Sparse, but Sometimes Rambling
We say "rambling" out of love, of course, because this story's got some of the best darn rambling you're ever going to see. That said, you won't read many authors this side of Henry James who are as quite adverse to the period as Proulx. Just take a look:
They seized each other by the shoulders, hugged mightily, squeezing the breath out of each other, saying, son of a b****, son of a b****, then, and easily as the right key turns the lock tumblers, their mouths came together, and hard, Jack's big teeth bringing blood […] (47)
And it goes on like that for another five or six lines.
Why? We suspect it's in keeping with the characters' overall lack of control over their lives (and in this moment, a particular lack of control over their feelings). Just like their love careens this way and that with no sign of halting or slowing down, so does Proulx structure her sentences to convey how they might feel at the time. Go back and read that sentence we cited in paragraph 47 and see how it ends. You'll get what we mean.
On the other hand—or perhaps because of the rambling style—Proulx tends to get right to the point without a whole lot of embellishment. She doesn't use a lot of descriptions and skips through anything she doesn't feel is absolutely necessary, say, for instance when Jack dies and she tells us with a simple "Ennis didn't know about the accident for months" (123). That, too, is in keeping with the characterization in the story, since both Ennis and Jack are men of two worlds.