Glacial Processes & Landforms of Erosion
The Last Ice Age in the UK
At the peak of the last glacial period, around 20,000 years ago, ice covered approximately two-thirds of the UK. The ice sheet reached as far south as the Bristol Channel in the west and roughly the Humberside coast in the east. Everything north of a line from the Severn Estuary to the Vale of York — Scotland, most of Wales, northern England and all of Ireland — lay beneath ice up to 1 km thick in places. The Scottish Highlands were buried entirely, with only the highest peaks (nunataks) projecting above the glacier surface. By around 10,000 years ago the ice had retreated, leaving behind a landscape completely reshaped: the Lake District, Snowdonia, the Cairngorms and the Yorkshire Dales all carry the unmistakable signature of glacial erosion and deposition.
Glacial Processes: Weathering and Erosion
Freeze-thaw weathering (also called frost shattering) is the starting process. When temperatures fluctuate around 0°C, water in cracks freezes overnight and expands by about 9%, widening the crack. Repeated cycles shatter the rock into angular fragments called scree, which pile up at cliff bases and are collected and carried by glaciers as tools of erosion. On the exposed, ice-free peaks and ridges above the glacier — the nunataks — freeze-thaw was relentless.
Once the glacier is moving, two main erosion processes operate. Abrasion is like sandpapering: rock fragments embedded in the base of the moving glacier grind against the bedrock below, smoothing and scratching it. The scratches left in the bedrock are called striations and show the direction of ice movement. Plucking (also called quarrying) happens where meltwater at the base of the glacier freezes around jointed bedrock; as the glacier moves on, it tears away entire blocks of rock. Plucking is most effective on the downstream side of a rock obstacle where pressure is lower and refreezing occurs.
Abrasion smooths bedrock using embedded rock fragments (leaving striations); plucking tears away jointed blocks where meltwater refreezes on the downstream side. Both processes operate at the base of the glacier.
Glacial Movement: Rotational Slip and Bulldozing
Rotational slip is the movement responsible for creating corries — the characteristic armchair-shaped hollows on mountain faces. As ice accumulates in a pre-existing hollow, it begins to rotate rather than simply slide forward, so the base grinds deep into the rock floor while the ice pivots over the rim. Bulldozing (pushing) occurs at the glacier's snout: material that has been scraped or freeze-thaw shattered and fallen onto the glacier is pushed ahead of the moving ice like a bulldozer blade. Bulldozing is one way till accumulates at the snout as moraine. Till is deposited directly by the ice and is therefore unsorted — boulders and clay particles jumbled together. Outwash is deposited by meltwater streams beyond the glacier's snout; water sorts the sediment by size, so outwash is layered and gets finer with distance from the glacier.
Erosional Landforms: Corries, Arêtes and Pyramidal Peaks
A corrie (also cwm in Welsh, cirque in French) forms when snow accumulates in a sheltered hollow — usually on a north- or northeast-facing slope — and compacts into glacier ice. The glacier rotates by rotational slip, cutting the rock floor downward into a deep basin. Freeze-thaw and plucking attack the backwall, making it steep and high. The lip at the entrance to the corrie is eroded less because the rotating ice presses down less there. When the glacier melts, the basin fills with water to form a tarn (or corrie lake). Red Tarn, below Helvellyn in the Lake District, is a classic UK example.
When two corries form on opposite sides of a mountain, the backwalls erode towards each other, leaving a narrow, knife-edged rocky ridge between them — an arête. The characteristic jagged profile of Striding Edge on Helvellyn or Crib Goch in Snowdonia results from freeze-thaw shattering on both sides simultaneously. When three or more corries erode back into a mountain from different sides, the backwalls create a sharp, pyramid-shaped summit — a pyramidal peak. Snowdon in Wales and Ben Nevis in Scotland have forms shaped partly by this process; the classic European example is the Matterhorn in the Alps.
1. Snow accumulates in a sheltered hollow, usually on a north- or northeast-facing slope where sunlight is weakest. 2. Snow compacts under its own weight into glacier ice. 3. Rotational slip begins: the ice rotates, grinding the floor downward by abrasion and deepening the basin. 4. The backwall is attacked by freeze-thaw from above and plucking from the ice below, making it tall and steep. 5. The lip at the entrance is eroded less because ice pressure drops there. 6. When the ice melts, the basin holds a tarn. Red Tarn on Helvellyn is the result of this sequence.
Erosional Landforms: Glacial Troughs, Ribbon Lakes and Hanging Valleys
As glacier ice flows down from its source and fills pre-existing river valleys, it transforms their shape dramatically. Rivers cut V-shaped valleys; glaciers cut glacial troughs — U-shaped valleys with broad, flat floors and steep, near-vertical sides. The glacier erodes both the floor (by abrasion and plucking) and the sides, removing the interlocking spurs that a river would weave between. These truncated spurs are cut off, leaving sheer cliff faces on the valley walls — the sheer walls of Borrowdale in the Lake District or Nant Gwynant in Snowdonia are truncated spurs.
The flat floor of the glacial trough is often not perfectly flat — the glacier over-deepens sections where the rock is weakest. When the ice retreats, these over-deepened sections fill with water to form ribbon lakes: long, narrow and deep. Windermere in the Lake District is England's longest lake (18 km) and reaches a depth of 64 m — a ribbon lake in a classic glacial trough. Smaller tributary glaciers flow into the main glacier from the sides but cannot erode as deeply because they carry less ice. When the ice melts, these tributary valleys are left hanging high above the main trough floor — a hanging valley. The stream from the hanging valley drops down to the main valley as a waterfall. Aira Force waterfall in the Lake District falls from a hanging valley into the Ullswater trough.
Glacial troughs are U-shaped valleys with flat floors and steep sides. England's longest ribbon lake is Windermere (18 km, 64 m deep). Hanging valleys form where smaller tributary glaciers could not erode as deeply as the main glacier — their streams now fall as waterfalls into the trough below.
A river is only as wide as its water and erodes only the narrow channel floor, creating a V-shape. A glacier fills the entire valley and exerts enormous pressure on the floor and both sides simultaneously. Abrasion and plucking widen the floor and straighten the walls, while the interlocking spurs (the zigzagging ridges between which the river wound) are cut through or planed off entirely. The result is a broad, straight valley with a flat floor — a U-shape. After the ice retreats, a small river (called a misfit stream) may flow along the vast flat floor, looking comically small for the valley it occupies.