Glaciers and Glacial Erosion: Terminology for UPSC Geography Optional

Glaciers are bodies of ice that flow under their own weight, and glacial erosion is the carving of bedrock by that moving ice. This family of terms covers how glaciers form, gain and lose mass and move, and the landforms they cut: cirques, arêtes, horns, troughs and fjords. Himalayan and Karakoram glaciers make it a topic of both Paper I and Paper II.

Each entry gives a definition first, then mechanism, examples and a sketch line. The first cluster explains glacier behaviour; the next three explain erosion and its landforms, from small rock forms up to whole valleys. UPSC asked about truncated spurs in 2024 and about Indian glaciers under climate change in Paper II in 2024.

Quick Revision Table

Term	Meaning in one line	Example
Glacier	Mass of ice formed from compacted snow that flows under its own weight	Siachen Glacier, Karakoram
Firn (névé)	Granular old snow that has survived a melt season, midway to ice	Accumulation basins of the Gangotri Glacier
Snowline & equilibrium-line altitude	Lower limit of permanent snow; line where annual gain equals loss	Snowline at sea level in Antarctica
Glacier types	Valley, cirque, piedmont glaciers; ice caps, ice sheets, ice shelves	Malaspina Glacier, Alaska (piedmont)
Thermal regime	Warm-based ice slides and erodes; cold-based ice is frozen to its bed	Cold-based ice of interior Antarctica
Accumulation & ablation zones	Zones of net gain and net loss; their balance is the mass balance	Chhota Shigri Glacier, Himachal Pradesh
Glacier movement	Flow by internal deformation, basal sliding and bed deformation	Jakobshavn Isbræ, Greenland
Glacier surge	Short phase of flow ten to a hundred times faster than normal	Kutiah Glacier, Karakoram, 1953
Crevasse & bergschrund	Fractures in brittle surface ice; the gap at a cirque head	Khumbu Icefall, Everest
Debris-covered glaciers & the Karakoram anomaly	Rock-mantled tongues; Karakoram glaciers stable while others shrink	Baltoro and Siachen glaciers
Glacial plucking & abrasion	Ice tearing out jointed blocks; debris-laden ice scouring rock	Lee faces of roches moutonnées
Glacial protection vs erosion debate	Whether ice shields its bed or excavates it	Overdeepened Alpine lake basins
Striations & glacial grooves	Scratches and channels cut by debris in basal ice	Kelleys Island grooves, Ohio
Roche moutonnée	Rock knob smoothed on the up-ice side and plucked on the lee	Lembert Dome, Yosemite
Crag and tail	Resistant crag with a tapering tail of rock or till behind it	Castle Rock and Royal Mile, Edinburgh
Cirque & tarn	Armchair-shaped hollow at a valley head; the lake in it	Red Tarn, Helvellyn; Roopkund, Uttarakhand
Arête	Knife-edged ridge between two cirques	Striding Edge, Lake District
Col	Saddle where two cirque headwalls meet	Col du Géant, Mont Blanc massif
Horn	Pyramidal peak left by three or more cirques	Matterhorn; Shivling, Gangotri
Biscuit-board & fretted upland	Plateau notched by cirques; upland wholly consumed by them	Cairngorm plateau, Scotland
Nunatak	Rock peak projecting above an ice sheet	Nunataks of the Greenland ice sheet
U-shaped trough	Steep-walled, flat-floored valley deepened by a glacier	Yosemite Valley; Miyar valley, Lahaul
Truncated spur	Spur end planed off by a valley glacier into a steep facet	Lauterbrunnen valley, Switzerland
Hanging valley	Tributary valley left perched above a deepened main trough	Staubbach Falls, Lauterbrunnen
Ribbon lake	Long, narrow lake in an overdeepened trough	Windermere, Lake District
Paternoster lakes & glacial stairway	Chain of rock-basin lakes on a stepped valley floor	Grinnell Valley, Glacier National Park
Fjord	Glacial trough cut below sea level and drowned by the sea	Sognefjord, Norway

What a Glacier Is: Ice, Mass Balance and Movement

Glacier

A glacier is a large, long-lasting mass of ice formed on land by the accumulation and compaction of snow, which deforms and flows downslope or outwards under its own weight. It moves because the ice behaves as a very viscous fluid under stress, not merely because it slides.

• Formation: Snow survives summer melt above the snowline, compacts into firn and then into glacier ice.
• Key features: Glaciers and ice sheets cover about a tenth of Earth’s land area and hold roughly two-thirds of its fresh water; the Antarctic ice sheet averages about 2 km thick.
• Examples: Siachen Glacier, about 76 km long, the longest glacier in the Karakoram; Gangotri Glacier, about 30 km, source of the Bhagirathi.
• Significance: Himalayan glaciers feed the Indus, Ganga and Brahmaputra in the dry season.
• Don’t confuse with: A snowfield does not flow; a glacier does.

Firn (Névé)

Firn is granular, partly compacted snow that has survived at least one melt season and is in transition to glacier ice; névé is the French term, used also for the accumulation basin in which firn forms. Firn has a density of roughly 0.4–0.8 g/cm³, against about 0.05–0.3 for fresh snow.

• Formation: Melting, refreezing, sublimation and the weight of later snow round the crystals and squeeze out air.
• Key features: At a density of about 0.83 g/cm³ the pores close off, trapping air bubbles, and firn becomes glacier ice (up to about 0.917 g/cm³).
• Rate: The change takes a few years on warm, wet mountain glaciers but centuries to millennia in cold, dry interior Antarctica.
• Significance: Trapped air bubbles make ice cores archives of past atmospheres.

Snowline and Equilibrium-Line Altitude

The snowline is the lower limit of permanent snow cover, above which more snow falls than melts in a year; the equilibrium-line altitude (ELA) is the height on a particular glacier where annual accumulation exactly equals annual ablation, separating its accumulation and ablation zones.

• Key features: The snowline lies at sea level in polar regions and rises to more than 6,000 m in the dry subtropical Andes and Tibet, higher than at the equator because aridity outweighs warmth.
• Controls: Temperature, snowfall, aspect (lower on shaded slopes) and continentality.
• Significance: A rising ELA is one of the clearest signals of glacier imbalance; on a glacier in balance, the accumulation zone usually occupies about 60 per cent of its area.
• Don’t confuse with: The transient snowline on any given day is not the ELA, which is defined over a whole balance year.

Glacier Types (Valley, Cirque, Piedmont, Ice Cap, Ice Sheet, Ice Shelf)

Glacier types are classified by form and setting: valley and cirque glaciers are confined by relief, piedmont glaciers spread out where valley glaciers leave the mountains, and ice caps, ice sheets and ice shelves are unconfined masses that bury the landscape or float on the sea.

• Valley (alpine) glacier: Fed from cirques, flows down a trough; Siachen, Baltoro, Gangotri.
• Cirque glacier: Small ice mass confined to a cirque; most surviving glaciers of the Pyrenees are of this kind.
• Piedmont glacier: Lobe formed where a valley glacier spreads on a lowland; Malaspina Glacier, Alaska.
• Ice cap and ice sheet: Domes that flow outwards in all directions; below about 50,000 km² an ice cap (Vatnajökull, Iceland), above it an ice sheet (Antarctica, Greenland).
• Ice shelf: Floating extension of an ice sheet; Ross and Ronne–Filchner ice shelves, Antarctica.

Thermal Regime (Warm-Based and Cold-Based Glaciers)

The thermal regime of a glacier is the temperature of its ice relative to the pressure-melting point: warm-based (temperate) glaciers are at melting point at their beds and slide on meltwater, while cold-based (polar) glaciers are frozen to their beds; polythermal glaciers combine both.

• Mechanism: Basal temperature depends on ice thickness, geothermal heat, frictional heat and surface climate.
• Key features: Warm-based ice slides, plucks and abrades vigorously; cold-based ice moves almost wholly by internal deformation and erodes little.
• Examples: Temperate Alpine valley glaciers; cold-based ice over much of interior Antarctica.
• Significance: Thermal regime explains why some glaciated landscapes are deeply carved while others preserve pre-glacial surfaces almost untouched.

Accumulation and Ablation Zones (Glacier Mass Balance)

Glacier mass balance is the difference between the mass a glacier gains in its accumulation zone, mainly from snowfall and avalanches, and the mass it loses in its ablation zone by melting, sublimation and calving, measured over a balance year in metres of water equivalent (m w.e.).

• Mechanism: Ice flows from the accumulation zone to the ablation zone, carrying the surplus down-glacier; the snout advances when the balance is positive and retreats when it is negative.
• Key features: Positive balance thickens a glacier; negative balance thins it and later shortens it.
• Examples: Chhota Shigri Glacier, Lahaul, Himachal Pradesh, has the longest continuous mass-balance record in the Himalaya: a mean of about −0.47 m w.e. a year over 2002–2023, a cumulative loss of nearly 10 m w.e.
• Significance: Mass balance is the direct climate signal; snout position responds only after a lag. The resulting glacier retreat is treated with glacial deposition.

Glacier Movement (Basal Sliding and Internal Deformation)

Glacier movement is the downslope flow of ice driven by gravity, achieved by internal deformation of ice crystals (creep), by basal sliding over the bed where meltwater is present, and by deformation of soft, water-saturated sediment beneath the ice. The balance between these depends on ice thickness, slope and basal temperature.

• Internal deformation: Ice crystals glide along their basal planes; strain rate rises roughly with the cube of stress, the flow law of John W. Glen (1955), so thick, steep ice flows fastest.
• Basal sliding: Operates only under warm-based ice, aided by regelation (pressure melting and refreezing round obstacles) and high water pressure.
• Flow pattern: Fastest at the surface centre, slowest at the sides and bed; extending flow over steep beds and compressive flow at their feet, as John Frederick Nye showed.
• Rates: Most valley glaciers move tens to hundreds of metres a year; Jakobshavn Isbræ, Greenland, exceeded 17 km a year in 2012.

Glacier Surge

A glacier surge is a short-lived phase, lasting months to a few years, during which a glacier flows ten to a hundred times faster than normal, thinning its upper reaches and thrusting its snout forward, followed by a long quiescent phase of slow flow and rebuilding. Surges are internally triggered, not climate-driven advances.

• Mechanism: Either a hydrological switch, where subglacial drainage collapses and high water pressure lets the ice slide, or a thermal switch, where a frozen bed warms to melting point.
• Key features: Contorted medial moraines, heavily crevassed surfaces and sudden snout advance.
• Examples: The Kutiah Glacier, Karakoram, advanced more than 12 km in about three months in 1953; the Karakoram has one of the world’s densest clusters of surge-type glaciers.
• Hazard link: Surging Shisper Glacier, Hunza, dammed a meltwater lake whose outburst in May 2022 destroyed the Hassanabad bridge, an ice-dam failure.

Crevasse and Bergschrund

A crevasse is a deep, open fracture in the brittle upper layer of a glacier, formed where tension from uneven flow exceeds the strength of the ice; a bergschrund is the crevasse at the head of a glacier or cirque that separates moving ice from ice or snow still frozen to the headwall.

• Types: Transverse crevasses form where the bed steepens, longitudinal crevasses where ice spreads laterally, and marginal (chevron) crevasses from drag at the valley walls.
• Key features: Crevasses rarely exceed about 30–40 m deep in temperate ice, because below that depth ice flows plastically and closes them; icefalls and seracs form where the bed is very steep.
• Examples: The Khumbu Icefall below the Western Cwm of Everest.
• Significance: Willard Drake Johnson’s bergschrund hypothesis (1899) held that freeze–thaw at the foot of the bergschrund sapped cirque headwalls.

Debris-Covered Glaciers and the Karakoram Anomaly

Debris-covered glaciers are glaciers whose ablation zones are mantled by a continuous layer of rock debris, and the Karakoram anomaly, a term introduced by Kenneth Hewitt in 2005, is the stable or slightly positive mass balance of many Karakoram glaciers in the early twenty-first century while most glaciers worldwide lost mass.

Debris cover

• Mechanism: Rockfall and avalanches from steep valley walls feed debris onto the ice; as clean ice melts out, the debris concentrates on the tongue.
• Østrem curve: Gunnar Østrem (1959) showed that a thin debris layer, of a few centimetres or less, speeds melt by darkening the surface, while thicker debris insulates the ice and slows it.
• The debris-cover anomaly: Satellite work led by Andreas Kääb (2012) found debris-covered tongues in the Himalaya thinning about as fast as clean ice. Supraglacial ponds and bare ice cliffs act as melt hotspots, and the stagnant tongues receive little ice from above, so insulation does not protect them.
• Indian examples: Most glaciers of the Miyar basin, Lahaul, carry extensive debris in their ablation zones, as do the tongues of the Gangotri and Chorabari glaciers.

The Karakoram anomaly

• Evidence: A 2020 review led by Daniel Farinotti described balanced to slightly positive budgets, faster ice flow, stable or advancing snouts and widespread surging.
• Explanations: Most snow comes in winter with the westerlies, so warming affects accumulation less than in monsoon-fed ranges; very high catchments, summer cloud and cooling, and thick debris add to the effect; irrigation-driven cooling in the plains has also been proposed.
• Current position: Studies since 2023 report a decline in mass balance after about 2010, suggesting the anomaly is weakening, although some glaciers remain near balance. The 2020 review judged its long-term persistence unlikely under projected warming.
• Significance for India: Siachen and the eastern Karakoram lie within the anomaly zone, while Himalayan glaciers from Himachal to Sikkim are losing mass; national glacier policy cannot treat the two as one system.

How Glaciers Erode: Processes and Small-Scale Forms

Glacial Plucking (Quarrying) and Abrasion

Glacial plucking, or quarrying, is the removal of joint-bounded blocks of bedrock by moving ice, and glacial abrasion is the grinding, scratching and polishing of the bed by rock fragments frozen into the base of the ice. Together they are the main processes of glacial erosion.

• Plucking mechanism: Meltwater freezes into joints and pressure changes crack the rock; blocks are lifted out, mainly on the down-ice side of obstacles where pressure falls.
• Abrasion mechanism: Rock fragments in basal ice act as tools; its rate rises with ice pressure, sliding speed and the supply of hard debris.
• Products: Abrasion yields silt-sized rock flour that makes meltwater milky, as in the Bhagirathi at Gaumukh; plucking yields large angular blocks.
• Key features: Both require basal sliding, so they are effective only under warm-based ice.
• Examples: Plucked lee faces and abraded stoss faces of roches moutonnées.

Glacial Protection vs Glacial Erosion Debate

The glacial protection versus glacial erosion debate was the nineteenth-century controversy over whether glaciers merely cover and shield the ground, modifying valleys already cut by rivers, or actively excavate rock basins and troughs. Modern glaciology resolves it through the glacier’s thermal regime.

• Erosionist view: Andrew Crombie Ramsay argued in 1862 that glaciers excavated the rock basins of many lakes in Switzerland, Britain and North America.
• Protectionist view: Thomas George Bonney and others doubted that ice, softer than rock, could dig basins, and held that ice protected pre-existing forms; tributary valleys filled with ice were said to be sheltered while main valleys were cut by water.
• Resolution: Warm-based ice laden with debris erodes powerfully, overdeepening troughs and fjords far below sea level, while cold-based ice frozen to its bed preserves the surface beneath it; both camps described real cases.
• Significance: Explains the sharp contrast between deeply carved troughs and preserved plateaux in the same glaciated region.

Striations and Glacial Grooves

Glacial striations are fine, parallel scratches, millimetres to centimetres deep, cut into bedrock by debris held in the base of a sliding glacier, while glacial grooves are much larger channels, up to metres deep, gouged in the same way. Both record the direction of past ice flow.

• Formation: Hard clasts dragged across the bed under great pressure act like chisels.
• Key features: Associated forms include chatter marks and crescentic gouges; striated surfaces are often polished.
• Examples: The glacial grooves of Kelleys Island, Lake Erie, Ohio; smoothed and striated rock surfaces in the Miyar basin, Lahaul.
• Significance: Mapped striations reconstruct the flow of former ice sheets; crossing sets of striations reveal changes of flow direction between glacial phases.

Roche Moutonnée

A roche moutonnée is an asymmetric bedrock knob whose up-ice (stoss) side is smoothed and striated by abrasion and whose down-ice (lee) side is steep and jagged from plucking. Horace-Bénédict de Saussure coined the term in 1786, likening such rocks to wigs of his day smoothed with mutton fat, not to sheep.

• Formation: High pressure on the stoss side favours abrasion; pressure falls in the lee, where meltwater refreezes into joints and blocks are pulled away.
• Scale: From a metre to whole hills several hundred metres long; the large Swedish forms are called flyggbergs.
• Examples: Lembert Dome, Tuolumne Meadows, Yosemite, California.
• Sketch: Long profile with ice-flow arrow, smooth gentle stoss slope, steep plucked lee slope.
• Don’t confuse with: A drumlin is depositional and its steep end faces up-ice; a roche moutonnée is erosional with its steep end down-ice.

Crag and Tail

A crag and tail is a streamlined landform in which a resistant rock knob, the crag, faces the oncoming ice with a steep side, while a gently tapering ridge, the tail, of softer rock or till extends down-ice in its shelter. It forms where ice meets an isolated obstacle such as a volcanic plug.

• Formation: Ice erodes the crag’s up-ice face and flows round it; softer rock in its lee is protected, and till accumulates there.
• Key features: Steep end up-ice and a solid rock core; the tail may be bedrock, till or both.
• Examples: Castle Rock, Edinburgh, a volcanic plug, with the Royal Mile running down its tail.
• Sketch: Profile with ice arrow, steep crag and long tapering tail.

Erosional Landforms of Glaciated Uplands

Cirque (Corrie, Cwm) and Tarn

A cirque is a steep-walled, armchair-shaped hollow cut into a mountainside at the head of a glaciated valley, with a near-vertical headwall, an overdeepened rock basin and a raised lip (threshold) at its mouth; a tarn is the small lake that fills the basin once the ice melts. It is corrie in Scotland, cwm in Wales, kar in Germany and botn in Norway.

• Formation: A snow patch deepens a hollow by nivation; once ice forms, rotational flow scours the basin, plucking and freeze–thaw steepen the headwall, and the lip survives where ice pressure is lower.
• Key features: In the northern hemisphere cirques face mostly north and north-east, where shade and drifting snow favour ice.
• Examples: Red Tarn below Helvellyn, Lake District; Roopkund, at about 5,020 m in a bowl-like hollow at the foot of the Trishul massif, Uttarakhand.
• Sketch: Cross-section: headwall, bergschrund, rotational flow arrows, rock basin, lip, tarn.

Arête

An arête is a sharp, knife-edged rock ridge formed where two cirques or two glacial troughs erode back towards each other from opposite sides until only a narrow crest remains between them. The word is French for fish-bone or ridge.

• Formation: Headwall retreat by plucking and frost shattering narrows the divide.
• Key features: Serrated crests with jagged pinnacles; arêtes often radiate from horns.
• Examples: Striding Edge, Helvellyn, Lake District; the ridges of the Mont Blanc massif.
• Significance: Arêtes are the ridges from which horns and cols develop as cirque retreat continues, so they mark a mature stage of upland glaciation.
• Sketch: Two cirques back to back with a narrow ridge between them.

Col

A col is a saddle-shaped pass or low point on an arête, formed where the headwalls of two cirques on opposite sides of a ridge cut back and intersect. Cols are glacial breaches of a divide and are often used as mountain passes.

• Formation: Continued headwall retreat lowers part of the arête into a gap.
• Key features: U-shaped or V-shaped notch in a ridge crest, often with a cirque on each side.
• Examples: Col du Géant, Mont Blanc massif, on the France–Italy border.
• Don’t confuse with: A wind gap is a dry notch left by river capture; a col is carved by cirque headwalls.
• Significance: Many high Alpine and Himalayan passes follow cols, because they are the lowest crossings of otherwise continuous ridges.

Horn (Pyramidal Peak)

A horn is a sharp, pyramidal mountain peak with steep faces and radiating arêtes, formed where three or more cirques erode headward into the same summit from different sides. It takes its name from the Matterhorn in the Swiss–Italian Alps.

• Formation: Headwall retreat by plucking and frost shattering in each cirque steepens the faces until they meet at a point.
• Key features: Three or more steep faces separated by arêtes, often with a cirque at the foot of each face.
• Examples: Matterhorn (4,478 m), Alps; Shivling, overlooking Gaumukh at the snout of the Gangotri Glacier, which early European explorers called the Matterhorn Peak for its pyramid form.
• Sketch: Plan view: three cirques round a central peak with arêtes between them.

Biscuit-Board and Fretted Upland

A biscuit-board (scalloped) upland is a plateau whose edges have been bitten into by cirques while its central surface remains intact, and a fretted upland is the later stage in which cirques have consumed the plateau entirely, leaving only arêtes, cols and horns. William Herbert Hobbs described this sequence in his cycle of mountain glaciation (1910).

• Formation: Progressive headward retreat of cirques from several sides.
• Key features: Biscuit-board uplands retain flat summit remnants; fretted uplands are all ridges and peaks.
• Examples: The Cairngorm plateau, Scotland, notched by corries (biscuit-board); the Mont Blanc massif (fretted).
• Significance: The contrast depends on how long, and how actively, cirque glaciers worked on the upland.

Nunatak

A nunatak is an isolated rocky peak or ridge that projects above the surface of an ice sheet or ice field, surrounded by ice on all sides. The term is Greenlandic Inuit, and nunataks are sometimes called glacial islands.

• Formation: Summits stand above the ice surface; frost shattering attacks them while the ice below erodes their flanks.
• Key features: Sharp, frost-shattered summits above smoothed, ice-scoured lower slopes, separated by a trimline.
• Examples: Nunataks along the margins of the Greenland ice sheet; the Transantarctic Mountains.
• Sketch: Cross-section of an ice sheet with a peak projecting through it, trimline marked on its flanks.
• Significance: Nunataks may have sheltered plants and animals through glaciations, the basis of the nunatak hypothesis of biotic survival.

Erosional Landforms of Glaciated Valleys

U-Shaped (Glacial) Trough

A U-shaped glacial trough is a straight, deep valley with steep, often near-vertical walls and a broad, flat or gently concave floor, formed when a glacier occupies and reshapes a pre-existing river valley. Its cross-section approximates a parabola, the shape that lets ice flow with the least friction.

• Formation: Ice fills the valley to a great depth and erodes its whole cross-section, widening and deepening the floor and steepening the walls.
• Key features: Overdeepened basins along the floor, truncated spurs, hanging tributary valleys and trimlines marking the former ice surface.
• Examples: Yosemite Valley, California; Lauterbrunnen valley, Switzerland; the Miyar valley, Lahaul, Himachal Pradesh.
• Sketch: A V-shaped river valley with the U-shaped trough superimposed, labelled trimline and overdeepened floor.
• Don’t confuse with: A river valley’s V-shape comes from vertical cutting plus slope wasting.

Truncated Spur

A truncated spur is a steep, often triangular rock face at the end of a valley-side ridge, formed where a valley glacier has cut off the projecting tip of an interlocking spur while straightening and widening its valley. It is the diagnostic sign that a winding river valley has been converted into a straight glacial trough.

Where they form

• Setting: In glaciated mountain valleys that were first cut by rivers and later occupied by valley glaciers, typically in Pleistocene or present-day alpine glaciation.
• Examples: The Lauterbrunnen valley, Switzerland; Yosemite Valley, California; the Miyar valley, Lahaul, Himachal Pradesh, where truncated spurs, trimlines and U-shaped valley form are used to identify the oldest glacial stage.
• Associated forms: Hanging valleys between the spurs, trimlines and a U-shaped cross-section.

How they form

1. Pre-glacial stage: A river winds between alternating interlocking spurs in a V-shaped valley.
2. Glacier occupation: A valley glacier, far thicker and wider than the river and unable to bend round each spur, fills the valley.
3. Planation: Abrasion and plucking attack the protruding spur tips, where ice pressure and velocity are concentrated; the valley is straightened.
4. Result: After the ice melts, the spur ends stand as steep facets or cliffs aligned along the trough walls, often with tributary hanging valleys between them.

Glacial truncated spurs compared with other types

• Fault-truncated (faceted) spurs: Triangular facets along an active normal-fault front, where uplift and fault movement cut spur ends; the Wasatch Front, Utah, is the classic example, and such facets are used in seismotectonic mapping.
• Marine truncated spurs: Where a rising sea or wave attack cuts cliffs across the ends of coastal spurs.
• Fluvial trimming: Meander loops migrating down a mature valley can cut back spur ends, but the valley stays winding; only ice straightens it.

• Sketch: Plan view before (river winding between interlocking spurs) and after (straight trough with flat-faced spur ends and hanging valleys).

Hanging Valley

A hanging valley is a tributary valley whose floor lies high above the floor of the main glacial trough it joins, so that after deglaciation its stream enters the main valley by a waterfall or steep cascade. It results from unequal glacial deepening of main and tributary valleys.

• Formation: The thick trunk glacier erodes its valley far deeper than the thinner tributary glaciers can erode theirs; the tributary floors are left perched when the ice melts.
• Alternative view: Protectionists argued that ice-filled tributaries were shielded while the main valley was deepened, but differential glacial erosion is now the accepted explanation.
• Examples: Staubbach Falls, Lauterbrunnen valley, drops 297 m from a hanging valley; Bridalveil Fall, Yosemite; hanging valleys in the Miyar basin, Lahaul.
• Sketch: Block diagram of a main trough with a tributary valley at a higher level and a waterfall at the junction.
• Don’t confuse with: A hanging-valley waterfall is one of several waterfall types.

Ribbon Lake

A ribbon lake is a long, narrow, often deep lake occupying an overdeepened rock basin on the floor of a glacial trough, commonly dammed at its lower end by a rock bar or a terminal moraine. It forms where a glacier eroded one section of its valley more deeply than the rest.

• Formation: Overdeepening happens where ice is confined, where tributary glaciers join, or where rock is weaker or more jointed.
• Key features: Depth often exceeds the height of the lip at the outlet.
• Examples: Windermere, Lake District, England; Loch Morar, Scotland, the deepest lake in Britain.
• Don’t confuse with: A tarn is a small lake in a cirque; a ribbon lake lies in the trough below.

Paternoster Lakes and Glacial Stairway

A glacial stairway is a longitudinal valley profile of alternating steep rock steps (riegels) and overdeepened flatter basins, and paternoster lakes are the chain of small lakes that occupy those basins after the ice melts, linked by a stream like beads on a rosary.

• Formation: Differential erosion where rock varies in jointing or resistance, and extra deepening below tributary junctions where ice thickens.
• Key features: Steps from a few metres to hundreds of metres high; streams cascade from lake to lake.
• Examples: The paternoster lakes of the Grinnell Valley, Glacier National Park, Montana.
• Sketch: Long profile of a stepped valley floor with a lake in each basin.

Fjord

A fjord is a long, narrow, deep, steep-sided coastal inlet formed when a glacial trough, eroded far below present sea level by a valley glacier or outlet glacier, is flooded by the sea after the ice retreats. Fjords are often deepest inland and shallow at a submerged rock or moraine sill near their mouths.

• Formation: Thick outlet glaciers descending from coastal ice fields eroded their beds below sea level; post-glacial sea-level rise drowned the troughs; many follow fault or joint zones.
• Key features: Hanging valleys and waterfalls on the walls, great depth, a threshold at the mouth where the ice thinned and spread.
• Examples: Sognefjord, Norway, about 205 km long and 1,308 m deep; Milford Sound, New Zealand.
• Significance: Fjords shape the fjord coast type of Norway, Greenland, Chile and British Columbia.
• Don’t confuse with: A ria is a drowned river valley, V-shaped in section and shallowing inland.

PYQs Built on These Terms

• What are ‘truncated spurs’? Where and how are they formed? (2024)
• Discuss the variations in nature of glaciers in India and the emerging issues due to climate change. (Paper II, 2024)
• Discuss the evolution and characteristics of the land forms of glaciated regions. (1987)

Frequently Asked Questions

What is the difference between an interlocking spur and a truncated spur?

An interlocking spur is still whole; a truncated spur has had its tip cut off. Interlocking spurs alternate from each side of a young, winding river valley. When a glacier occupies the valley it cannot bend round them, so it planes off their ends, leaving steep truncated spurs along a straight U-shaped trough.

How do glaciers erode their valleys?

Glaciers erode mainly by plucking and abrasion. Plucking tears out joint-bounded blocks, especially on the down-ice side of obstacles; abrasion scours and polishes rock with debris frozen into the ice base. Both need a warm-based glacier sliding on meltwater. Frost shattering on exposed walls supplies extra debris, and the result is deepened, widened and straightened troughs.

What is the Karakoram anomaly?

The Karakoram anomaly is the stable or slightly growing state of many Karakoram glaciers in the early twenty-first century while most mountain glaciers shrank. It was named in 2005 and is linked to winter snowfall from western disturbances, very high catchments and cooler summers. Studies since 2023 suggest it is weakening as warming continues.

How is a cirque different from a tarn?

A cirque is the landform and a tarn is the water in it. A cirque is an armchair-shaped rock hollow at the head of a glaciated valley, with a steep headwall and a raised lip. After the ice melts, water collects in its overdeepened basin behind the lip, forming a small lake called a tarn, such as Red Tarn below Helvellyn.

What is a hanging valley and how is it formed?

A hanging valley is a tributary valley perched high above the floor of a main glacial trough. The thick main glacier deepened its valley much more than the smaller tributary glaciers could, so after deglaciation the tributary floor is left hanging, and its stream enters by a waterfall, as at Staubbach Falls in Switzerland.

Why do debris-covered glaciers still lose mass quickly?

Thick debris insulates ice and should slow melting, yet satellite measurements show debris-covered Himalayan tongues thinning almost as fast as clean ice. Supraglacial ponds and bare ice cliffs melt rapidly, and the stagnant, debris-laden tongues receive little new ice from above. Only a thin debris layer of a few centimetres actually speeds melting.