Answer:
by foreshadowing a change in nature
Explanation:
I did the test
What is your question what do you not get?
They change shape due to a number of reasons.
The easiest reason to understand why they change is that they erode.
The coasts around the the world erode due to waves hitting cliffs, the wind batting against the coasts etc. Over a long period of time we can see that they change in shape.
(sorry I don't know why they move)
The best way according to him is to make a good fence. The poem's final lines says that good fences make good neighbors. That can mean that if you stick your nose in your yard you will have good neighbors. Of course, this is all figuratively and refers to much greater things in life.