www.americanpoems.com/poets/Billy-Collins/8970 --Will this link? Dunno.
This is the third poem in the book, and having read some two-thirds of the collection, I would say this is the cornerstone. The obvious theme is our struggle with mortality, the excellent cover art features a dog barking at the ocean in a futile attempt to chase it off. The picture is repeated elsewhere as a negative, which gives the appearance of a now-white dog still barking away well into the night. So, to further state the general theme, "things we do to try to distract ourselves from our inevitable demise, or at least things we do to comfort ourselves in the face of the lurking inevitable".
I consider this poem one of my favorite in the book. Later, perhaps as filler, coping with mortality flutters too often around the author's personal consuming interest in writing poetry, and name and phrase-dropping of other well-known poets, which becomes irksome, especially when applied to an already bloated subject, five takes on "gleaning my teaming brain" where one would suffice.
That grumble aside, this one was well balanced for me, the slow, rambling pace appropriate here, as opposed to unapplied, pointless rambling elsewhere. The setting is autumn(what else), and there are plenty of pleasant facets of the metaphor used, beginning with "a heap of rocks, probably pushed down during the horrors of the Ice Age", which calls to mind a barrow. The journey takes one over "the small footbridge with the broken railing, and for an even more bittersweet tone, "you might have to grab hold of a sapling when the going gets steep."The end is "a long stone ridge", but while there, the distractions of life fade to where one can hear "a sprig of birdsong" (one of my favorite phrase in the piece). Another favorite is "how the earth holds us painfully against its breast made of humus and brambles".
The poem only gets better with rereading, as the images sink in via the gentle voice used.