It is in unrhymed iambic pentameter--making it blank verse--so it has specific meter (or pattern of beats). This is not to be confused with rhythm, which is the actual form those beats take (think of it like this: if climate is what you expect and weather is what you get, "meter" is the climate and "rhythm" is the weather). The entire poem is in quatrain stanza--stanzas (the poetic version of paragraphs) of four lines each.
The poem is rife with imagery: "sunalive weekends" (3), bacon "strips festooned / On a poplar prong" (12-13), and a glittering "alien prairie" (20).
As well, Birney uses simile (they "won" the snow of the slopes above the timberline "like fire in the sunlight" (16-17) and the "peak was upthrust / Like a fist..." (17-8), and its more muscular big brother metaphor (the "dawn was a floating / Of mists" (15-6)).
He loves personification, as well: the sun has "retreats" (8), the Mount Gleam has "shoulders" (11), and the "pines thrust at the stars" (15). There is hyperbole: they spend an "endless hour in the sun" (31) and "valleys the moon could be rolled in (19); synesthesia: they "feast" on the sight before them (36)--a feat actually impossible to do with one's eyes; and enjambment as well as end-stopped lines. Lines 37-40 are a good example of how both are employed:
...By the fading shreds of the shattered stormcloud. LingeringThere it was David who spied to the south, remote,And unmapped, a sunlit spire on Sawback, an overhangCrooked like a talon. David named it the Finger.