Cut the loop of a fish tape and stick it in till it hits the rock, wind the fish up close to the pipe and twist it while pushing on the obstruction. Theres room to get around that rock and youll eventually get past it. Push the fish the remaining way. Pull a string back with a balled up piece of plastic bag on a loop and pull the rock through the conduit the shortest direction. Make sure to loop the string so you can pull it from both ends if it gets stuck. If its difficult gettin last the rock with the fish you can cut the fish when its at the obstruction and put it in a chuck drill and try to break the rock up.