A cirque or bowl-shaped valley, especially one carved by glaciers in mountainous regions, commonly found in Wales and other glaciated areas.
From Welsh 'cwm,' of Celtic origin. It entered English in the 18th-19th centuries through descriptions of Welsh geography and geology.
Cwm is famously one of the few English words with no traditional vowels—it's pronounced with a 'oo' sound entirely from consonants, and Welsh speakers knew it long before English borrowed it.
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