Back in the day, stones were commonly used to construct major roads for travel. We still have phrases that reference that relationship. In "The Layers," stone imagery occurs twice, which is no coincidence. Kunitz utilizes the parallels between stones, time, and distance to reflect on the journey of life from the speaker's perspective.
Line 11: "Milestones" are shrinking as they approach the horizon, symbolizing the great distance the speaker has traveled over a long period of time. The speaker has lived "many lives" after all, and he's now approaching a unique crossroads. He can pass further milestones (also known as turning points or life stages) by applying the knowledge he's gained so far, or he can dwell in the past and halt his progress. Although milestones are functioning in some capacity as physical objects, they're primarily intended as imagery to mark the expansiveness of the speaker's conscious journey.
Line 30-31: In these lines, the speaker says that he cherishes "every stone on the road" that has led him to where he is today. Previously in the poem, the speaker was struggling over hardships in his past. Now, he's embracing all of his life experiences as learning experiences. Each "stone" is a metaphor for a life experience, leading him one step closer to a vibrant and internally fulfilling future.