Something there is that doesn't love a wall, That sends the frozen-ground-swell under it, And spills the upper boulders in the s
un; And makes gaps even two can pass abreast. The work of hunters is another thing: I have come after them and made repair Where they have left not one stone on a stone, But they would have the rabbit out of hiding, To please the yelping dogs. The gaps I mean, No one has seen them made or heard them made, But at spring mending-time we find them there. Whom does the speaker blame for the gaps in the wall?
The speaker is talking about the water. During winter, the water enters the cracks and freezes and expands. It breaks the stone and makes cracks and when the water melts the holes are bigger. What he is describing is mechanical weathering which is common in nature.
