… [I]n the last lines of the poem the absolute is mythologized not as the spirit Father whose Christmas Son seems not yet to have redeemed nature but as Nature the Terrible Mother whose "rocky breasts" feed her offspring chilling fire, whose "cold hard mouth" engorges what her womb has birthed. [Robert] Lowell tried to convince Bishop to excise the image of the mother, and in the journal notes of "Geographical Mirror" [working title of an earlier version of the poem] she had herself canceled out the phrase "a great rocky breast." By the time she was finishing the poem, however, she realized that she had to have as an image of the absolute fact of nature and mortality an image that was not just "awful but cheerful" but also sublime and awesome. In time and history the knowledge of the absolute is rendered in the elision from the present participle to the past: "flowing and drawn" becomes, in the metamorphosis of an internal rhyme, "flowing, and flown," knowing and known.
From Albert Gelpi, "Wallace Stevens and Elizabeth Bishop at Key West: Ideas of Order," in Wallace Stevens Journal 19.2 (Fall 1995), 163-164.