Whilst belaying my partner as he led P2 of Nightmare another team were up and right of us finishing the E1 alternate finish of that route. The second dislodged loose rock at the start of the headwall which bounced off the ledges below before thudding to the ground 15ft to the right of the start of the route. A couple of minutes before two climbers had been walking there looking at routes to climb, only one of whom had a helmet on at the time. Fortunately we had radios so I could check on my partner and he was ok. Team that dislodged the rock are an experienced pair and no one was injured. Wall was in parts and recent poor weather may have contributed to loose rock on lesser-climbed lines.