It sifts from Leaden Sieves -
It powders all the Wood
It fills with Alabaster Wool
The Wrinkles of the Road -
It makes an Even Face
Of Mountain, and of Plain -
Wool comes from sheep and is often fluffy and white (alabaster) . 
 Alabaster is a type of stone. In  a single compressed metaphor, Dickinson is implying that that snow feels like wool that’s cold as stone.
It sifts from Leaden Sieves - 311