Read the sentence.The canyon, formed by years of erosion, was a breathtaking sight to behold.

“Formed by years of erosion” is a phrase that adds extra information about the canyon; it does not identify which canyon is meant. The commas show that this added detail is nonessential, so it is a nonrestrictive phrase. A restrictive phrase would be needed to distinguish the canyon from others, and a clause would contain a subject and verb.

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