EE & co-workers were in the process of cleaning rock off of the LW head drive. Member was lying on his belly with upper body positioned under the #2 LW shield. A rock approx 8"" x 6"" x 1/2"" fell off the shield tips striking his right calf. He walked to the mantrip & was brought to the surface. He was taken to Boone Memorial. He was diagnosed with a deep contusion to right calf.