Location 5th West Mains, Entry 1, x-cut 7. Employee inspected area, scaled rib-line and roof area. EE pulled bolter in entry and started the first row of bolts. After installing the 2nd bolt the rib started to slough off for a distance of approximately 35'. A rock hit the employee's leg, causing ee to be pinned against the roof bolter, resulting in a fracture to left leg.