Employee was dusting travel way & the hose busted on rock duster. EE went to get repairman. EE was waiting on car to dump on feeder & noticed a piece of rock hanging loose on top. EE didn't feel comfortable leaving it, so used a slate bar to pull the piece that was hanging. EE did not anticipate what was coming with it. An 80""x 44""x 10"" piece of rock fell on him.