flickering trees

I recently passed out in the back of someones car, it was relaxing a good way to get from one place to another, not at all like the bus, every once in awhile I would wake up, stare out the window for a bit, at the beginning there were these trees, and the sun was setting, the shadows where all laying down, the trees were flickering by, the shadows retained their position for miles, leaving me to wonder which was more substantial the trees, or the shadows.