What’s all this nonsense about the “silence” of the lambs? The sheep who populate nearly every frame of Sweetgrass are bleat merchants of a spectacularly boisterous order. They may seem gentle and easily persuaded, as they cluster in a vast, moving carpet of snowy wool, kept in regimented order by attentive sheep dogs. But these hooved beasts are louder than bombs. That incessant bahhhh, in all its variations, makes a hypnotic soundtrack for this absorbed and absorbing documentary about the end of an era: The camera follows Montana rancher Lawrence Allested, his posse of cowboys and one big-ass herd on its last grazing run across 200 miles of the Rockies in the summer of 2001. There’s no narration. Often, there’s no dialogue, aside from some salty campfire banter, cowboy jokes and all that ovine crosstalk.

Sounds a tad dry, right? It’s not. Even if husband-wife team Lucien Castaing-Taylor (who shot the footage) and Ilisa Barbash (who produced) are professors of ethnographic film at Harvard, their marathon observations are wholly keyed into the transcendental elements—and the inherently earthy humor—of life on the trail. The film’s poetic sweep comes with the territory. The waves of alabaster sheep, the endless blue horizon, the green grassy hills: It’s a pastoral wet dream of American splendor. You get the feeling, though, that Castaing-Taylor didn’t take any of this as a gimme. He really threw himself headlong into the fray, so committed is the camerawork. (According to production notes, the rigor of those efforts caused him to develop advanced degenerative arthritis.) Sweetgrass represents only a fraction of the material gathered by the filmmakers over the better part of the decade. They have eight more films in the works. And if they have more footage of newborn lamb tossing, I’d love to see it.