I do a lot of riding up rivvi and would love to catch these doing this sort of stuff. If its not nails then just where the stone path starts they push the grass over and put rocks down in the hope you will come off. There is a difference between not liking people sharing these locations on bikes and potentially injuring people an their livelyhoods etc.
They put stones across the bit at the start of where you go over to great hill and turn left to lead miners aswell. Some miserable fekkers about.