The pilot reported that during the takeoff ground roll from the private grass airstrip, the left main tire followed by the tailwheel, struck a rock that was partially hidden by the tall grass. The pilot continued the takeoff, proceeded to his destination and landed uneventfully. After landing, the airplane was examined and the rudder was found substantially damaged. The pilot reported no preimpact mechanical failures or malfunctions with the airplane or engine that would have precluded normal operation.