Glacial Landscapes: How Ice Shapes Mountains and Valleys

Cold environments and the glacier system

Cold environments are places where ice and frozen ground shape the land. There are four kinds:

• Polar: near the poles, like Antarctica and Greenland. Huge ice sheets.
• Alpine: high mountains anywhere, like the Himalaya, Alps and Andes.
• Glacial: land covered by moving ice.
• Periglacial: land next to ice, where the ground stays frozen (permafrost) and only the top thaws in summer.

Winters are long and very cold. Soils are thin and poor. Plants are low: mosses, lichens, small shrubs (tundra).

About 20,000 years ago, in the last cold stage of the Pleistocene (the "Ice Age"), ice covered much of northern Europe and North America. Many landscapes we see today were made then.

A glacier is a system

A system has inputs, stores and outputs.

• Inputs: snow, avalanches, rock falling on the ice.
• Store: the ice itself.
• Outputs: melting, evaporation, calving (ice blocks breaking off), meltwater.

The top part where more snow is added than lost is the zone of accumulation. The lower part where more is lost is the zone of ablation. The line between them is the equilibrium line.

The glacier budget = accumulation − ablation. Positive budget: the snout advances. Negative: it retreats. Zero: it stays still. Ice still flows downhill even when the snout retreats.

Feedback: when a glacier shrinks, darker rock is exposed, it absorbs more heat, and more ice melts. This is positive feedback (it speeds up the change).

How ice erodes, moves and drops rock

Weathering and erosion

• Freeze-thaw weathering: water gets into a crack, freezes and expands by about 9%. Repeated many times, it splits the rock. Loose pieces collect as scree.
• Plucking: meltwater freezes onto the rock under the glacier. As the ice moves, it tears out blocks.
• Abrasion: rocks stuck in the base of the ice scrape the bedrock like sandpaper, leaving scratches called striations.

How ice moves

• Basal sliding: a thin film of meltwater lets the glacier slide over its bed.
• Internal flow: ice crystals slowly deform and slip past each other under their own weight.
• Rotational movement: in a corrie, ice turns like a spoon scooping out the hollow.

Transport and deposition

Ice carries rock on top, inside and below it. When it melts, it drops this load. Deposits from ice are unsorted (big and small mixed). Deposits from meltwater are sorted (in layers by size).

Erosional landforms

• Corrie (also cirque or cwm): an armchair-shaped hollow with a steep back wall and a lip. Snow collects in a north-facing hollow (in the northern hemisphere), turns to ice, and rotational movement deepens it. After the ice melts, a small lake called a tarn may fill it.
• Arête: a knife-edge ridge between two corries that eroded back to back.
• Pyramidal peak: a sharp peak where three or more corries cut into one mountain. The Matterhorn in the Alps is a famous example.
• U-shaped (glacial) trough: a wide valley with steep sides and a flat floor, carved when a glacier fills an old river valley.
• Truncated spurs: ridges of land that the ice cut off, leaving steep cliffs along the trough.
• Hanging valley: a small side valley left high above the main trough, often with a waterfall.
• Ribbon lake: a long, narrow lake in a deeper part of the trough floor where softer rock was eroded more.
• Roche moutonnée: a rock smoothed by abrasion on the up-stream side and made jagged by plucking on the down-stream side.
• Fjord: a U-shaped valley flooded by the sea after the ice melted (Norway, New Zealand, Chile).

Depositional and periglacial landforms

Dropped by ice (till)

• Till: unsorted clay, sand and boulders dropped by ice.
• Moraine: a ridge of till. Lateral moraine forms along the sides, medial where two glaciers join, terminal at the furthest point the snout reached, and recessional where the snout paused while retreating.
• Drumlin: an egg-shaped hill of till. The steep, blunt end faces where the ice came from; the gentle end points the way the ice moved. Drumlins often occur in groups.
• Erratic: a boulder carried far and dropped on different rock. It shows the direction of old ice flow.

Built by meltwater (sorted)

• Outwash plain: a flat spread of sorted sand and gravel in front of the snout.
• Esker: a long, winding ridge of sand and gravel left by a river that flowed in a tunnel under the ice.
• Kame: a mound of sorted material dropped at the ice edge or in hollows on the ice.

Periglacial landforms

• Permafrost: ground frozen for at least two years in a row. Only the top active layer thaws in summer.
• Ice wedges: cracks in the frozen ground fill with water that freezes; year after year the wedge grows.
• Patterned ground: frost pushes stones up and sideways, sorting them into circles or polygons.
• Pingo: a dome-shaped hill with an ice core that pushes the ground up (up to about 50 m high).
• Solifluction: the wet active layer slowly slides downhill over the frozen ground.

Process, time and landscape: a glaciated valley is the work of many cold stages. Each landform is a clue to how ice moved, how thick it was and when it retreated.

People in glaciated uplands and cold environments

Land use

• Farming: mostly sheep and cattle on thin soils; crops only on flat valley floors.
• Forestry: conifers on steep valley sides.
• Quarrying: hard rock for building and roads.
• Water and energy: ribbon lakes and dams store drinking water and make hydroelectric power.
• Tourism: hiking, skiing, climbing and sightseeing.

Tourism pressures and management

Many visitors bring jobs and money, but also traffic jams, footpath erosion, litter and high house prices for locals. Managers build stone paths, park-and-ride buses, visitor centres and set limits on numbers.

Fragile cold environments

Plants grow very slowly in the cold, so damage lasts for decades. Plants and animals adapt: low cushion plants, thick fur, fat layers, migration. Opportunities include minerals, oil and gas, fishing and tourism. Challenges include extreme cold, permafrost that moves under buildings, and remoteness. Pipelines are raised on stilts so their heat does not melt permafrost.

Climate change

Most glaciers are shrinking. This raises sea level, changes river flow (more floods at first, then less water), and can cause glacial lake outburst floods. Thawing permafrost releases greenhouse gases, adding to warming (positive feedback). International agreements, protected areas and cutting emissions all help.

Try it: make your own glacier

1. Mix a handful of sand and small stones with water in a small plastic tub and freeze it overnight. This is your "glacier" with rocks in its base.
2. Press it onto a bar of soap or a block of soft clay and push it slowly along. Look at the scratches: this is abrasion making striations.
3. Leave it to melt on a tray tilted slightly. Notice the stones dropped in a mixed heap (till) and the sand washed further away in layers (outwash).

Then use the slider in the 3D step 6 to predict: if summers warm by 2 °C, does the snout move forward or back?

Key formulas and definitions

• Glacier budget = accumulation − ablation
• Positive budget → snout advances; negative → snout retreats
• Accumulation zone: more snow gained than lost
• Ablation zone: more ice lost than gained
• Equilibrium line: where gain = loss
• Water expands about 9% when it freezes (freeze-thaw)
• Till = unsorted (dropped by ice); outwash = sorted (dropped by meltwater)
• Permafrost = ground frozen for 2+ years in a row

Worked examples

Common mistakes

• Thinking a retreating glacier is flowing backwards. The ice still moves downhill; the snout retreats because melting is faster than flow.
• Mixing up till and outwash. Till is unsorted (ice); outwash is sorted in layers (meltwater).
• Calling every lake in mountains a ribbon lake. A tarn sits in a corrie; a ribbon lake is long and lies in a trough floor.
• Drawing a drumlin the wrong way round. The steep blunt end faces up-ice; the gentle tail points down-ice.