The poem starts with rock. Sandstone breaks back into sand, loosened by frost or the chemistry of fungi, one grain at a time. The speaker hesitates to call this "metamorphosis," since it is so slow, and then notes that it is even slower than an hourglass and far harder to undo.
From there the poem turns to people. Why, the speaker asks, do we only value changes that come from pressure, the dramatic and sudden kind? The poem points instead to years of weather that "unfather us" and to small, repeated blows that shape our addictions. Change here is gradual and wearing, not heroic.
The last two stanzas get more personal. The speaker says they've been "baptised / in a saline solution," which suggests tears, sweat, or the sea, and that every ambiguity has "a labyrinthine root." The mood is thoughtful and a little rueful: calm in tone, but aware of loss. Nothing is announced; we are left to feel how slowly we come apart and how tangled the causes are.