Come my tan-faced children, Follow well in order, get your weapons ready, Have you your pistols? have you your sharp-edged axes? (1-3)
This opening stanza tells us a lot about the relationship between these pioneers and the natural world. Their tan faces tell us they're the outdoorsy type, exposed to the sun and elements. And their need of weapons suggests that there is a sort of battle going on. Their connection to nature is not all skipping through meadows and picking buttercups; there's also hunting and chopping through the undergrowth, carving out a space in the natural world for people to move in.
Down the edges, through the passes, up the mountains steep, Conquering, holding, daring, venturing as we go the unknown ways, (22-23)
Those words "conquering" and "holding" also have military associations. Our speaker is presenting the work of these pioneers as a battle fought for territory. Not only do they have to conquer new land, but they have to hold it (we guess to keep if from being overrun again by the wilderness).
We primeval forests felling, We the rivers stemming, vexing we and piercing deep the mines within, We the surface broad surveying, we the virgin soil upheaving, (25-27)
Um, violent much, Walt? We are cutting down whole forests and vexing and piercing the natural world. And of course that word "virgin" makes the word "piercing" become pretty sexual. Although it's violent (and perhaps not all that consensual), we have to note that the sexual metaphor suggests something beyond just conquering or subduing the earth. It suggests creating something new through the combination of human work and the land and resources of the natural world.
Colorado men are we, From the peaks gigantic, from the great sierras and the high plateaus, From the mine and from the gully, from the hunting trail we come, (29-31)
Even with all the competition and battle imagery from earlier in the poem (all that conquering and felling), these lines imply that the natural world is extremely important. These men are defined by the geography where they live, by the peaks and plateaus. They are also defined by what they carve out of that geography: the mines and the trails.