Glacial Deposition and Glacial Lake Outburst Floods: Terminology for UPSC Geography Optional

Glacial deposition is the laying down of rock debris carried by ice and its meltwater, and a glacial lake outburst flood is the sudden release of water ponded by that debris or by ice. This family of terms runs from till, moraines and drumlins to eskers, outwash and varves, and ends with the glacier retreat, glacial lakes and outburst floods now reshaping the Himalaya.

Each entry opens with a definition, then formation, examples and a sketch line. Terms move from deposits laid directly by ice, to meltwater deposits, to lakes and floods. UPSC asked for basket-of-eggs topography in 2016 and for the causes of glacial lake outburst floods in 2025; drumlins and GLOFs deserve full answers.

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Deposits Laid Directly by Ice

Glacial Drift (Stratified vs Unstratified)

Glacial drift is the collective term for all sediment transported and deposited by glaciers or their meltwater. It is divided into unstratified drift, or till, laid directly by ice without sorting, and stratified drift, laid by meltwater streams and lakes in sorted, bedded layers.

• Origin of the term: “Drift” dates from the early nineteenth century, when these deposits were thought to have drifted in on floating ice during a great flood.
• Unstratified drift: Till, forming moraines, drumlins and till plains.
• Stratified drift: Ice-contact deposits (eskers, kames) and proglacial deposits (outwash, valley trains, lake clays).
• Examples: Drift sheets up to tens of metres thick bury the pre-glacial relief of the North American Midwest.
• Significance: Drift makes the soils, aquifers and building aggregates of formerly glaciated lands.

Glacial Debris Transport (Supraglacial, Englacial and Subglacial)

Glacial debris transport is the carriage of rock waste by a glacier in three positions: supraglacial debris on the ice surface, englacial debris within the ice, and subglacial debris at its bed. Where the debris travels decides how worn it becomes and which deposit it later forms.

• Supraglacial: Rockfall from valley walls, carried passively; stays angular; forms lateral and medial moraines and ablation till.
• Englacial: Buried surface debris or debris lifted from the bed along shear planes.
• Subglacial: Crushed and abraded against the bed; rounded, faceted and striated clasts in a fine matrix of rock flour; forms lodgement till.
• Examples: The debris-mantled lower tongue of the Gangotri Glacier; the dark medial moraines of the Baltoro Glacier, Karakoram.

Till (Lodgement and Ablation Till) and Boulder Clay

Till is unsorted, unstratified sediment deposited directly by glacier ice, a chaotic mixture of clay, silt, sand, gravel and boulders, commonly with angular to subangular, striated clasts. Boulder clay is the older British name for clay-rich till; lithified ancient till is called tillite.

• Lodgement till: Plastered on to the bed beneath sliding ice; dense, compact, with long axes of stones aligned with ice flow.
• Ablation (melt-out) till: Let down as debris-rich ice melts at the surface or in place; loose, coarser, less compact.
• Examples: The boulder-clay cliffs of Holderness, Yorkshire, among Europe’s fastest-eroding coasts; Permo-Carboniferous Talchir tillite in peninsular India.
• Significance: Till fabric reveals the direction of ice flow; clay-rich tills are poorly drained and prone to slumping.

Glacial Erratic

A glacial erratic is a rock fragment, from a pebble to a house-sized block, carried by ice and deposited on bedrock of a different type, often far from its source. Erratics were among the first evidence that ice sheets had once covered northern Europe and North America.

• Formation: Blocks fall on to or are plucked by a glacier and are dropped when the ice melts.
• Key features: Indicator erratics of distinctive rock trace ice-flow paths; lines of them form boulder trains.
• Examples: The Okotoks Erratic, Alberta, a 16,500-tonne quartzite block carried from the Jasper area; microgranite erratics from Ailsa Craig, Firth of Clyde, scattered across Wales, Ireland and north-west England.
• Significance: Jean Louis Rodolphe Agassiz used erratics and polished rock in 1840 to argue for a former ice age.

Moraine (Lateral, Medial, Terminal, Recessional, Ground and Push Moraines)

A moraine is a ridge, mound or sheet of till deposited directly by a glacier, named from its position relative to the ice. Moraines mark former ice margins, so they are the main evidence for mapping past glacier extent.

• Lateral moraine: Ridge along a glacier’s side from rockfall and marginal debris; Chorabari Glacier above Kedarnath is flanked by high lateral moraines.
• Medial moraine: Stripe formed where two lateral moraines merge at a glacier confluence; Baltoro Glacier below Concordia.
• Terminal (end) moraine: Arcuate ridge at the maximum advance; the Ronkonkoma and Harbor Hill moraines form the spine of Long Island, New York.
• Recessional moraine: Ridge left during a halt in retreat, inside the terminal moraine.
• Ground moraine: Undulating till sheet laid beneath the ice.
• Push moraine: Ridge of older sediment bulldozed and folded by an advancing snout.
• Hazard link: Young, steep, ice-cored end moraines dam many Himalayan glacial lakes.

Drumlin

A drumlin is a smooth, elongated, streamlined hill of till, or of till over a rock core, moulded beneath actively flowing ice and aligned with the direction of flow, with a blunt, steep up-ice (stoss) end and a gently tapering down-ice (lee) end. The name comes from the Irish droimnín, “little ridge”, first used by James Bryce in 1833.

• Dimensions: Usually about 250–1,000 m long and 5–50 m high, with length about two to four times the width.
• Formation hypotheses: Subglacial deposition of till where overloaded ice lodges debris round an obstacle; moulding and streamlining of a deforming, water-saturated till bed; erosion of pre-existing sediment into streamlined remnants; and, controversially, sculpting by catastrophic subglacial meltwater floods.
• Current view: Drumlins, flutes and mega-scale glacial lineations form a continuum of subglacial bedforms beneath fast-flowing ice, and many researchers now treat them as the product of an instability in the ice–bed interface, a debate not yet settled.
• Examples: The New York State drumlin field, with more than 10,000 drumlins between Rochester and Syracuse; drowned drumlins forming islands in Clew Bay, County Mayo, and Strangford Lough, County Down; Dodge County, Wisconsin.
• Indian context: Drumlin fields form beneath lowland ice sheets, so they are essentially absent from the Himalaya, whose glaciers are confined valley glaciers.
• Sketch: Profile and plan: ice arrow, blunt stoss end, tapering lee end.
• Don’t confuse with: A roche moutonnée is an erosional rock knob whose steep face is on the lee side.

Basket-of-Eggs Topography

Basket-of-eggs topography is the landscape formed where drumlins occur in dense swarms or fields, hundreds or thousands of rounded, parallel hills packed closely together so that, seen from a height, they resemble eggs lying in a basket. The drumlin is the individual form; basket-of-eggs topography is the whole swarm landscape.

• Origin of the image: Charles Smith, describing County Down in 1744, compared its hills to “wooden Bowls inverted, or Eggs set in Salt”; the phrase “basket of eggs” grew from such descriptions.
• Formation: A broad lobe of ice, often a fast-flowing ice stream, streamlines the whole of its till bed at once, so drumlins in a field share a common alignment and often an en échelon arrangement.
• Key features: Parallel long axes pointing along former ice flow; small lakes, bogs and poorly drained hollows between the hills; deranged drainage; roads and field boundaries that wind between the hills.
• Examples: Clew Bay, County Mayo, where the drowned swarm stands as hundreds of islands; County Down and the drumlin belt of Northern Ireland; the New York State drumlin field between Lake Ontario and the Finger Lakes.
• Significance: Drumlin alignment maps the flow of the last ice sheets, and denser swarms usually mark faster-flowing ice; the waterlogged hollows shape settlement, farming and road patterns, as in the lake-strewn drumlin belt of south Ulster.
• Sketch: Plan of many small ovals, all with blunt ends up-ice, and an ice-flow arrow across the field.

Knob-and-Kettle Topography

Knob-and-kettle topography is a hummocky landscape of irregular till mounds (knobs) and enclosed depressions (kettles), many holding ponds, formed where debris-rich ice stagnated and melted in place or along broad end-moraine belts. It is also called hummocky moraine or knob-and-basin topography.

• Formation: Debris on and within stagnant ice is let down unevenly as the ice wastes; buried ice blocks leave hollows when they melt.
• Key features: Chaotic relief with no single alignment, disordered drainage and many small lakes.
• Examples: The Kettle Moraine, Wisconsin, an interlobate moraine between two ice lobes.
• Don’t confuse with: Basket-of-eggs topography is streamlined and aligned; knob-and-kettle relief is chaotic.

Meltwater Deposits: Glacio-Fluvial and Glacio-Lacustrine Forms

Glacio-Fluvial Landforms

Glacio-fluvial landforms are landforms built by glacial meltwater from sand and gravel that is sorted, rounded and layered, unlike till. They divide into ice-contact forms, laid against or within ice, and proglacial forms, laid beyond the ice front.

• Ice-contact forms: Eskers, kames, kame terraces and crevasse fillings; their beds often collapse as supporting ice melts.
• Proglacial forms: Outwash plains, valley trains, kettles in outwash, and deltas in glacial lakes.
• Key features: Bedding, sorting and rounded clasts separate them from moraine.
• Examples: Skeiðarársandur, Iceland; braided gravel floors below Himalayan glacier snouts.
• Significance: Glacio-fluvial gravels are major aquifers and aggregate sources in formerly glaciated lands.

Esker

An esker is a long, narrow, sinuous ridge of stratified sand and gravel deposited by meltwater flowing in a tunnel beneath, within or on a glacier, and left standing as a ridge when the ice melted. The word comes from the Irish eiscir, a ridge.

• Formation: Pressurised subglacial streams fill their ice-walled tunnels with sediment; because the water is under pressure, eskers can run uphill across low divides.
• Key features: Steep sides; lengths from hundreds of metres to hundreds of kilometres; beaded eskers swell into mounds where sediment was dumped at stages of retreat.
• Examples: The Esker Riada, an ancient routeway across central Ireland; the Uppsala esker, Sweden.
• Significance: Eskers are dry, raised routes through boggy country and valuable sources of sand and gravel.
• Don’t confuse with: A medial or lateral moraine is unsorted till; an esker is sorted, bedded gravel.

Kame and Kame Terrace

A kame is an irregular mound or short ridge of stratified sand and gravel deposited by meltwater in contact with stagnant ice, in a crevasse, a moulin or an ice-marginal pond, which slumps into a hill once the ice melts; a kame terrace is a flat-topped bench of the same material laid between a glacier margin and the valley wall.

• Types: Delta kames formed in ice-marginal lakes; moulin (perforation) kames in vertical shafts; kame terraces along valley sides.
• Key features: Collapsed, faulted bedding where supporting ice melted; kames often mingle with kettles as kame-and-kettle topography.
• Examples: Carstairs Kames, South Lanarkshire, Scotland.
• Sketch: Valley cross-section showing a glacier, meltwater ponded at its side, and the terrace left on the valley wall after melting.

Kettle and Kettle Lake

A kettle is an enclosed depression in glacial drift, formed where a block of stagnant ice buried in outwash or till later melted and the overlying sediment collapsed; a kettle lake is the pond or lake that fills it. Kettles range from a few metres to several kilometres across.

• Formation: Ice blocks detach from a retreating glacier, are buried by outwash, and melt slowly; the ground subsides into the space.
• Key features: Steep sides, no surface outlet, often in clusters that pit an outwash plain.
• Examples: Walden Pond, Massachusetts; the kettle lakes of the Kettle Moraine, Wisconsin; pitted outwash in front of Icelandic glaciers.
• Significance: Kettle lakes and mires hold sediment and pollen records spanning the postglacial period.

Outwash Plain (Sandur) and Valley Train

An outwash plain, or sandur (plural sandar, from Icelandic), is a broad, gently sloping plain of sorted sand and gravel laid by braided meltwater streams beyond a glacier’s terminal moraine; a valley train is the same deposit confined between the walls of a mountain valley.

• Formation: Meltwater leaves the snout loaded with debris; with falling gradient it spreads, braids and drops its load, coarse near the ice and finer downstream.
• Key features: Braided channels, kettles, and fining downstream; wind lifts silt from bare outwash, one source of glacial loess.
• Examples: Skeiðarársandur, south of Vatnajökull, Iceland; valley trains of braided gravel below Himalayan glacier snouts, such as the Bhagirathi valley below Gaumukh.
• Sketch: Plan view: snout, terminal moraine, braided channels fanning across the plain, kettles.

Varves

Varves are annual pairs of sediment layers deposited in a glacial or proglacial lake: a thicker, lighter, coarser layer of silt and fine sand laid in summer, when melt streams are active, and a thin, darker layer of clay settled in winter under ice cover. Counting varves gives year-by-year chronologies.

• Mechanism: Summer inflow brings coarse sediment; in winter, with inflow stopped and the lake frozen, only fine clay settles.
• Coined by: The Swedish geologist Gerard Jakob De Geer, who from the 1880s built the Swedish varve chronology, the first time scale of deglaciation measured in years.
• Examples: The glacial varves of Ragunda, Indalsälven valley, Sweden; glacial-lake clays of the North American Great Lakes region.
• Significance: Varves date deglaciation and record meltwater, climate and seismic events; their wider use as climate archives is treated with Quaternary palaeoclimate proxies.

Glacier Retreat, Glacial Lakes and Outburst Floods

Glacier Retreat (Himalayan Glacier Mass Loss)

Glacier retreat is the upslope recession of a glacier’s snout and the thinning and shrinking of its ice under sustained negative mass balance, when ablation exceeds accumulation year after year. It is the landform-scale response to a climate signal first recorded in mass balance.

• Evidence: Hindu Kush Himalayan glaciers lost mass 65 per cent faster in 2011–2020 than in 2000–2010, and could lose up to 80 per cent of their present volume by 2100 on current emission paths, according to a June 2023 assessment by the International Centre for Integrated Mountain Development.
• Geomorphic effects: Retreat exposes fresh moraine and bare, unstable valley walls (paraglacial slopes), and leaves water ponded behind moraines and in overdeepened basins.
• Correction of a myth: The claim in the Intergovernmental Panel on Climate Change’s 2007 assessment that Himalayan glaciers could vanish by 2035 was acknowledged as an error in January 2010; loss is rapid, but on a timescale of a century.
• Regional contrast: The Karakoram has lost far less ice than the central and eastern Himalaya, the Karakoram anomaly.
• Significance: The United Nations declared 2025 the International Year of Glaciers’ Preservation.

Proglacial and Moraine-Dammed Lakes

A proglacial lake is a lake formed at or near the front of a glacier, in contact with the ice or just beyond it; a moraine-dammed lake is one impounded behind a terminal or lateral moraine, the most common and most hazardous type in the Himalaya. Lakes may also be dammed by ice, cut into bedrock, or sit on the glacier surface.

• Formation: As a snout retreats from its moraine, meltwater fills the overdeepened hollow behind; supraglacial ponds on debris-covered tongues can coalesce into large lakes.
• Indian inventory: Satellite comparison released by the Indian Space Research Organisation in April 2024 found 2,431 glacial lakes larger than 10 ha in the Himalayan river basins in 2016–17, of which 676 had grown since 1984; 130 of these lie in India (65 Indus, 7 Ganga, 58 Brahmaputra), and 307 of the expanding lakes are moraine-dammed.
• Examples: Ghepan Ghat lake, Lahaul, Himachal Pradesh, grew by 178 per cent, from about 36 to 101 ha, between 1989 and 2022; South Lhonak Lake, Sikkim.
• Significance: Moraine dams are loose, steep and often ice-cored, which makes them the chief source of GLOFs.

Glacial Lake Outburst Flood (GLOF)

A glacial lake outburst flood (GLOF) is the sudden release of a large volume of water from a lake dammed by moraine, glacier ice or bedrock at a glacier margin, producing a flood surge, often laden with debris, that can reach peak discharges far above any rainfall flood in the same valley within minutes to hours.

Causes

• Moraine-dam failure: Loose, steep moraine fails by overtopping and erosion, or slumps as its buried ice core melts.
• Piping: Seepage through the dam carries away fines and hollows out tunnels until it collapses.
• Displacement waves: Rock, ice or moraine avalanches plunging into the lake raise waves that overtop the dam.
• Ice-dam failure: The ice dam floats once the lake is deep enough, or tunnels open beneath it.
• Rain and snowmelt: Extreme rain or rapid melt raises the lake and saturates the dam.
• Seismic triggering: Shaking fractures dams or sets off slope failures.
• Cascades: One lake’s outburst can breach a second lake downstream.

Indian cases

• Kedarnath, Uttarakhand, June 2013: After about 390 mm of rain between 10 and 17 June and rapid melting of snow, the small moraine-dammed Chorabari lake (Gandhi Sarovar), fed by rain and snowmelt rather than glacier melt, breached on 17 June; more than 6,000 people died across the state, most of them in this disaster.
• South Lhonak, Sikkim, 3–4 October 2023: At about 22:13 on 3 October, some 14.7 million m³ of frozen lateral moraine collapsed into the lake, raising a wave about 20 m high that breached the moraine dam and released about 50 million m³ of water. About two hours later the surge destroyed the 1,200 MW Teesta-III dam at Chungthang; 55 people died and 74 went missing. A study led by Ashim Sattar in 2025 traced the collapse to permafrost thaw.
• Not a GLOF: The February 2021 Chamoli disaster was a rock–ice avalanche, not a lake outburst.

Global cases

• Other outbursts: Lake Palcacocha, Huaraz, Peru, 13 December 1941 (estimates of 1,600–4,000 deaths); Dig Tsho, Nepal, 4 August 1985, set off by an ice avalanche; Luggye Tsho, Bhutan, 7 October 1994; Shisper’s ice-dammed lake, Hunza, 7 May 2022; Thyanbo lake above Thame, Nepal, 16 August 2024; and the flood of 8 July 2025 from Tibet that destroyed the Rasuwagadhi border bridge.

• Sketch: Moraine-dammed lake behind a breached dam, avalanche above, surge path and dam downstream.

Jökulhlaup

A jökulhlaup, Icelandic for “glacier run”, is a sudden outburst flood from a lake dammed by glacier ice or from water stored beneath a glacier, often released when geothermal or volcanic heat melts the ice from below. The term is sometimes used loosely for any GLOF, but strictly refers to ice-dammed and subglacial sources.

• Mechanism: Water accumulates until it lifts or breaks through the ice seal, then enlarges tunnels by melting their walls, so discharge rises rapidly to a peak and then falls.
• Examples: The Gjálp eruption beneath Vatnajökull (30 September–13 October 1996) filled the subglacial Grímsvötn lake, which burst in early November 1996, sending tens of thousands of cubic metres per second across Skeiðarársandur and destroying parts of the Ring Road and two bridges. Suicide Basin, dammed by the Mendenhall Glacier, Alaska, has drained at least 39 times since 2011, and on 13 August 2025 produced a record crest at Juneau.
• Pleistocene scale: The repeated failure of ice-dammed glacial Lake Missoula scoured the Channeled Scablands of Washington, a case argued by J Harlen Bretz.

GLOF Risk Mitigation

GLOF risk mitigation is the combination of structural measures that reduce the volume or instability of a hazardous glacial lake, and non-structural measures such as inventories, monitoring, early warning and land-use control that reduce exposure downstream. Most effort now goes into identifying which lakes matter before a trigger arrives.

• Structural measures: Controlled lowering by siphons or excavated outlet channels: Tsho Rolpa, Nepal, was lowered 3 m in 2000 and Imja Tsho about 3.4 m in 2016, though scientists argue far deeper lowering is needed; siphoning at South Lhonak in 2016 removed only about 150 litres a second.
• Non-structural measures: Satellite inventories and hazard ranking, automatic weather and water-level stations, sirens and community drills, and keeping settlements and hydropower out of flood paths.
• India: The National Disaster Management Authority issued Guidelines on Management of GLOFs in October 2020. On 25 July 2024 a High-Level Committee chaired by the Union Home Minister approved the ₹150 crore National GLOF Risk Mitigation Programme for Himachal Pradesh, Uttarakhand, Sikkim and Arunachal Pradesh. The Central Water Commission now monitors 2,485 lakes larger than 10 ha; a 2025 assessment of 189 high-risk lakes placed 56 in the very high risk class. Multi-agency teams surveyed South Lhonak and Shako Cho, Sikkim, in 2023–24. By July 2026 no end-to-end GLOF early-warning system had yet been commissioned.
• Significance: As Chungthang showed, the siting of infrastructure decides losses as much as the lake does.

PYQs Built on These Terms

• Explain the causes of glacial lake outburst floods. (2025)
• Describe the “Basket of Eggs topography”. (2016)
• Examine the problems of Glacial Lake Outburst Flood (GLOF) in India. (Paper II, Contemporary Issues)
• Discuss the evolution and characteristics of the land forms of glaciated regions. (1987)

Frequently Asked Questions

What is the difference between a drumlin and an esker?

A drumlin is a streamlined oval hill of unsorted till moulded beneath moving ice and aligned with its flow; an esker is a long, winding ridge of sorted, bedded sand and gravel laid by meltwater flowing in a tunnel in or under the ice. Drumlins are ice-moulded; eskers are meltwater deposits.

Why is it called basket-of-eggs topography?

It is called basket-of-eggs topography because a dense swarm of drumlins, all rounded and aligned in the same direction, looks from above like eggs packed in a basket. Charles Smith in 1744 compared the drumlins of County Down to “Eggs set in Salt”. Clew Bay in Ireland and the New York State drumlin field are classic examples.

What causes a glacial lake outburst flood?

A GLOF occurs when the dam holding a glacial lake fails. Moraine dams fail by overtopping, piping or melting of buried ice; ice dams float or are tunnelled through. Triggers include rock or ice avalanches that raise waves, heavy rain, rapid snowmelt and earthquakes. At South Lhonak in 2023, a moraine collapse into the lake produced a 20 m wave.

What is the difference between a GLOF and a jökulhlaup?

A jökulhlaup is one kind of GLOF. GLOF is the general term for any outburst from a glacial lake, whether dammed by moraine, ice or rock. A jökulhlaup, strictly, is an outburst from an ice-dammed or subglacial lake, often melted out by volcanic heat, as at Grímsvötn in Iceland. Most Himalayan GLOFs come from moraine-dammed lakes, not jökulhlaups.

What is the difference between a moraine and outwash?

Moraine is deposited directly by ice, so it is unsorted till with angular, mixed sizes, piled at the glacier’s sides, snout or bed. Outwash is deposited by meltwater beyond the ice, so it is sorted, rounded and bedded, spreading as braided gravel plains or sandar in front of the terminal moraine.

What is India doing to reduce GLOF risk?

India follows the National Disaster Management Authority’s 2020 guidelines on GLOFs and, since July 2024, a ₹150 crore National GLOF Risk Mitigation Programme covering four Himalayan states. It inventories glacial lakes by satellite, ranks high-risk lakes, surveys them in the field, and is installing monitoring and early-warning stations, though none was fully commissioned by mid-2026.