Why are so many Welsh valleys called Cwm?

If you've looked at a map of Wales, you'll have noticed cwm cropping up everywhere—Cwm Idwal, Cwm Rhondda, and dozens more. It's the Welsh word for a valley or cirque, and in place names it's typically followed by whatever distinguishes that particular cwm from all the others: a person, a feature, or just what it looks like. Simple enough, once you know what you're looking at.