"That sends the frozen-ground-swell under it..."
See in text (Mending Wall)
Frost wrote “Mending Wall” in blank verse: unrhymed iambic pentameter. The pentameter line is often acknowledged as the natural line in English poetry. Its length is roughly the length of a breath, giving pentameter poetry a thoughtful, conversational tone. Frost’s choice in meter makes sense here, given the conversation at the poem’s heart.