IE was bolting 2 Left (HG-17) and had hog panel 3 pieced to help cover the top. IE pinned EE's side than helped EE's partner finish EE's row. They advanced to the next row and as IE got underneath EE's canopy. A rock fell out between the atrs pad and last permanent row. Rock hit the roof bolter tray than hit IE's elbow. IE was diagnosed with a contusion.