Glacial Erosional Features: Corries, Arêtes, and U-Valleys

Glacial erosional features are landforms carved by moving ice, including corries (armchair-shaped hollows), arêtes (sharp ridges), pyramidal peaks, U-shaped valleys, hanging valleys, truncated spurs, ribbon lakes and fjords. In brief: glaciers erode by plucking (ripping out blocks of rock that are frozen to the ice) and abrasion (scraping by rock fragments embedded in the ice). A V-shaped river valley becomes a broad, steep-sided U-shaped valley when it is filled with a glacier. Where two corries meet back to back, they leave an arête, and where three or more form around a summit, they create a pyramidal peak such as the Matterhorn.

These landforms are the most striking signs of past and present glaciation in mountains and coasts. This guide explains the erosion processes, describes each landform and how it forms, compares them in a table, works through some calculations, outlines the history of their study and corrects common mistakes.

How do glaciers erode?

Plucking happens when meltwater seeps into cracks in the bedrock and freezes onto the glacier. As the ice moves, it pulls away blocks of rock. Abrasion occurs when the rock pieces carried in the base of the ice act like sandpaper, polishing the surface and leaving scratches called striations. Freeze-thaw weathering above the ice breaks up cliffs, and the debris falls onto the glacier and is carried away. Erosion is most effective in warm-based glaciers that move fast, where there is plenty of rock debris and the rock is jointed or weak.

What landforms form in the uplands?

Corries

A corrie (cirque in North America, cwm in Wales) is an armchair-shaped hollow with a steep back wall, a flat or hollowed floor and often a raised lip. It starts as a small hollow in which snow collects and turns into ice. The ice moves in a rotational way, plucking the back wall and abrading the floor, and the hollow deepens. When the ice melts, a round lake, called a tarn, often fills the basin, as at Cwm Idwal in Snowdonia.

Arêtes and pyramidal peaks

When two corries form on opposite sides of a ridge and erode backward, the ridge between them becomes narrow and jagged, which is an arête, as with Striding Edge on Helvellyn in the English Lake District. If three or more corries erode back around a mountain top, they leave a steep, pointed pyramidal peak (horn), such as the Matterhorn in the Alps or Mount Everest in its upper parts.

How are glacial valleys formed?

A river valley is usually V-shaped and winding. A glacier is thick and powerful, and it cuts the sides and floor, straightening and deepening the valley into a U-shaped (glacial trough) with a wide, flat floor and steep sides. Yosemite Valley in California is a famous example. Associated features include:

• Truncated spurs: the ends of interlocking ridges (spurs) that stuck out into the old river valley are cut off by the glacier, leaving steep cliffs.
• Hanging valleys: smaller tributary valleys with smaller glaciers are left high above the main valley, ending in steep drops, where streams now form waterfalls, such as Bridalveil Fall in Yosemite.
• Ribbon lakes: long, narrow lakes that occupy a hollow deepened by the glacier in an area of weak rock or where it was joined by another glacier, such as Windermere in the Lake District.
• Misfit streams: small rivers on the floor of a large glacial valley, which are too small to have formed it.

What other erosional features occur?

Fjords are deep, narrow inlets of the sea in glacial troughs that were flooded after the ice melted and sea level rose, such as Sognefjord in Norway, which is about 205 km long and over 1,300 m deep, and the fjords of Chile, New Zealand, Alaska and Greenland. They often have a shallow lip (threshold) at the mouth, where the ice was thinner and eroded less. Roches moutonnées are bedrock hills smoothed on the side the ice came from by abrasion and rough on the downstream side where plucking occurred. Other features are glacial striations, rock basins and erratic boulders left by the ice.

Erosional features compared

Worked example: erosion rates and valley shape

The numbers are hypothetical, except where the data of Sognefjord are given.

1. Erosion rate: a glacier erodes its bed by an average of 1 mm per year. Over 100,000 years, the lowering is 1 × 100,000 = 100,000 mm = 100 m.
2. Valley cross-section: a glacial valley is 2,000 m wide and 600 m deep. The width-to-depth ratio is 2,000 ÷ 600 = 3.3. A river valley of 300 m depth and 1,500 m width has a ratio of 5. A lower ratio suggests a steeper, deeper trough.
3. Side slope: if the valley side rises 500 m over a horizontal distance of 400 m, the slope is arctan(500 ÷ 400) = arctan(1.25) = 51°, which is steep, typical of a U-shaped valley.
4. Waterfall height: if the main valley floor is at 400 m and the tributary valley floor at 900 m above sea level, the waterfall at the lip of the hanging valley has a drop of 900 − 400 = 500 m (it may be split into several steps).
5. Fjord depth: Sognefjord is over 1,300 m deep and about 205 km long. If its average width is 4.5 km and the average depth is 700 m, the water volume is about 205 × 4.5 × 0.7 = 646 km³ (a rough estimate).
6. Time scale: the last glacial maximum was about 20,000 years ago. At 1 mm per year, an ice sheet could erode about 20 m since then, but repeated glaciations over 2.6 million years added up to much more.

How did we learn about glacial landforms?

Until the early 19th century, many large boulders and valleys were thought to have been moved by floods, including the biblical flood. Naturalists in Switzerland noticed that boulders, striations and moraines in the lowlands resembled those next to living glaciers. Louis Agassiz presented his glacial theory in 1837–1840 and later studied glaciers in Scotland and North America. In the later 19th and 20th centuries, geologists mapped glacial landforms in Europe, North America and Asia and linked them to repeated advances and retreats of ice over the last 2.6 million years. Today, satellite and laser measurements, and dating of rock surfaces, help determine when and how fast landscapes were shaped.

Which mistakes are common?

Mistake 1: Saying that glaciers “bulldoze” valleys

Glaciers erode mainly by plucking and abrasion, helped by freeze-thaw weathering, and not by simple pushing.

Mistake 2: Mixing up V-shaped and U-shaped valleys

Rivers cut V-shaped valleys. Glaciers produce U-shaped valleys with flat floors and steep sides.

Mistake 3: Thinking that the lakes in corries are made by the glacier melting alone

The hollow is made by glacial erosion. Meltwater and rain then fill it to form a tarn.

Mistake 4: Confusing fjords with ordinary valleys

A fjord is a glacial trough drowned by the sea. Its depth often exceeds that of the adjacent sea.