the new lucinda williams release, 'Minneapolis', contains the line: "I wish my thoughts were pure like the driven snow", which led me to wonder: why is driven snow poetically considered the most pure form of snow? what prezactly is "driven" snow? i'm presuming it's a reference to driving winds, rather than the greyish brown muck left on the side of the road after fresh snow's been driven.

Hello, guest!...and welcome to our Band of Merry Wordsters!

I would think the degree of purity of "driven" is enhanced by the fact it still hasn't touched the ground and been tarnished by the Earth. Just guessin'...no citation.

snowed by drifting in the purest google sense:

"I used to be pure as the driven snow, but then I drifted!". - Mae West(?)

Mae West's quote was actually "I used to be snow white... but I drifted." Actress Tallulah Bankhead, however, is supposed to have said "I'm as pure as the driven slush."

perhaps it is a Shakespearean Conflation:
Shakespearian, although it doesn't appear in that form in any of his writing. Shakespeare used snow as a symbol for purity and whiteness in several plays. In The Winter's Tale - Autolycus: Lawn as white as driven snow. In Macbeth - Malcolm: black Macbeth will seem as pure as snow. - The Phrase Finder

I agree with W'ON. Snow that is driven by the wind is still falling and would be the purest of snows--leastways as long as it's not falling through smog or smoking chimneys, but that's getting the phrase off-track.