IE walked over to check the CM that was down in E2 at 14+18. As the IE approached the CM, a rock measuring (2.5-4 W x 4 L x 5-1 T) fell out from between the roof bolts. The rock struck the IE's head and right forearm pushing IE forward. To prevent from falling forward, the IE planted right leg in front of self. The rock hit the IE's right knee as it fell to the floor.