What Are a Scottish “Corrie” and “Tarn”?

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Corrie and Tarn, Wales.

Often called “cirques” (French, from Latin word “circus”) because of their theatre-shaped valleys formed by glacial erosion, “corries” (from Scottish Gaelic “coire” meaning pot or cauldron) are bowl-shaped indentures found in certain peak and valley formations that often fill with snow and icemelt to form “tarns” (small lakes). The gelogical formation is also called a “cwm” (Welsh for “valley” and pronounced “coom”).  A corrie may also be a similarly shaped landform arising from fluvial erosion.

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Formation of a corrie = cirque = cwm.

The concave theatre shape is open on the downhill side corresponding to the flatter area of the stage, while the cupped seating section is generally made up of steep, cliff-like slopes down which ice and glaciated debris combine and converge from the three or more higher sides.

The floor of the corrie ends up bowl-shaped as it is the complex convergence zone of combining ice flows from multiple directions and their accompanying rock burdens, hence it experiences somewhat greater erosion forces, and is most often overdeepened below the level of the corrie’s low-side outlet (stage) and its down-slope (backstage) valley.

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If the corrie is subject to seasonal melting, a tarn will form within its bowl behind the moraine, glacial till, or bedrock lip; marking the downstream limit of glacial overdeepening of the basin, which serves as a dam at the outlet.’

Another source…

‘Corries are mountain valley heads which have been shaped into deep hollows by the erosion of small glaciers. In the United Kingdom, many corries were last filled by glacier ice around 12,000 years ago; but these corries have held glaciers on many occasions during the last 2.4 million years. In high mountains elsewhere in the world corries still hold glaciers today.

Bowscale_Tarn_on_Bowscale_Fell-1

Bowscale Tarn on Bowscale Fell in the English Lake District.

Godard plotted the orientations of all the corries in the Northern Highlands of Scotland, and found that 71% of them faced between north and east. This indicates that in Scotland, corrie glaciers develop in preglacial hollows on shaded slopes that receive snow blow from prevailing south-westerly winds. Snow blow is particularly important in feeding glacier development because corrie glaciers are relatively small glaciers that form during periods only just cold enough for glaciation. Under more intense conditions ice caps and ice sheets form and the corries may feed ice into larger glacial systems.’

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Corrie in the Scottish Highlands with tarn formed by melting snow and ice.

(Sources: Wiki; http://www.bbc.co.uk; http://www.geologywales.co.uk; http://www.southlakes-uk.co.com)

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