Glaciation and Glacial Landforms: KS3 Geography

A glacier is a slow-moving ice mass that forms when snowfall exceeds snowmelt over many years. As it moves downhill under gravity, a glacier erodes the landscape powerfully, carving distinctive landforms — corries, arêtes, U-shaped valleys, drumlins — that still shape the uplands of Scotland, Wales, and the Lake District.

How do glaciers form and move?

Glaciers begin in hollows on high mountain slopes where snow accumulates year-round. As snow layers build up, the weight of overlying snow compresses lower layers into dense glacial ice (firn or névé in intermediate stages). Once the ice mass is thick enough — typically 30–40 metres — it begins to flow downhill under gravity.

Glaciers move in two ways:

Internal deformation: ice crystals at the base of the glacier deform and flow plastically under pressure, like extremely slow-moving putty.

Basal sliding: meltwater at the base of the glacier (created by the pressure of the overlying ice, which lowers the melting point of ice) acts as a lubricant, allowing the glacier to slide over the bedrock.

Glaciers move slowly — typically 1–3 metres per day, though some outlet glaciers in Greenland can move 20–30 metres per day. This movement is invisible to the human eye but, over thousands of years, produces dramatic landscape changes.

What erosion processes do glaciers use?

Glaciers erode the landscape through two main processes.

Plucking (quarrying): Meltwater at the base of the glacier seeps into cracks in the bedrock and refreezes, bonding the ice to the rock. As the glacier moves forward, it pulls (plucks) fragments of rock away from the bedrock. Plucking tends to produce rough, jagged surfaces on the downstream side of rock obstacles.

Abrasion: Rock fragments embedded in the base of the glacier act like sandpaper, grinding and scratching the bedrock beneath. This produces a smoothed, striated (scratched) surface on bedrock and creates glacial flour — finely ground rock powder that gives glacial meltwater its characteristic milky blue-green colour. Abrasion is most effective where the glacier carries large amounts of embedded rock.

The material picked up and transported by a glacier is called moraine. Moraine carried on the glacier's surface is called lateral moraine; moraine at the front of the glacier is terminal moraine.

What upland glacial landforms are created by glaciers?

Corrie (Cwm / Cirque)

A corrie is a bowl-shaped hollow cut into a mountainside, open on one side and enclosed by steep, curved "armchair" walls (the backwall and headwall). Corries form where snow accumulates in a pre-existing hollow on a slope. Freeze-thaw weathering breaks up the rock at the back of the hollow; plucking removes it; abrasion deepens the floor. The rotational movement of the ice deepens the centre of the hollow more than its lip, often leaving a rock lip at the front. When the glacier melts, this hollow may fill with water to form a small circular lake called a tarn (or corrie loch in Scotland).

UK example: Red Tarn on Helvellyn in the Lake District, Glaslyn on Snowdon in Wales.

Arête

Where two corries erode back-to-back on either side of a ridge, the ridge between them is gradually narrowed into a narrow, knife-edged ridge called an arête. Striding Edge on Helvellyn is a classic example.

Pyramidal peak

Where three or more corries erode back into a mountain from different sides, the remaining summit is sharpened into a steep-sided pyramidal peak (sometimes called a horn). The Matterhorn in the Alps is the world's most famous example; in the UK, Snowdon shows this form.

Landform | How it forms | UK example
Corrie | Rotational erosion in a sheltered hollow | Red Tarn, Lake District
Tarn | Lake left in a corrie hollow after glaciers melt | Glaslyn, Snowdon
Arête | Narrow ridge between two back-to-back corries | Striding Edge, Helvellyn
Pyramidal peak | Summit sharpened by 3+ corries eroding from different sides | Snowdon summit
U-shaped valley | Former river valley deepened and widened by a valley glacier | Nant Ffrancon, Snowdonia
Hanging valley | Tributary glacier valley left high above the main valley floor | Numerous examples in the Alps

U-shaped valley (glacial trough)

A river valley has a V-shape, carved by a river eroding downward. When a glacier occupies the same valley, it erodes much more powerfully — both downward (deepening the floor by abrasion) and sideways (widening the walls by plucking). The result is a valley with a flat floor and steep, straight sides: a U-shaped valley or glacial trough.

The shape contrasts sharply with the interlocking spur pattern of a river valley. Where tributaries once joined the main river, their smaller glaciers eroded to a lesser depth, leaving hanging valleys — tributary valleys that enter the main valley high up on its sides. Waterfalls often cascade down from hanging valleys into the main glacial trough.

What lowland landforms do glaciers deposit?

As a glacier melts, it deposits the material it has been carrying. These depositional landforms are found in lowland areas beyond the glaciers' maximum extent.

Till (Boulder clay): Unsorted mixture of clay, sand, and rocks of all sizes deposited directly from melting ice. The North Sea plain and much of lowland England south of a line from the Thames estuary to the Severn Estuary was covered by glacial till during the last ice age — the foundation of much of England's agricultural soil.

Drumlins: Smooth, oval, whale-backed hills of till, streamlined in the direction of ice flow. Their upstream end is steeper (the "stoss" end) and the downstream end tapers gently (the "lee" end). Drumlins typically occur in "swarms" — fields of hundreds of aligned hills. Parts of the Eden Valley in Cumbria display classic drumlin fields.

Erratics: Boulders transported by glaciers and deposited far from their origin, sometimes on completely different rock types. The Norber Erratics in the Yorkshire Dales — dark Silurian greywacke boulders resting on pale limestone — are a famous example. They were carried by ice from a valley about 2 km to the north.

Frequently asked questions

What was the last ice age and when did it end?

The most recent major glacial period in Britain is called the Devensian glaciation. At its maximum extent, around 20,000–25,000 years ago, ice covered all of Scotland, Wales, most of England north of the Thames, and much of Ireland. The glaciers began melting approximately 15,000 years ago, and by around 10,000 years ago Britain was largely ice-free — though a brief cold snap called the Younger Dryas (around 12,900–11,700 years ago) caused a temporary re-advance of glaciers in Scotland. The landscapes of upland Britain — the Lake District, Scottish Highlands, Snowdonia, and the Pennines — were shaped almost entirely by glacial processes during this period.

Why is it important to understand glacial landforms for geography?

Glaciation shaped the landscape of most of upland Britain. Understanding why Snowdon looks the way it does — its pyramidal summit, the corrie lakes on its flanks, the U-shaped valleys radiating from it — requires understanding glacial processes. The British Geological Survey publishes detailed maps showing glacial deposits across the UK, which are directly relevant to practical decisions about land use, flooding, and building foundations. Glacial processes are also an active area of study today because glaciers worldwide are retreating rapidly due to climate change.

How do geographers study glaciers today?

Modern glaciologists use a range of techniques: satellite imagery to measure changes in glacier area over time; ground-penetrating radar to measure ice thickness; GPS sensors to measure glacier movement speed; ice cores drilled from deep within glaciers, which preserve records of past climate (trapped air bubbles contain ancient atmosphere). The British Antarctic Survey and the British Geological Survey both contribute to this research. The data they collect shows that glaciers worldwide have been losing mass at an accelerating rate since the mid-twentieth century.

What is the difference between a glacier and an ice sheet?

A glacier is a valley or mountain ice mass, typically hundreds of metres to a few kilometres wide. An ice sheet is an enormous dome of ice covering a continental-scale area — the two present-day ice sheets are in Antarctica and Greenland. Ice sheets can be kilometres thick and flow outward from a central dome. During ice ages, ice sheets covered much of the northern hemisphere. Valley glaciers form within the ice sheet system or independently in mountain regions at lower latitudes.

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