When children dream of becoming cowboys, they're probably envisioning the Hollywood archetype: lots of horsebackery, some lassoing, all the bacon you can eat, and maybe the occasional gunfight. Scratch that; do 21st-century kids even aspire to cowboydom anymore? Used to be that a laconic loner riding the open range represented the ultimate in manly, mythic cool, but if modern society would pull its collective nose out of the Blackberry for a second, it would be very apparent that the traditional American cowboy is an increasingly rare breed. A vital, vivid tribute to a dying way of life, "Sweetgrass" follows a couple of these intrepid souls as they lead a herd of sheep into the mountains of Montana for summer grazing, the final time anyone will be allowed to do so.

For more than a century sheepherders had been driving their flocks into the Absaroka-Beartooth Mountains under a federal permit, and in 2001 visual anthropologists Lucien Castaing-Taylor and Ilisa Barbash began to chronicle the real-life equivalent of one of cinema's favorite subjects, the "one last job." But with no expository background information and zero narration, we're left to piece together what's happening. First we spend time at the ranch with a few thousand noisy sheep, watching as they birth their wobbly lambs and get efficiently shorn of their voluminous wool. We also meet their much quieter caretakers, two of whom will spend the summer under that big sky so their charges can dine in the delicious mountain meadows.

A tiny sheep pin adorning his well-worn hat, John Ahern is the craggy veteran, partial to afternoon naps and long bouts of contemplative silence when he's not cooing to his "girls." Pat Connolly is the younger, chattier one, though his wordiness takes a dark turn once exhaustion sets in and he lets loose with an R-rated stream of profanity directed at an apathetic flock. "Sweetgrass" observes as these men go about their demanding job of making sure that 3000 sheep, along with a pack of horses and some very talented dogs, stay safe, often at great peril to their own lives. (There's mutton-loving bears in them thar hills.) One moving scene finds Connolly phoning his mom from the top of a rock, the tearful exhaustion bubbling up as he breaks down about his frustrations at work.

No one gets a director's credit on "Sweetgrass," instead using the more accurate term "recordist" to describe Castaing-Taylor's long takes. This practice, which involves long stares into the faces of various sheep along with some stunning panoramas, may strike a viewer as leisurely headed toward boring, but it actually makes for a purer viewing experience, allowing the scenes to unfold in an organic fashion without the agenda of an edit. Then it's back down the mountain, and perhaps time to face the future. "I didn't want to think about it for a week," Ahern replies when asked what he might do, but the faint twinkle in his tired eyes suggests that he ain't ready to mothball his saddle quite yet. You can take the boy out of the wild, but good luck taking the wild out of the boy.