Employee was operating the RS-20 while standing on shield #61, as EE lowered shield #67, a rock measuring 12""X 12"" X 12"", fell from between shields 67 & 66. The rock landed on the cable tray and slid down the face to shield 61 where it fell off striking EE in the right ankle, resulting in a hairline fracture to the ankle. EE was released to modified duties.