I saw this at about 9000 ft in Broads Fork canyon in Utah near Salt Lake City
You found a caterpillar of the Spotted tussock moth.
Tussock caterpillars have the tufts like you see on this one…the hairs usually vary in length.
This caterpillar is found across western North America as well as some eastern Appalachian states.
The caterpillar feeds on foliage of a wide number of shrubs and trees including birches, maples, oaks, poplars, willows and alders. There is one generation per year.
They overwinter in the pupal stage so this caterpillar is probably close to pupation. They form a cocoon using silk and the hairs. The adults emerge in early summer.