Glacial Landscapes (Optional Context)

Glacial landscapes are an optional part of the AQA GCSE Geography specification for Physical Landscapes in the UK. If your school has chosen this option, you need to understand the processes of glacial erosion and deposition, and the distinctive landforms they create. Even if you are not studying this as your optional topic, understanding glaciation helps explain the broader UK physical landscape.

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What Is Glaciation?

Glaciation refers to the shaping of the landscape by glaciers and ice sheets. During the Pleistocene epoch (roughly 2.6 million to 10,000 years ago), the UK experienced several glacial periods (ice ages) when ice sheets covered much of Scotland, Wales, northern England, and the Midlands.

The most recent glacial maximum occurred approximately 20,000 years ago, when ice covered the UK as far south as a line roughly from the Bristol Channel to the Wash.

How glaciers form

1. Snow accumulates in a hollow on a mountainside.
2. Over many years, the weight of accumulated snow compresses the lower layers into firn (compacted, granular snow).
3. Further compression turns firn into glacier ice — dense, blue ice with air squeezed out.
4. When the ice becomes thick enough (typically 30-50 metres), it begins to flow downhill under its own weight due to gravity.

How glaciers move

Glaciers move by two main mechanisms:

Mechanism	Description
Basal sliding	The base of the glacier melts slightly due to pressure and friction with the bedrock. The glacier slides over a thin film of meltwater. This is the main mechanism of movement.
Internal deformation	Individual ice crystals within the glacier deform and flow under pressure, like a very slow-moving liquid. The surface moves faster than the base (due to friction).

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Glacial Erosion Processes

Glaciers erode the landscape through two main processes:

Process	Description
Plucking	As the glacier moves, meltwater at its base seeps into cracks in the bedrock and refreezes, bonding to the rock. As the glacier moves forward, it plucks (pulls) chunks of rock away from the bedrock. This is most effective on the downstream side of outcrops, where the ice pulls away from the rock.
Abrasion	Rocks and debris frozen into the base of the glacier scrape across the bedrock like sandpaper, wearing it smooth and creating striations (scratches). The debris acts as an abrasive tool.

Freeze-thaw weathering

Freeze-thaw weathering works alongside glacial erosion:

1. Water seeps into cracks in rock exposed above the glacier.
2. At night or during cold spells, the water freezes and expands by approximately 9%.
3. The expansion widens the crack.
4. Repeated cycles of freezing and thawing shatter the rock into angular fragments called scree or frost-shattered debris.
5. This debris falls onto or into the glacier and can be used as a tool for abrasion.

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Erosional Landforms

Corries (Cirques / Cwms)

A corrie is an armchair-shaped hollow on a mountainside, with steep back and side walls and a flat or scooped-out floor. It is the birthplace of a glacier.

How a corrie forms:

1. Snow accumulates in a north-facing hollow (shaded from the sun, so snow persists).
2. The snow compresses into firn and then glacier ice.
3. The glacier erodes the floor of the hollow by plucking and abrasion, deepening it.
4. Freeze-thaw weathering steepens the back wall above the glacier (the bergschrund — the gap between the glacier and the back wall — allows freeze-thaw to operate).
5. The glacier rotates within the hollow due to gravity, deepening the floor further.
6. When the ice melts, a corrie lake (or tarn) may form in the hollow, often dammed by a lip of moraine or resistant rock at the front.

UK Example: Red Tarn on Helvellyn in the Lake District is a classic corrie lake.

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Aretes and Pyramidal Peaks

• An arete is a narrow, knife-edge ridge formed when two corries erode back-to-back on either side of a ridge. The ridge between them becomes progressively narrower and sharper.
• A pyramidal peak is a pointed, horn-shaped mountain formed when three or more corries erode around a single peak. The peak is sharpened from all sides.

UK Example: Striding Edge on Helvellyn is a famous arete. Snowdon in Wales has a pyramidal peak shape (though modified by later erosion).

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Glacial Troughs (U-Shaped Valleys)

A glacial trough is a wide, flat-floored valley with steep sides, carved out by a glacier. It has a distinctive U-shape (compared with the V-shape of a river valley).

How a glacial trough forms:

1. A glacier fills a pre-existing river valley.
2. The glacier is much larger and more powerful than the river, so it erodes the valley floor by abrasion and plucking, deepening it.
3. The glacier also erodes the valley sides, widening them and making them steeper.
4. The interlocking spurs of the original river valley are truncated (cut off) by the glacier, leaving truncated spurs with steep, cliff-like faces.
5. When the glacier melts, the valley is left with a flat floor and steep, almost vertical sides — the classic U-shape.

UK Example: Borrowdale and Langdale in the Lake District are classic glacial troughs.

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Ribbon Lakes

A ribbon lake is a long, narrow lake that fills the floor of a glacial trough. It forms in areas where the glacier eroded more deeply, either because:

• The rock was softer and more easily eroded, or
• The glacier was thicker or more confined (e.g., where tributary glaciers joined the main glacier).

UK Example: Lake Windermere (the largest natural lake in England at 17 km long) is a ribbon lake in a glacial trough.

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Hanging Valleys

A hanging valley is a smaller, shallower valley high up on the side of a glacial trough. It forms because:

1. A tributary glacier occupies a smaller side valley.
2. The tributary glacier has less erosive power than the main glacier.
3. The main glacier erodes its valley much more deeply than the tributary glacier.
4. When both glaciers melt, the tributary valley is left "hanging" above the main valley floor.
5. A stream from the hanging valley often descends to the main valley floor as a waterfall.

UK Example: The valley above Lodore Falls in Borrowdale (Lake District) is a classic hanging valley.

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Depositional Landforms

When a glacier melts (retreats), it deposits the material it has been carrying. This material is called glacial till (or boulder clay) — an unsorted mixture of clay, sand, gravel, and boulders.

Moraines

Moraine is the general term for material deposited by a glacier. There are several types:

Type	Location	Description
Lateral moraine	Along the sides of the glacier	Material that has fallen from the valley sides onto the edges of the glacier. Forms a ridge along the valley sides after the glacier melts.
Medial moraine	Down the centre of the glacier	Forms when two glaciers merge and their lateral moraines combine in the middle. Visible as a dark stripe down the centre.
Terminal moraine	At the snout (end) of the glacier	A ridge of material deposited at the furthest point the glacier reached. Marks the maximum extent of the glacier.
Ground moraine	Under the glacier	Material deposited across the valley floor as the glacier retreats. Creates an undulating, hummocky landscape.
Recessional moraine	Between terminal moraine and source	Ridges formed during temporary pauses in glacier retreat.

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Drumlins

A drumlin is a smooth, elongated, egg-shaped hill made of glacial till. Drumlins have a steep end (stoss) facing the direction the glacier came from and a gentler, tapering end (lee) pointing in the direction the glacier moved.

• Drumlins usually occur in groups called swarms or drumlin fields.
• They are typically 25-50 metres high and several hundred metres long.

UK Example: The Eden Valley in Cumbria and the area around Ribblehead in the Yorkshire Dales contain excellent drumlin fields.