Cirques

Glacial cirques, known locally as corries or coires (Scotland) and cwms (Wales), are large-scale erosional features common to many mountainous regions^1,2. Classic cirques take the form of armchair-shaped hollows (see image below), with a steep headwall (which often culminates in a sharp ridge, or arête) and a gently-sloping or overdeepened valley floor (see diagram below).

In actively glacierized terrain, cirques are important basins for the accumulation of snow. They may host small cirque glaciers (see image below) that are confined to their bedrock hollows, or act as the source area for larger valley glaciers.

In other mountainous areas, such as the British uplands, the occurrence of ice-free cirques (see image below) serve as a reminder of past glacier activity by recording former sites of glacier build-up^3,4,5.

Types of cirques

Far from being the same in all mountain areas, a wide range of cirque types occur. The most common are^1,6:

• Simple cirques, which are distinct and independent features
• Compound cirques, where the upper part of a cirque basin contains two similarly sized simple cirques
• Cirque complexes, where the upper part of a cirque basins contains more than two similarly sized simple cirques
• Staircase cirques, where one cirque occurs above another
• Cirque troughs, where a cirque basin occurs at the upper end of a glacial trough

The formation and growth of cirques

Cirques form through the gradual expansion of mountainside hollows associated with earlier fluvial, volcanic, or mass movement (e.g. landsliding) activity⁷. When these hollows become filled with snow⁸ they start to enlarge by nivation (a group of processes that includes freeze-thaw activity, chemical weathering, and seasonal snow melt)⁹.

True cirque growth only occurs once the thickness of snow patches increases to a point at which glacier ice can form by compaction. Once formed, glaciers widen and deepen cirques by subglacial abrasion and quarrying of the hollow floor and lower headwall³ (see diagram below). Cirques can also grow by backwards headwall erosion (wear back) due to frost-action, free-thaw, and mass movement^3,10.

Case study: glacial cirques of Snowdonia

The glacial cirques of Snowdonia formed over several glaciations, and have a long history of investigation, first being visited by Charles Darwin over 150 years ago¹¹. The most recent period of glacier activity in Snowdonia was during the mountain glaciation of upland Britain in the Loch Lomond Stadial (between ~12 and 10 thousand years ago)^5,12,13.

Why are cirques important?

Because cirques are areas of snow accumulation, the direction in which they point (their aspect) can tell us something about the links between climate and glacier growth in the past^2,14.

If looking from above (see image above), an interesting observation is that most cirques in Snowdonia face to the north or east¹⁴ and these also held most (as well as the largest) Loch Lomond Stadial glaciers^5,12.

Controls on cirque aspect

This is due to two factors. Firstly, north-facing cirques receive less solar radiation than south-facing cirques (in the Northern Hemisphere), resulting in lower air temperatures and less ice-melt across the year¹⁵.

Secondly, where prevailing winds blow mainly from the west, the snow on high ground will be blown down into east-facing cirques, adding to glacier mass^5,15.

Further reading

• Cirque landsystem of Upland Britain
• Glacial Erratics
• Younger Dryas glaciation of Britain
• Alpine icefield landsystem