Depends on what kind of fence. In rocky fields in Kansas and Montana you'd often see double posts like two poles of a teepee with the rails running parrallel to the ground. I've seen chain link fences with holes drilled in the rock and set in cement. That method would work wth any wire fence. I've also seen anchor bolts set in rock and the posts bolted down with them. Some of these had bracing also bolted to the rock surface.I haev a fence that keeps falling over too. The problem is that it sits on a rock ledge so I can't drive posts! Any ideas?