Cirques

Glacial cirques, known locally as corries or coires (Scotland) and cwms (Wales), are large-scale erosional features common to many mountainous regions1,2. 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).

Classic glacial cirque basin. Cwm Clyd in the Glyderau mountains of Snowdonia. Image from GoogleEarth.
Cross-section of a classic glacial cirque with an overdeepened (and lake filled) valley floor and a steep headwall mantled with slope deposits, such as scree. Image created by J. Bendle based on Barr & Spagnolo (2015; ref. 2)

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.

Cirque glacier (Styggebrean) in Jotunheimen National Park, Norway. Photo: J. Bendle.

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-up3,4,5.

Cwm Cau, a formerly glacierized cirque basin in Snowdonia, Wales. Photo: J.Bendle.

Types of cirques

Far from being the same in all mountain areas, a wide range of cirque types occur. The most common are1,6:

  • 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
Different types of glacial cirques. The top three examples are drawn in plan view, whereas the bottom two are drawn in cross-section. Image created by J. Bendle based on Barr & Spagnolo (2015; ref. 2)

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 movement3,10.

Cirque glaciers erode their hollows by subglacial plucking and abrasion, which are most effective under a warm-based, sliding glacier. Meltwater that drains to the bed through the randkluft (the gap between the glacier and headwall), bergshrund (a large crevasse near, but not touching, the headwall) or other crevasses, promotes subglacial erosion. Periglacial erosion (e.g. freeze-thaw) occurs on the headwall and in the randkluft. Image created by J. Bendle.

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.

Loch Lomond Stadial (~12 to 10 thousand years ago) cirque glaciers in Snowdonia, North Wales. Image from Bendle & Glasser (2012; ref. 5)

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 past2,14.

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 glaciers5,12.

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 mass5,15.

Activities

Using GoogleMaps or GoogleEarth, enter “Snowdon” in the navigation search bar and explore the cirques of Snowdonia.

Try to identify different cirque types (e.g. ‘simple’, ‘compound’, ‘complex’), and compare their sizes, shapes, and aspects.

References

[1] Benn, D.I. and Evans, D.J.A., 2010. Glaciers and Glaciation. Hodder Arnold.

[2] Barr, I.D. and Spagnolo, M., 2015. Glacial cirques as palaeoenvironmental indicators: their potential and limitations. Earth-Science Reviews151, 48-78.

[3] Evans, I.S., 2006. Allometric development of glacial cirque form: geological, relief and regional effects on the cirques of Wales. Geomorphology80, 245-266.

[4] Ballantyne, C.K., 2007. Loch Lomond Stadial glaciers in North Harris, Outer Hebrides, North-West Scotland: glacier reconstruction and palaeoclimatic implications. Quaternary Science Reviews26, 3134-3149.

[5] Bendle, J.M. and Glasser, N.F., 2012. Palaeoclimatic reconstruction from Lateglacial (Younger Dryas Chronozone) cirque glaciers in Snowdonia, North Wales. Proceedings of the Geologists’ Association123, 130-145.

[6] Gordon, J.E., 1977. Morphometry of cirques in the Kintail-Affric-Cannich area of northwest Scotland. Geografiska Annaler: Series A, Physical Geography59, 177-194.

[7] Turnbull, J.M. and Davies, T.R., 2006. A mass movement origin for cirques. Earth Surface Processes and Landforms 31, 1129-1148.

[8] Sanders, J.W., Cuffey, K.M., MacGregor, K.R. and Collins, B.D., 2013. The sediment budget of an alpine cirque. Geological Society of America Bulletin125, 229-248.

[9] Thorn, C.E., 1976. Quantitative evaluation of nivation in the Colorado Front Range. Geological Society of America Bulletin87, 1169-1178.

[10] Sanders, J.W., Cuffey, K.M., Moore, J.R., MacGregor, K.R. and Kavanaugh, J.L., 2012. Periglacial weathering and headwall erosion in cirque glacier bergschrunds. Geology40, 779-782.

[11] Darwin, C.R., 1842. Notes on the effects produced by the ancient glaciers of Caernarvonshire, and on the boulders transported by floating ice Lond. Edinb. Dublin Philos. Mag. J. Sci. 21, 180-188.

[12] Gray, J.M., 1982. The last glaciers (Loch Lomond Advance) in Snowdonia, N. Wales. Geological Journal17, 111-133.

[13] Hughes, P.D., 2009. Loch Lomond Stadial (Younger Dryas) glaciers and climate in Wales. Geological Journal44, 375-391.

[14] Evans, I.S., 2006. Local aspect asymmetry of mountain glaciation: a global survey of consistency of favoured directions for glacier numbers and altitudes. Geomorphology73, 166-184.

[15] Evans, I.S., 1977. World-wide variations in the direction and concentration of cirque and glacier aspects. Geografiska Annaler: Series A, Physical Geography59, 151-175.

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