A contract employee was helping move roof bolts from the rear of a roof bolter to the side boom tray. A rock measuring 2'8"" X 2'2"" X 1'6"" (approximately 380 lbs) fell from the roof at a trimmed corner. When this corner was trimmed, an arched kerf was left from the head of the miner. Part of the arched kerf fell and struck the miner. The rock fell between the roof bolts and the rib.