TRUCK WAS BEING LOADED; SHOVEL OPERATOR NOTICED A LARGE BOULDER AND PLACED SOFT MATERIAL IN BOTTOM BED. WHEN HE LOADED THE BOULDER, THE ANGLE OF THE DIPPER CHANGED, DROPPING THE BOULDER ON THE LEFT REAR CORNER OF THE BED, JARRING THE TRUCK. TRUCK DRIVER WAS DIAGNOSED AT THE ER WITH NECK STRAIN, GIVEN MEDICATIONS, 3 DAYS OFF WORK AND TOLD TO SEE HIS PERSONAL PHYSICIAN FOR A RELEASE.