This is a deceptively difficult poem. While most of the language is familiar, it's relatively long, skittish with its subject matter, and full of references to historical events. It's hard to keep your finger on the pulse, because Lowell keeps pulling the rug out from under us without warning.
While he does faithfully return to each of his juggled subjects, he never really unlocks any of their mysteries, so they remain swimming around in our brains long after the poem is over. To help make sense of what Lowell does give us, it's a good idea to keep an encyclopedia next to you to look up the historical references in this poem, as well as the geographical ones. In other words, bring tools and dress in the proper snow gear, because this is going to be a hard one to tackle.