In chapter 29, Mr. Birkway reads the Longfellow poem "The Tide Rises, The Tide Falls."
The poem is deeply distressing to Phoebe. It calls to mind her mother's disappearance, and she begins to imagine a nefarious reason for it.
The fact of the matter is that neither Sal nor Phoebe were able to prevent their mothers from leaving. For Sal, her mother's death was beyond her ability to prevent. Meanwhile, Phoebe's mother returns, but her reappearance is fraught with tension.
In the poem, the last line of each of the three stanzas references the rising and falling of the tides. The references highlight the inevitability of the flow and ebb of life. In the second stanza, the incoming tide "effaces the footprints in the sand."
Ominously, we are told that, even though the day returns, "nevermore returns the traveler to the shore." Loss occurs, despite our wishes otherwise. This is highlighted in the story. Sal eventually realizes that she has to make peace with her mother's disappearance and tragic death. Neither was in her ability to prevent. All she can do is remember that life continues, even in the face of tragedy.
Essentially, the poem highlights the dichotomy of life: the tides may wipe out the footprints in the sand, but its waves are as gentle as "soft, white hands." Essentially, life is filled with both joy and tragedy. At the end of the novel, Sal makes her peace with this.
Meanwhile, Phoebe also learns the same thing. By the end of the novel, she has to learn to accept the presence of a half-brother in her life. She also has to come to terms with the hidden side of her mother's personality. For Phoebe, the vicissitudes of life have caught her unawares. To thrive, she must "go with the flow" and learn how to embrace change.