While backing the miner out of the face, cap rock fell in. The cap rock broke off at the last row of roof bolts landing on the miner. A portion of the slab measuring 3' x 4' x 6' broke off when it struck the boom. As the slab broke it pivoted back landing on his foot. He was standing at the 2nd to the last row of roof bolts facing the shuttle car when the incident occurred.