Glacial Processes and Landforms of Erosion

How ice shaped the UK

Many British uplands contain broad valleys, sharp ridges and mountain hollows that rivers alone cannot explain. These landscapes were shaped by glaciers: large masses of ice that move slowly under gravity. Snow accumulated over many years and was compressed into glacial ice. Where ice spread across a large area, it formed an ice sheet.

The last ice age reached its peak about 20,000 years ago and ended about 11,500 years ago. At its maximum extent, ice covered Scotland, Northern Ireland, Wales and northern England, reaching into the Midlands. Ice did not cover the whole UK. Its former extent helps explain why glacial landscapes are especially widespread in northern and western upland Britain.

Glaciers changed the landscape through four connected processes: weathering broke rock apart; erosion removed and wore away rock; transportation moved the material; and deposition left it behind.

Freeze-thaw weathering supplies loose rock

Water enters cracks in exposed rock. When the temperature falls below freezing, the water freezes and expands, putting pressure on the sides of the crack. When temperatures rise, the ice thaws and water can enter again. Repeated cycles widen the crack until fragments break away.

This is weathering, because the rock is broken down in place. It is particularly effective where temperatures repeatedly rise above and fall below freezing, rather than remaining permanently below freezing. The fragments can fall onto a glacier and become material that it transports or uses to erode other rock.

Moving ice erodes and transports material

A glacier moves downhill under gravity. It carries rock and sediment on its surface, within the ice and at its base. Two important processes erode the bedrock beneath and beside it.

Plucking occurs when meltwater freezes onto rock, including rock loosened along cracks. As the glacier moves, the attached ice pulls pieces away. This can leave steep, rough rock faces.

Abrasion occurs when fragments embedded in the moving ice scrape and grind against bedrock. Imagine coarse sandpaper pressed firmly against a surface: the glacier's weight presses its rocky load against the valley floor and sides, wearing them away.

Within a mountain hollow, ice can move along a curved path called rotational slip. This helps concentrate erosion in the hollow. At the glacier's front, or snout, moving ice can also push loose debris ahead of it. This is bulldozing: material is transported by being pushed, rather than only carried inside the ice.

Why glaciers leave sediment behind

When glacial ice melts, the material it was carrying is released and deposited. Sediment deposited directly by ice is called till. It is unsorted: fine clay, sand, stones and large boulders can all be mixed together because the ice carries them together rather than separating them by size.

Meltwater streams also carry sediment away from the glacier. When their energy decreases, they deposit this material as outwash. Flowing water sorts sediment more effectively than ice, so outwash is usually arranged in layers of different particle sizes. The important distinction is direct deposition by ice versus deposition by meltwater.

Corries: erosion begins in a mountain hollow

A corrie, also called a cwm or cirque, is an armchair-shaped hollow with a steep back wall, a deep basin and a raised lip at its front. Cwm Idwal in Wales is an example.

Snow collects in a sheltered hollow, often on a north-facing slope where it melts less readily. Over time, accumulated snow becomes ice. As the ice thickens, it moves by rotational slip.

Freeze-thaw weathering loosens rock around the hollow. Plucking removes rock from the back wall, making it steeper, while abrasion deepens the basin. Together, these processes enlarge the original hollow into a corrie. A raised lip remains at its outer edge. After the glacier melts, water may collect behind this lip to form a small lake called a tarn; Red Tarn in the Lake District is an example.

Arêtes and pyramidal peaks link neighbouring corries

When two corries erode back towards one another, the ridge between them becomes narrower and steeper. The result is an arête: a sharp, knife-edge ridge. Striding Edge in the Lake District is an example.

Where three or more corries erode back towards a mountain summit from different directions, they leave a pointed pyramidal peak. Snowdon in Wales is an example. These features are connected: corrie erosion shapes both the hollows and the ridges or peaks left between them.

Glacial troughs and truncated spurs

A valley glacier often follows an existing V-shaped river valley. Unlike a river, which concentrates much of its erosion in a narrow channel, a glacier fills much of the valley and erodes its floor and sides. Abrasion and plucking deepen and widen it, producing a glacial trough with steep sides and a broad, relatively flat floor. Its cross-section is U-shaped. Nant Ffrancon in Wales is an example.

Before glaciation, a river may have wound around interlocking spurs: projecting ridges on alternate sides of the valley. The much larger glacier cuts through their ends as it moves down the valley. These cut-off ridges become truncated spurs, with steep, cliff-like faces.

Hanging valleys and ribbon lakes

Smaller tributary glaciers can join a larger main-valley glacier. The larger glacier erodes its valley more deeply, leaving the tributary valley floor at a higher level. When the ice melts, the tributary valley appears to hang above the main trough: a hanging valley. A stream flowing out of it may fall over the steep drop as a waterfall. Cwm Dyli in Wales is an example.

Erosion also varies along a valley. Where a glacier crosses less resistant rock, it can erode a deeper basin than in neighbouring harder rock. Harder rock may remain as a rock step. After the glacier melts, water fills the elongated basin, creating a ribbon lake. Windermere in the Lake District is an example.

A ribbon lake therefore occupies a long valley basin, whereas a tarn occupies the smaller hollow of a corrie. Both are water-filled depressions left after glacial erosion, but their settings and shapes differ.

Ice cover and processes

• Maximum extent: about 20,000 years ago; Scotland, Northern Ireland, Wales and northern England, extending into the Midlands—not the whole UK.
• Freeze-thaw: water enters cracks → freezes and expands → repeated cycles break rock apart.
• Plucking: ice freezes onto rock → moving glacier pulls it away.
• Abrasion: embedded fragments scrape and grind bedrock.
• Movement: gravity moves ice downhill; rotational slip is curved movement in a hollow.