I always wonder why "northern plains". Is it because the cold and wildness of the northerm plains? There used to be a thread on IMDB PT discussing the meanings of some lines in the short story. I loved that thread.
Well, for me the idea of the "northern plains" just suggests vast loneliness. It seems to me to be characterized by large expanses of open space where a person could feel very tiny and isolated. Also plains are the opposite of mountains... so Ennis here is feeling oppressed by plains (flat land) as opposed to his ideal place, a mountain (i.e. Brokeback mountain, to which he can never return... not even to return Jack's ashes). Since this line comes following Jack's death, well, it just makes me weep for Ennis.
OK, this is more than a "line" but it always blows me away. Talk about the power of nature as an active component/ symbol in the relationship.
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The next week Joe Aguirre sent word to bring them down - another, bigger storm was moving in from the Pacific - and they packed in the game and moved off the mountain with the sheep, stones rolling at their heels, purple clouds crowding in from the west and the metal smell of coming snow pressing them on. The mountain boiled with demonic energy, glazed with flickering broken-cloud light, the wind combed the grass and drew from the damaged krummholz and silt rock a bestial drone. As they descended the slope Ennis felt he was in a slow-motion, but headlong irreversible fall."
I love that the wind (the symbol many of us see as Jack) and earth/ stone (the symbol many of us think of as Ennis) are so significant here... and that the wind makes the stones sing!