EE had been operating the miner & a piece of roof rock (approx 5'L x 3 1/2'W x 6"" thick) was on top of the machine. The rock was on the miner overhanging slightly over the right side. He backed the miner up under supported roof to the left side of the entry to remove the rock. As he approached the mid-point of the miner, the rock slid off on its own striking his left foot.