It's hard to find a McGuane quote that both evokes Montana and showcases that writing style ... but here's a paragraph that I like. It's from a short story called "Flight," featured in his 1986 anthology To Skin A Cat:
Every time the dirt road climbed to a new vantage point, the country changed. For a long time, a green creek in a tunnel of willows was alongside us; then it went off under a bridge, and we climbed away to the north. When we came out of the low ground, there seemed no end to the country before us: a great wide prairie with contours as unquestionable as the sea. There were buttes pried up from its surface and yawning coulees with streaks of brush where the springs were. We had to abandon logic to stop and leave the truck behind. Dan beamed and said, "Here's the spot for a big nap." The remark frightened me.