Glaciers & Glacial Landforms

Ice is a geologic agent that carved Yosemite, built Long Island, and shaped almost every landscape at high latitudes.

Intro GeologyUni Year 1
⏱️ About 18 min
Glaciers & Glacial Landforms — illustration
Illustrative image (AI-generated).

During the last ice age, ice sheets up to 3 km thick buried much of North America and Europe. When they melted, they left behind scratched bedrock, U-shaped valleys, and ridges of dumped sediment that now form Long Island and Cape Cod. Glaciers are not just frozen water — they are geologic machines that erode, transport, and deposit on a continental scale.

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The big idea: Glaciers are masses of ice that move under their own weight, eroding by plucking and abrasion and depositing distinctive landforms. Alpine (valley) glaciers carve U-shaped valleys, cirques, horns, and arêtes; continental ice sheets reshape entire landscapes. A glacier's mass balance — the balance between accumulation and ablation — determines whether it advances or retreats.
🎯 By the end, you'll be able to
  • Distinguish alpine (valley) glaciers from continental ice sheets and describe their settings
  • Explain glacier mass balance, including the accumulation zone, ablation zone, and equilibrium line
  • Contrast the two flow mechanisms: internal plastic flow and basal slip
  • Identify glacial erosional and depositional landforms and explain how each forms
📎 Helpful to know first

Two kinds of ice

Glaciers form where snow accumulation exceeds melting over many years. There are two main types:

  • Alpine (valley) glaciers: Confined to mountain valleys, flowing downhill under gravity. They start in cirques (bowl-shaped hollows) and follow pre-existing valleys, carving them deeper and wider. Examples: glaciers in the Alps, the Himalayas, and Alaska.
  • Continental ice sheets: Vast, dome-shaped masses of ice covering large land areas, spreading outward in all directions. Today they exist only in Greenland and Antarctica, but during ice ages they covered much of North America and northern Europe.

Mass balance: the glacier's budget

A glacier is a system with inputs and outputs:

  • Accumulation: Snow and ice added, mostly at high elevations or high latitudes where temperatures stay low year-round.
  • Ablation: Loss of ice through melting, sublimation, and calving (ice breaking off into water).

The boundary between the accumulation zone and the ablation zone is the equilibrium line. Above this line, the glacier gains mass; below it, the glacier loses mass. If accumulation exceeds ablation, the glacier advances. If ablation wins, the glacier retreats.

✨ Retreat is a budget deficit, not a reversal
When a glacier 'retreats,' its terminus moves uphill, but the ice itself still flows downhill. Retreat means ablation at the toe outpaces the delivery of new ice from above — the glacier is shrinking from the bottom, not flowing backward.

How glaciers move

Glaciers move by two mechanisms:

  • Internal (plastic) flow: Under the weight of overlying ice, crystals deform and slide past one another. This is slow, continuous movement within the ice mass.
  • Basal slip: The entire glacier slides over its bed on a thin film of meltwater. This can be much faster than plastic flow and is responsible for surges in some glaciers.

Glacier speeds range from a few centimetres per day for sluggish ice sheets to more than 10 metres per day for fast-flowing outlet glaciers.

Glacial erosion: plucking and abrasion

Glaciers erode in two ways:

  • Plucking: The glacier freezes onto bedrock fragments and yanks them loose as it moves. This is most effective on fractured or jointed rock.
  • Abrasion: Rock fragments embedded in the glacier's base scrape the bedrock beneath, polishing and scratching it. The scratches are called striations and they point in the direction of ice flow.
⚠️ Rivers make V-shaped valleys; glaciers make U-shaped valleys
A common misconception is that glaciers carve sharp, V-shaped valleys. In fact, rivers cut V-shaped valleys with narrow bottoms and steep sides. Glaciers carve broad, U-shaped valleys with flat floors and steep walls. The glacier fills the entire valley and erodes the sides as well as the bottom, widening the channel into a characteristic U profile.
Cross-section comparison of a V-shaped river valley and a U-shaped glacial valley V-shaped — River narrow bottom steep sides steep sides U-shaped — Glacier broad, flat floor steep walls steep walls River vs glacial valley cross-sections
Rivers carve V-shaped valleys; glaciers carve U-shaped valleys by eroding the sides as well as the bottom.

Erosional landforms

Glaciers sculpt distinctive features:

  • Cirque: A bowl-shaped hollow at the head of a valley glacier, carved by plucking and frost wedging. After the glacier melts, the cirque may hold a tarn (small lake).
  • Horn: A sharp, pyramid-shaped peak formed where several cirques erode back into a mountain from different sides. The Matterhorn is the classic example.
  • Arête: A narrow, jagged ridge between two cirques.
  • U-shaped valley: A steep-walled, flat-floored valley carved by an alpine glacier.
  • Roche moutonnée: A smooth, polished bedrock bump with a gentle upstream side and a steep, plucked downstream side.

Depositional landforms

When glaciers melt, they deposit the sediment they have carried — collectively called drift. Landforms include:

  • Moraines: Ridges of till (unsorted, unstratified debris) deposited at the glacier's edge. Lateral moraines form along valley sides; medial moraines form where two valley glaciers merge; terminal moraines mark the furthest advance.
  • Eskers: Long, winding ridges of sand and gravel deposited by meltwater streams flowing inside or beneath the glacier.
  • Drumlins: Smooth, elongated hills of till shaped beneath flowing ice, with a steep stoss (upstream) end and a tapered lee end.
  • Outwash plain: A broad plain of stratified sand and gravel deposited by meltwater streams beyond the glacier's terminus.
Alpine glacial landscape with cirque, horn, arête, U-shaped valley, and moraines Horn Arête Cirque + Tarn U-shaped valley floor Lateral moraine Terminal moraine Esker Alpine glacial erosional and depositional landforms
Glacial erosional and depositional landforms: cirques, horns, arêtes, U-shaped valleys, moraines, eskers, and drumlins.

Check your understanding

1. Which valley shape is produced by glacial erosion?
Glaciers fill entire valleys and erode the sides as well as the bottom, producing broad, U-shaped valleys with flat floors and steep walls.
2. What is the equilibrium line on a glacier?
The equilibrium line separates the zone where the glacier gains mass (accumulation) from the zone where it loses mass (ablation).
3. What is the difference between plucking and abrasion?
Plucking is the freezing and lifting of bedrock fragments by the glacier. Abrasion is the scraping and polishing of the bed by rock debris carried at the glacier's base.
🔎 Related lessons
Mass Wasting Groundwater, Aquifers & Karst Deserts & Wind
✅ Key takeaways
  • Alpine glaciers occupy mountain valleys; continental ice sheets cover large landmasses.
  • Glacier mass balance compares accumulation (gain) to ablation (loss). The equilibrium line separates the two zones.
  • Glaciers move by internal plastic flow and basal slip; retreat means the terminus shrinks, not that ice flows backward.
  • Glacial erosion produces cirques, horns, arêtes, and U-shaped valleys (not V-shaped — those are river-cut).
  • Depositional landforms include moraines, eskers, drumlins, and outwash plains.
➡️ Ice and water are powerful sculptors, but where water is scarce, wind takes over. In deserts, wind erodes, transports, and deposits sediment through eolian processes — creating dunes, yardangs, and vast dust deposits that can travel halfway around the world.
Want to test yourself on this? Try the Science practice tests →
🎓 Go deeper: university courses & trusted references

Handpicked external material for this module — for when you want the full university treatment of surface processes.

External sites are listed for reference only. This course is independent and has no affiliation with, or endorsement from, the institutions named.