The previous answer is generally quite insightful and helpful, but I must correct its mistaken understanding of the poem's meter. It is not in iambic pentameter; in fact, the meter is extremely irregular, varying from line to line. Line 2, for example, is in anapestic tetrameter with an initial iamb. An anapest is two unstressed syllables followed by a stressed syllable, while the root "tetra" means four, like a tetrahedron. So there are four anapests in a row. Anapests create the feeling of galloping along. We are familiar with anapestic meter that begins with an iamb from the limerick form:

There WAS a young LAdy named MAY
Who READ a love STOry each DAY

There are only two anapests in each of these lines, but it's the same gallop that makes the limerick funny, though humor is not always the effect. Sometimes, the gallop can simulate a heart beating faster, for example. So when Birney writes, "All WEEK in the VALley for WAGes in AIR that was STEEPED," he is simulating the feeling of their hard work's rhythm.

Line 9 creates even more of a galloping feeling, with no initial iamb for a deep breath: "Into VALleys the MOON could be ROLLed in. ReMOTEly unFURLing." Note the association of anapests with valleys for Birney. This line also has what's called a feminine ending—that is, a single unstressed syllable attached to the end of the line.

The lines vary radically throughout the poem: line 13 alternates anapests and iambs; line 21 gets into dactyls—the opposite of iambs, with one stressed syllable before two unstressed; line 33 goes back to the same meter as number 2. All this is to say that metrical regularity is definitely not an element of this poem, even as a standard framework against which the rhythm can ebb and flow. I could not find even a single iambic pentameter line in it, at least not in the first five sections. So be sure not to assume a metrical regularity when you write about this piece.

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. Lingering There it was David who spied to the south, remote, And unmapped, a sunlit spire on Sawback, an overhang Crooked like a talon. David named it the Finger.