The EE had just loaded a number of 6' roof bolts onto his operator side boom tray when a rock fell from the top striking him on the head. The rock striking him on the head caused his head to hit the tray that was full of bolting supplies. The rock then slid down his back to the mine floor. The rock measured approx. 36"" x 24"" x 6"" thick. The rock fell out from between bolts.