A walker moves along a trail in the woods and notices small things: birch roots standing like stilts, a hollow where a stump used to be, a path the deer have worn across the trail. Nothing here is dramatic. The speaker simply looks, and the poem's short, spaced-out lines feel like pauses in a slow walk.
Then the attention narrows to one rock, held up in the claw of an uprooted tree. It looks ordinary, a piece of smooth sandstone. The last lines make a sudden leap, though: the "empty seas of the first great extinction." The poem suggests that the sandstone carries a memory of ancient seas, and that a stone you could pass without a glance comes from a time almost beyond imagining.
The speaker's feeling seems to be quiet wonder, with a touch of humility. The mood is calm and observant, but the ending opens it up, so a small patch of forest suddenly holds an enormous stretch of time.