Glacial Landforms: Erosional and Depositional

• A glacier is a mass of ice moving slowly downslope or outward under its own weight, formed where snowfall over many years exceeds melting.
  • The word comes from the French glace (ice); glaciers are often called “rivers of ice”.
• Glaciers now cover about one-tenth of the Earth’s land; during the Pleistocene they covered nearly one-third.
  • Glacial landforms therefore also occur where no ice exists today, as relics of past glaciation.
• As an exogenic agent, ice erodes the highlands and deposits in the lowlands, producing a distinctive suite of erosional, depositional and glacio-fluvial landforms.

Glaciers: Formation, Types and Movement

Snow-line and the Making of Glacier Ice

• The snow-line is the lower limit of permanent snow, the level where the mean temperature of the warmest month stays below freezing.
  • It lies at sea level in the polar regions and rises towards the tropics.
  • It is highest (above 6,000 m) in the dry subtropical Andes and Tibet, not at the equator, because low snowfall there matters more than temperature.
• Snow accumulating in snowfields above the snow-line is transformed step by step:
  • Fresh snow (fluffy, very low density) → granular snow (compacted by overlying snow).
  • Firn or névé: granular snow that has survived at least one melt season; daytime melting and night refreezing harden it.
  • Glacier ice: dense, bubble-poor ice formed by further compaction and recrystallisation.
• Once thick enough, the ice begins to flow downslope under gravity; this marks the birth of a glacier.
• A glacier has an accumulation zone (gain exceeds loss) above an ablation zone (melting, evaporation and calving exceed gain).
  • Its lower end is the snout, toe or terminus; it advances when accumulation exceeds ablation and retreats when ablation wins.

Types of Glaciers

• Glaciers are broadly mountain (valley) glaciers or continental glaciers (ice sheets).
  • Hans Wilhelmsson Ahlmann later grouped them morphologically into ice-sheet types, valley (Alpine) types and piedmont or shelf types, with eleven sub-types.

Type	Form	Example
Ice sheet	Dome over 50,000 km², buries relief, flows outward	Antarctica, Greenland
Ice cap	Smaller dome, under 50,000 km²	Iceland (Vatnajökull), Arctic Canada
Valley (Alpine) glacier	Tongue of ice confined by valley walls	Siachen, Gangotri, Gorner (Switzerland)
Piedmont glacier	Valley glaciers spreading and merging at the mountain foot	Malaspina (Alaska)
Ice shelf	Floating extension of an ice sheet, fixed to the coast	Ross, Ronne-Filchner
Cirque / niche glacier	Small ice body in a cirque or on a steep rock face	High Himalaya, Alps

• Ice sheets are the largest glaciers; their centre is the ice dome, from which ice radiates in all directions.
  • The Antarctic ice sheet covers about 14 million km², averages over 2 km in thickness and reaches about 4.9 km.
  • The Greenland ice sheet covers about 1.7 million km², roughly 80% of the island, and is over 3 km thick at its centre.
• Valley glaciers occur on every continent except Australia; they descend below the snow-line until melting stops them.
  • Himalayan and Karakoram glaciers are mostly of this type, many with thick debris cover on their lower tongues.
  • Siachen (about 76 km) is the longest glacier of the Karakoram; Gangotri (about 30 km), source of the Bhagirathi, is among the largest in the Indian Himalaya.
• By thermal regime, glaciers are warm-based (temperate), with ice at melting point and meltwater at the bed, or cold-based (polar), frozen to the bed.
  • Warm-based ice slides and erodes vigorously; cold-based ice moves mainly by creep and can even protect its bed.

Ice Ages and Floating Ice

• An ice age is a long period of low global temperature in which ice sheets and valley glaciers expand; it contains colder glacials and warmer interglacials.
  • The Earth is still in the Quaternary ice age; at the Last Glacial Maximum (about 26,000–19,000 years ago) ice sheets covered much of North America and northern Europe.
• Where ice sheets reach the sea they float as ice shelves; blocks that break off (calving) drift as icebergs.
  • Only about one-ninth to one-tenth of an iceberg’s mass shows above water.
  • Melting icebergs drop their frozen debris on the sea floor as ice-rafted debris.

Movement of Glaciers

• A glacier moves because the weight of overlying ice sets up shear stress, which depends on ice thickness and surface slope.
  • Hydrostatic pressure acts equally in all directions and does not drive flow; the shear stress makes ice crystals slip past one another.

Mechanism	Process	Typical of
Internal deformation (creep)	Ice crystals deform and slide within the mass	Cold-based glaciers; all glaciers to some degree
Basal sliding	Ice slides over a bed lubricated by meltwater	Warm-based glaciers
Extending and compressive flow	Ice stretches over steep bed sections and thickens over gentle ones	Valley glaciers with stepped beds

• Velocity is greatest at the centre and surface and least at the sides and bed, where friction retards the ice.
  • A straight line of stakes across a glacier bends downvalley into a curve over time.
• Most glaciers move a few metres to a few hundred metres a year; thicker, steeper and warm-based ones move faster.
  • Cold-based Meserve Glacier (Antarctica) moves only a few metres a year; Franz Josef (New Zealand) about 300 m a year.
  • Jakobshavn Isbræ (Sermeq Kujalleq), Greenland, one of the fastest, exceeded 17 km a year in 2012 before slowing after 2016.
• A glacier surge is a sudden, short-lived acceleration, many times the normal speed, often in Karakoram glaciers.
• Where ice passes over bends or steep slopes it cracks into crevasses; the bergschrund is the crevasse at the glacier head.

Glacial Erosion and Erosional Landforms

Processes of Glacial Erosion

• Clean ice is a weak eroder; it becomes powerful when rock debris frozen into its base acts as a tool.
• Plucking (quarrying): meltwater freezes into joints of the bedrock and the moving ice tears out whole blocks.
• Abrasion: debris-laden ice scratches, grinds and polishes the bed, leaving striations, grooves and fine rock flour.
• Frost shattering of valley walls above the ice supplies further debris, a link with periglacial processes.
• Glacierization means the covering of land by ice; glaciation means the work of that ice on the land.
• Protectionist vs erosionist debate:
  • Protectionists held that ice, softer than rock, shields its bed and only modifies pre-glacial forms.
  • Erosionists treat glaciers as potent agents that deepen and reshape valleys.
  • The current view is that both are right in part: cold-based ice protects, while warm-based, sliding ice erodes strongly.

Landforms of Mountain (Valley) Glaciation

Cirque (Corrie, Cwm)

• A cirque is an armchair- or amphitheatre-shaped, steep-walled hollow at a glaciated valley head.
  • Local names: corrie (Scotland), cwm (Wales), kar (Germany), botn (Norway).
• Its three components:
  • Headwall: near-vertical rock wall, often 600–900 m high.
  • Basin (floor): over-deepened by rotational ice flow.
  • Threshold (lip): a rock bar at the outer edge.
• Types: simple, compound, hanging and nivation cirques.
• Favourable conditions: widely spaced pre-glacial valleys, heavy snowfall and fairly uniform rock.
• After the ice melts, water ponded behind the lip forms a tarn (cirque lake), such as Hemkund in Uttarakhand.

Theory	Proponent	Main process
Bergschrund (basal sapping)	Willard Drake Johnson	Freeze-thaw at the foot of the bergschrund undercuts the headwall
Glacial protection	Edmund Johnston Garwood	Hollows made by frost and water before glaciation; ice later occupies and protects them
Cyclic (mountain glaciation)	William Herbert Hobbs (1910)	Hollows enlarge through a cycle; cirques recede into cols, arêtes and fretted uplands
Meltwater	William Vaughan Lewis	Meltwater entering the headwall joints aids freeze-thaw
Rotational slip	Later glaciologists	Rotational flow of ice over-deepens the basin by abrasion

• Bergschrund theory is criticised because most bergschrunds are small and do not reach the headwall.
• The current view combines nivation and frost-shattering at the start with rotational sliding and abrasion that deepen the basin.

• The bergschrund (rimaye) is a deep crack where moving ice pulls away from the ice still frozen to the headwall.
  • It opens widest in summer, when outflow is not replaced by fresh snow; several may form, and they are major obstacles to climbers.

Arêtes, Cols and Horns

• Cirque recession by headwall retreat carves the uplands between cirques:
  • Arête: a knife-edged ridge between two cirques cutting back on opposite sides; a saw-toothed arête is a serrate ridge.
  • Col: a saddle or pass where two opposed cirques breach the arête.
  • Horn (pyramidal peak): a triangular-faceted peak left where three or more cirques recede together, named after the Matterhorn; K2 and Everest are Himalayan examples.
• A partly dissected upland is a scalloped (biscuit-board) upland; full dissection gives a fretted upland.
  • The crest of arêtes and horns forms col-and-peak topography; arêtes radiating from one peak are star-fish arêtes.

U-shaped Valley (Glacial Trough)

• A glacial trough has steep, concave walls and a broad, flat floor, giving a U-shaped cross-section.
  • The ice cuts off the interlocking spurs of the former river valley, leaving truncated spurs.
  • Over-deepened sections later hold long ribbon (trough or finger) lakes.
• Most troughs are pre-existing river valleys widened and deepened by ice rather than valleys dug by ice alone.
• Indian examples: the Sonamarg and Liddar valleys of Kashmir, and the troughs of Zanskar and the upper Bhagirathi.

Hanging Valley

• A hanging valley is a tributary valley whose floor lies well above the main trough; its stream plunges as a waterfall.
• Two explanations:
  • Glacial erosion school: the larger trunk glacier over-deepens its valley far more than the smaller tributaries.
  • Glacial protection school: water deepened the main valley while ice-filled tributaries were protected from down-cutting.
    • Evidence cited: not every tributary hangs, and hanging valleys are common where tributaries rise higher than the main glacier.
• Hanging valleys give natural heads of water for hydroelectric power, as in Norway and the Alps.

Rock Basins, Glacial Stairways and Paternoster Lakes

• Ice excavates its bed unevenly, according to rock resistance, joint density and ice thickness.
  • Rock basins form where erosion is greatest, often below tributary junctions where ice thickens.
  • Rock steps mark bands of harder rock or fault scarps.
• A long sequence of steps separated by cliffs of 30–300 m is a glacial (giant) stairway.
• Lakes in the basins along a stairway, linked by one stream like beads, are paternoster lakes; recessional moraines can dam similar chains.

Fjord

• A fjord is a glacial trough drowned by the sea: long, narrow, steep-walled and very deep, often with hanging valleys.
  • A shallower threshold near the mouth, often a submerged terminal moraine or rock bar, separates the deep inner basin from the sea.
• Fjords need coastal uplands, pre-existing valleys along structural lines and thick seaward-moving ice.
• Three views of origin:
  • Troughs cut above sea level and later drowned by post-glacial sea-level rise.
  • Troughs cut below sea level by thick ice when the sea stood lower in the Pleistocene.
  • Tectonic submergence of coastal grabens later modified by ice.
• Norway, Greenland, Alaska, British Columbia, southern Chile and New Zealand have classic fjord coasts.

Landforms of Ice-sheet Scouring and Bedrock Forms

Roche Moutonnée

• A roche moutonnée is an asymmetric rock hummock: a smooth, gently sloping stoss (up-ice) side and a steep, rough lee (down-ice) side.
  • The stoss side is abraded and polished; the lee side is plucked, because pressure falls as ice passes over the crest and meltwater refreezes in the joints.
• The term was coined by Horace-Bénédict de Saussure (1786), for rocks resembling a resting sheep.
• Sizes range from a metre to whole hills; several occur in the glaciated valleys of Kashmir.
• Related smoothed bedrock forms are whalebacks (abraded on all sides) and rock drumlins.

Crag and Tail

• A crag and tail is a resistant rock mass (often a volcanic plug) with a steep crag facing the ice and a gentle tail of softer rock and debris in its lee.
  • It is larger than a roche moutonnée but reverses its profile: the steep face is up-ice.
  • Edinburgh’s Castle Rock and Royal Mile are the classic example.

Nunatak

• A nunatak is a peak or ridge projecting above an ice sheet or glacier, a “glacial island”, worn down slowly by frost and lateral ice erosion.

Glacial Transport, Deposition and Glacio-fluvial Landforms

Glacial Drift, Till and Erratics

• All material carried and laid down by ice and its meltwater is glacial drift.
• By position during transport, debris is supraglacial (on the surface), englacial (within the ice) or subglacial (at the bed).

Drift	Sorting and layering	Deposited by	Landforms
Lodgement (basal) till	Unsorted, compact, clay-rich	Ice plastering debris at its base	Ground moraine, drumlins
Ablation till	Unsorted, loose, fewer fines	Melting ice letting debris down	Hummocky moraine
Ice-contact stratified drift	Sorted, layered, often slumped	Meltwater in contact with ice	Eskers, kames, kame terraces
Outwash	Well sorted and bedded	Meltwater beyond the ice front	Outwash plain, valley train

• Till (boulder clay) mixes boulders, clay and rock flour; spread as a sheet it forms gently undulating till plains, whose fertility depends on the parent material.
• Erratics are boulders carried far from their source and left on unlike bedrock; balanced ones are perched blocks.
  • They trace the source and direction of ice movement, though large numbers hinder farming.

Moraines

• Moraines are ridges or sheets of till laid down directly by ice, classed by their position:

Moraine	Position	Form
Terminal (end)	Across the valley at the farthest snout	Crescent-shaped ridge, concave up-valley
Recessional	Behind the terminal moraine	Ridges marking halts during retreat
Lateral	Along the valley sides	Long, narrow, steep-sided ridges
Medial	Down the centre, below a confluence	Two inner lateral moraines merged
Ground	Spread over the valley floor	Undulating till sheet
Push	At an advancing snout	Bulldozed and folded older sediment

• V. K. Prest (1968) grouped moraines by their orientation to ice flow:
  • Transverse: end, recessional, push, ribbed and De Geer moraines.
  • Parallel: fluted and drumlinized ground moraine, lateral and medial moraines.
  • Non-oriented: ablation and disintegration moraines, often with knob-and-basin (hummocky) topography.
• Moraine ridges dam many Himalayan lakes; the failure of such dams at Chorabari (Kedarnath, 2013) and South Lhonak (Sikkim, 2023) shows the link to glacial lake outburst floods.

Drumlins

• Drumlins are smooth, oval hills of till, like an inverted spoon or half-buried egg, elongated in the direction of ice flow.
  • The blunt, steeper end faces up-ice; the tapering tail points down-ice.
  • Most are a few hundred metres to about 2 km long and several to tens of metres high.
• They occur in swarms (drumlin fields), giving “basket of eggs” topography with poorly organised drainage.
  • Classic fields lie in Canada, Ireland (Clew Bay), Finland, Sweden, and New York and Wisconsin in the USA.
• Views on their origin:
  • Reshaping of older moraines by a later ice advance.
  • Lodgement of till around obstacles beneath overloaded ice.
  • Moulding of till mounds by meltwater erosion.
  • Current view: deformation of a soft, water-saturated bed under sliding ice, with some drumlins cut by subglacial meltwater floods; no single mechanism explains all.

Feature	Drumlin	Roche moutonnée
Material	Till (depositional)	Bedrock (erosional)
Steep side	Up-ice	Down-ice (lee)
Gentle side	Down-ice tail	Up-ice (stoss)
Occurrence	Swarms on lowlands	Singly or in groups on bedrock

Glacio-fluvial Landforms

• Melting (ablation) at the snout releases meltwater streams that sort and bed the debris; their deposits are glacio-fluvial.
• Esker: a long, narrow, sinuous ridge of sand and gravel laid down by meltwater in tunnels beneath the ice, roughly parallel to ice flow.
  • Some run for over 100 km; in Canada and Finland they carry roads across swampy ground.
  • Other views link them to surface channels or to channels issuing from the snout; the subglacial-tunnel view is now standard.
  • Swellings along the ridge at intervals give beaded eskers.
• Kame: a mound of bedded sand and gravel dumped by meltwater at or near a stagnant ice margin.
  • Built as small fans on land they are cone kames; built into ice-marginal lakes they are delta kames.
  • Kame terraces form between the ice and the valley side; moulin kames fill holes in decaying ice.
• Kettle: a hollow left when a buried block of stagnant ice melts; filled with water it is a kettle lake.
  • Mounds and kettles together form kame-and-kettle (hummocky) topography.
• Outwash plain (sandur): coalescing fans of well-sorted sand and gravel beyond the terminal moraine, crossed by braided channels.
  • A confined outwash in a valley is a valley train; sandy outwash soils suit crops such as potatoes.

Glacial Cycle of Erosion

• William Morris Davis (1900, 1906) extended his cycle of erosion to mountain glaciation.

Stage	Main features
Youth	Cirques form in pre-glacial hollows and grow; arêtes and horns begin; hanging valleys faint
Maturity	Valley glaciers join trunk glaciers; hanging valleys, stairways and nunataks clear; cirques at maximum size and begin to merge
Old age	Peaks rounded and lowered; U-shaped troughs broad; moraines, drumlins, eskers, kames and outwash spread; relief subdued

• Criticism:
  • Glaciated areas were too inaccessible to test the cycle, and ice lacks a clear base level, so a “glacial peneplain” cannot be defined.
  • Glaciations are short and repeated, so landscapes are polycyclic mixtures of glacial, periglacial and fluvial forms.
  • It remains a descriptive model rather than a proven sequence.

Glaciers and Glacial Landforms of India

• India’s glaciers lie in the Karakoram, Ladakh-Zanskar, Himachal, Uttarakhand and Sikkim Himalaya.
  • Western glaciers are fed mainly by winter westerly snow; eastern ones by summer monsoon snow, which makes them more sensitive to warming.

Landform	Indian example
Valley glacier	Siachen, Gangotri, Zemu (Sikkim), Milam
U-shaped trough, hanging valley	Sonamarg, Liddar, Zanskar, upper Bhagirathi
Roche moutonnée	Glaciated valleys of Kashmir
Moraines, moraine-dammed lakes	Gangotri snout area, Chorabari, South Lhonak
Horn	K2 (Karakoram)

• Recent change:
  • Gangotri has been retreating since at least 1780, faster since the 1970s.
  • Hindu Kush Himalayan glaciers lost mass 65% faster in 2011–2020 than in the previous decade and could lose up to 80% of their volume by 2100 on current emissions.
  • The UN marked 2025 as the International Year of Glaciers’ Preservation and made 21 March the annual World Day for Glaciers.