Final answer:
The underlined part of the sentence is best described as a nonrestrictive phrase that provides additional descriptive information.
Explanation:
Nonrestrictive phrase best describes the underlined part of the sentence. Nonrestrictive phrases provide additional information that is not essential to the meaning of the sentence. In this case, the phrase 'formed by years of erosion' adds descriptive detail about the canyon.
Learn more about Restrictive and nonrestrictive phrases in English grammar here:
https://brainly.com/question/30277284
