (facing the direction of ice advance) and a gentle lee end. Formed subglacially as the glacier streamlines the till. Often occur in groups called “drumlin swarms” or “basket of eggs” topography. Example: Ribble Valley, Lancashire; Clew Bay, Ireland. |
| Erratic | A large boulder of a rock type different from the local bedrock, transported by a glacier and deposited in a new location. Used to trace former ice movement paths. Example: the Norber Erratics, Yorkshire Dales. |
| Outwash plain (sandur) | A flat, extensive plain of stratified sediment deposited by meltwater in front of a retreating glacier. Sediment is sorted and stratified (unlike till). Example: Skeiðarársandur, Iceland. |
| Esker | A long, sinuous ridge of sand and gravel deposited by meltwater flowing through a tunnel within or beneath the glacier. |
| Kame | A mound of stratified sediment deposited by meltwater in a depression on the surface or at the margin of a retreating glacier. |
Periglacial Processes and Landforms
Periglacial Environments
Periglacial environments are found in areas of permafrost where freeze-thaw processes dominate. They cover approximately 25% of the Earth’s land surface, including much of Siberia, northern Canada, Alaska, and high mountain areas.
Key Periglacial Processes
- Freeze-thaw weathering (frost shattering): Water enters joints in rock, freezes (expanding by approximately 9%), and exerts pressure. Repeated cycles break rock into angular fragments. Most effective when temperatures fluctuate around 0°C (diurnal freeze-thaw).
- Frost heave: The upward movement of soil and rocks as groundwater freezes and expands, pushing material towards the surface. Forms patterned ground (stone polygons, stone stripes) as repeated frost heave sorts particles by size.
- Solifluction (gelifluction): The slow, downslope flow of water-saturated soil over impermeable permafrost. Occurs in the active layer during summer thaw. Produces lobate (tongue-shaped) landforms on hillsides. Rates are commonly 1–5 cm per year.
- Thermal contraction: Cracking of the ground surface as temperatures drop and frozen ground contracts. Infilling of cracks by water or sand forms ice wedges or sand wedges.
Periglacial Landforms
| Landform | Description |
|---|---|
| Pingos | Dome-shaped hills formed by the growth of a massive ice core within permafrost. Open-system pingos form where water under pressure injects into the permafrost; closed-system pingos form by freeze-down of a talik (unfrozen zone) after lake drainage. Example: Tuktoyaktuk, Canada. |
| Ice wedges | V-shaped bodies of ice extending downwards into permafrost, formed by repeated thermal contraction cracking and infilling with meltwater. Can form polygonal patterns on the ground surface (ice-wedge polygons). |
| Blockfields (felsenmeer) | Extensive sheets of angular, frost-shattered rock debris covering plateaus and upper slopes in periglacial environments. |
| Rock glaciers | Tongue-shaped masses of angular rock debris containing interstitial ice, flowing slowly downslope. |
| Thermokarst | Irregular, hummocky terrain formed by the melting of ground ice, creating depressions, thaw lakes, and collapsed surfaces. |
Climate Reconstruction
Using Glacial Evidence to Reconstruct Past Climates
Glacial landforms and deposits provide crucial evidence for understanding past climate conditions:
- Terminal moraines: Map the maximum extent of former ice sheets, indicating the boundary conditions for glacial climate.
- Erratics: Their lithology can be matched to source outcrops, revealing former ice movement directions. The distribution of the Blakeney Erratic in Norfolk traced ice flow from Scandinavia.
- Striations: Parallel scratches on bedrock surfaces indicate the direction of ice flow at the base of a glacier.
- Drumlins: Their orientation indicates the direction of ice flow; their shape and distribution provide information about basal conditions and ice velocity.
- Cirque floor altitude: The altitude of corrie floors provides an approximation of the former equilibrium line altitude (ELA). In the UK, cirque floors in Snowdonia are commonly at approximately 450–500 m, suggesting a lowering of the ELA by approximately 1,000 m during the last glacial maximum.
Other Proxy Evidence
- Ice cores: Locked air bubbles provide direct measurements of past atmospheric CO₂ and CH₄ concentrations. Oxygen isotope ratios (δ¹⁸O) in the ice indicate past temperatures. The Vostok and EPICA ice cores from Antarctica provide records spanning over 800,000 years.
- Ocean sediment cores: Foraminifera shells record oxygen isotope ratios, which reflect global ice volume and ocean temperature.
- Pollen analysis (palynology): Preserved pollen in lake sediments indicates past vegetation and therefore climate conditions.
- Dendrochronology: Tree ring width and density provide year-by-year climate records.
Case Studies
Case Study 1: The Lake District, England
The Lake District in north-west England provides an outstanding example of a glaciated upland landscape. During the last glacial maximum (Devensian, approximately 26,000–13,000 years ago), ice caps and valley glaciers covered the region, reshaping the pre-existing river landscape into the distinctive landforms visible today.
Erosional features: The valleys of the Lake District are classic U-shaped glacial troughs — for example, Borrowdale and Wasdale. These were originally V-shaped river valleys that were deepened and widened by glaciers, producing steep, rocky sides and flat valley floors. Many contain ribbon lakes: Wastwater (the deepest lake in England at 79 m), Windermere (the largest natural lake in England at 14.8 km²), and Coniston Water.
Corries are found on most of the higher fells. Red Tarn, beneath Helvellyn, occupies a classic corrie with a steep backwall and a moraine-dammed tarn. Striding Edge, adjacent to Red Tarn, is a spectacular arête separating two corries.
Hanging valleys are common where tributary glaciers were less erosive than the main valley glacier. For example, Sourmilk Gill flows over a 30 m waterfall where its hanging valley meets the main Borrowdale valley.
Depositional features: The lower valleys and lake margins show evidence of glacial deposition. Moraines are found in several valleys, including the Langdale Pikes area. The drumlin field around Grasmere and Rydal Water indicates streamlined subglacial deposition.
The glaciated landscape has significant economic and cultural value. The Lake District National Park receives approximately 15 million visitor days per year, contributing substantially to the local economy. The distinctive landscape, shaped by glaciation, was designated a UNESCO World Heritage Site in 2017.
Case Study 2: The Swiss Alps — Retreating Glaciers and Climate Change
The Swiss Alps contain approximately 1,400 glaciers, which have been retreating dramatically in recent decades due to rising global temperatures. The Aletsch Glacier, the largest glacier in the Alps (approximately 23 km long, covering approximately 80 km²), has retreated by approximately 3.2 km since 1870, with the rate of retreat accelerating.
Recent trends: Between 1973 and 2010, Swiss glaciers lost approximately 30% of their total volume. The extremely hot summers of 2003 and 2022 caused record losses. The Swiss Glacier Monitoring Network (GLAMOS) reported that Swiss glaciers lost over 6% of their remaining volume in 2022 alone — an unprecedented rate.
Impacts of retreat:
- Water resources: Alpine glaciers act as natural water stores, releasing meltwater during summer months. Their retreat threatens water supply for agriculture, hydroelectric power, and domestic use downstream. The Rhône Glacier feeds the Rhône River, which supplies water to much of southern France.
- Natural hazards: Retreating glaciers leave unstable, steep-sided valleys prone to landslides and rockfall, as the ice that previously supported valley walls disappears. Glacial lake outburst floods (GLOFs) pose risks when meltwater accumulates behind moraine dams that can fail catastrophically.
- Tourism and economy: Glaciers are major tourist attractions. The Jungfraujoch research station and the Glacier Express railway rely on glacier scenery. Ski resorts are investing heavily in artificial snow-making equipment and glacier blankets (geotextile covers that reflect sunlight and reduce melting) to preserve remaining ice.
- Ecological change: Newly exposed terrain is colonised by pioneer species, shifting ecosystems upslope as conditions warm.
Common Pitfalls
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Confusing erosional and depositional landforms: Students sometimes attribute drumlins to erosion or U-shaped valleys to deposition. Always remember: erosion produces smooth, streamlined, or hollowed features; deposition produces accumulations of unsorted till or stratified outwash.
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Describing glacial movement without explaining why it happens: Merely stating “the glacier moves” gains no credit. You must explain how — basal sliding (meltwater lubrication), internal deformation (ice crystal creep under pressure), or bed deformation — and the conditions required (warm-based vs. cold-based, thickness, gradient).
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Confusing periglacial and glacial environments: Periglacial zones are cold but not covered by ice. They are characterised by permafrost, freeze-thaw weathering, and solifluction — not by glacial erosion or deposition. Do not describe corries or U-valleys in periglacial contexts.
Worked Examples
Example 1: 9-Mark Question
“To what extent are glacial erosional landforms the product of pre-existing landscape features rather than glacial processes?”
Answer:
The character of glacial erosional landforms is strongly influenced by the pre-existing landscape, but glacial processes are the primary agents of modification. The pre-existing topography determines the location, shape, and scale of glaciated features, while the glacier itself provides the erosive power.
Pre-existing landscape features play a crucial role. Corries commonly develop in pre-existing hollows or depressions on north- or east-facing slopes where snow accumulates in the shade and is sheltered from prevailing winds. Without these initial hollows, glaciers may not nucleate. Similarly, U-shaped valleys develop from pre-existing V-shaped river valleys — the glacier exploits and modifies the existing valley rather than creating an entirely new one. The size and depth of the U-valley are influenced by the size of the original valley.
The underlying geology also exerts strong control. Weaknesses in the rock (joints, faults, bedding planes) are exploited by the glacier through plucking and enhanced abrasion. The Lithology affects resistance — hard, resistant rocks such as granite produce more rugged, angular landscapes than softer rocks.
However, glacial processes are the transformative agent. A V-shaped valley does not become U-shaped without the deepening and steepening action of glacial abrasion and plucking. The overdeepening of corrie floors, the creation of hanging valleys, and the formation of truncated spurs are all direct results of glacial erosion that could not occur through fluvial processes alone.
In conclusion, while pre-existing landscape features provide the initial conditions and geological framework, it is glacial processes that fundamentally reshape the landscape. The final landform is the product of both — the inherited landscape provides the template, and the glacier provides the transformation.
Example 2: 6-Mark Question
“Describe the formation of a drumlin.”
Answer:
A drumlin is an elongated, egg-shaped hill of glacial till formed beneath a moving glacier. Drumlins in most cases occur in groups called swarms, aligned parallel to the direction of ice flow.
They form subglacially when the glacier encounters a pre-existing obstacle or irregularity in the bed topography. Till is deposited around this nucleus and then streamlined by the flowing ice. The ice moulds the till into a smooth, elongated form: the stoss (up-ice) end is steep, where the glacier compresses and deposits material; the lee (down-ice) end is gentle and tapers away, where the ice extends and flows over the drumlin.
The long axis of the drumlin is parallel to the direction of ice flow, making drumlins valuable indicators of former ice movement. The sediment is characteristically unsorted till, though some drumlins contain a core of bedrock or stratified sediment. Examples include the drumlin fields around Ribblehead in the Yorkshire Dales and the extensive drumlin swarms around Clew Bay in County Mayo, Ireland.
Summary
- Glaciers form where accumulation exceeds ablation; they move via basal sliding, internal deformation, and bed deformation.
- Glacial erosion operates through plucking, abrasion, and freeze-thaw weathering, creating landforms such as corries, arêtes, U-valleys, and roches moutonnées.
- Depositional landforms include moraines, drumlins, erratics, eskers, and outwash plains, all composed of unsorted till.
- Periglacial environments feature permafrost, freeze-thaw weathering, solifluction, and distinctive landforms such as pingos and ice wedges.
- Glacial evidence (moraines, erratics, striations, cirque altitudes) is used to reconstruct past climate conditions and ice extent.
- Modern glacier retreat, as seen in the Alps, provides direct evidence of contemporary climate change and has significant environmental and economic consequences.
Sources: AQA Geography (7037) specification; Benn and Evans, Glaciers and Glaciation (2010); GLAMOS Swiss Glacier Monitoring; Evans, Glacial Geomorphology (2013); Lake District National Park data; IPCC AR6.