Long, flat whitish silver object between Clackamas and Troutdale, Oregon.While driving northbound from Clackamas to Troutdale, Oregon, I witnessed a large, very flat object, hovering in mid air. The size of the object isn't clear to me, but seemed to be the size of a medium size jet plane. It was very thin, almost looked flat and was bright white or silver in color.The object seemed to move very effortlessly and was not in any known flight path that I have seen in the area. It seemed to be gravitating towards other aircraft in the area but never got too close. I watched for roughly 10 minutes before arriving at a location I could pull over and take a photograph. At this point I had lost sight of the object for less than 30 seconds, since the last point it went out of view because of trees. It had completely disappeared upon my arrival to my destination.Was not an airplane or helicopter. Looked nothing like any drones I had ever seen.95 degrees, clear skies and sunny.((NUFORC Note: We spoke via telephone with the witness, and he seemed to us to be quite eloquent and sober-minded. We suspect he is a very good witness. PD))