travelling north on hwy 9, i caught a glimpse of what i thought was a dust devil rising ahead and to my right at appr 300 yds distance. it seemed there was a white wal-mart bag rising within the dust devil until i realized it was actually a silvery rectangular object i would describe as the size of a poster board. the object continued tumbling and rising to slightly above the tree line to appr 200 feet at which point it began to leisurely move west from the east towards the highway where i was driving north. this was when i moved over and stopped. during the next 6 min and 30 secs it crossed the highway and covered appr 3/4 mile then began to angle upwards at a 45% slope. it eventually diappeared behind the clouds. having flown extensively,i estimate the clouds to have been about 1000-1500 feet above the ground. during this period there were 4-5 cars pass, but unwilling to lose visual contact, i didn't wave anyone down. the reason i am so precise about the time is i was low on gas and was watching gas gauge and clock to see if i could make it to next station.