A corrie is a distinctive, armchair-shaped hollow found high on the side of a mountain, carved out by the erosive power of a small glacier. This deep, bowl-like landform is a relic of past glacial periods, representing the starting point of valley glaciers. The term corrie is most commonly used in Scotland, while the same feature is known as a cirque in the Alps and North America, and a cwm in Wales. The formation process involves a specific sequence of weathering and erosion that results in the characteristic steep-sided, rounded shape.
The Mechanism of Corrie Formation
The process of corrie formation begins with the accumulation of persistent snow in a pre-existing topographic depression on the mountainside. This initial snow patch deepens the hollow through a process called nivation, which combines meltwater erosion and freeze-thaw weathering beneath the snow cover. As the snow compacts under its own weight, it transforms into denser firn and eventually into glacial ice, initiating movement.
The developing glacier moves in a slow, rotational fashion within the hollow due to gravity and the weight of the ice mass. This movement, known as rotational slip, carves the basin shape by exerting maximum pressure and erosion at the center of the hollow. The ice’s movement is lubricated by a thin layer of meltwater produced by pressure melting at the glacier’s base.
As the ice rotates, two main erosional processes—plucking and abrasion—work to sculpt the corrie. Plucking occurs when the glacier freezes onto fractured rock on the back wall, pulling out large blocks as the ice moves away. This action continually steepens the back wall, creating a near-vertical cliff face. Simultaneously, the embedded rock fragments within the base of the glacier scrape against the bedrock in a sandpaper-like fashion, an action called abrasion, which deepens and smooths the corrie floor.
A large crack, or bergschrund, forms where the moving ice pulls away from the static rock of the back wall. Meltwater trickles down this crack, where intense freeze-thaw weathering shatters the rock, supplying more debris for the glacier to pluck and abrade. This combination of rotational slip, plucking on the back wall, and abrasion on the floor works in concert to gradually excavate the classic armchair shape of the corrie.
Distinctive Physical Features
A fully formed corrie exhibits three recognizable features.
The Steep Back Wall
The steep back wall is the highest and most prominent feature, typically a sheer cliff face resulting from intensive plucking and freeze-thaw processes. This curved wall marks the upper limit of the glacial erosion and is often jagged and rough due to mechanical weathering.
The Corrie Floor
The corrie floor is the large, relatively flat, and deep basin scoured and smoothed by glacial abrasion and rotational slip. This area represents the deepest point of erosion where the weight of the glacier was greatest. Once the ice melts, this depression frequently fills with water to create a small, circular lake known as a tarn, or a corrie lochan.
The Rock Lip
The rock lip is a raised threshold or ridge at the front of the corrie, which acts as a dam to contain the tarn. This lip is formed because the erosive power of the ice lessens toward the outer edge of the rotating glacier, leaving a rim of less-eroded, more resistant rock. The rock lip can also be a mound of deposited glacial till, or moraine, left behind as the glacier retreated.
Geographical Context and Nomenclature
Corries are characteristic landforms of high-altitude and high-latitude regions that experienced glaciation. They are predominantly found in mountain ranges globally, including the Scottish Highlands, the European Alps, the Rocky Mountains, and the Andes. In the Northern Hemisphere, corries frequently develop on north or northeast-facing slopes, as these aspects receive less direct sunlight, which promotes the sustained accumulation and preservation of snow and ice.
The variation in name is largely a matter of regional geography and language. While corrie is the term of choice in the British Isles, the French word cirque, meaning a circus-like arena, is the widely accepted international geological term. The Welsh counterpart, cwm, describes the same glacial feature within the Snowdonia mountains.
When multiple corries form in close proximity on different sides of the same mountain, their erosive action can lead to the creation of further dramatic landforms. If two corries erode back-to-back, they leave behind a narrow, knife-edge ridge known as an arête. If three or more corries erode into a single mountain peak, the resulting convergence of arêtes creates a sharply pointed mountain called a pyramidal peak.