I Believe The White Material is Volcanic Pumice.
….This is My First Encounter With This Fluffy Dust, Which Seems to Dominate Central Oregon Ahead.

….“As we move again down the side of the mountain we see the trees thin out even more … “(Cont.Next)

Sixteen & a half miles South of Baker City, OR.

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(Photo = 112-1220 ...... ZMM Page = 277 ...... WayPt = 314w 4210ft)