He saw those same rocks on the surface of the farm, built into the well structure. There were no holes within ten feet of that well. When he had ran into the golem for the first time, the magical flame of his burning brand had been sucked into them. That’s why the witch couldn’t escape, that’s why she had been sending magical blasts to the surface, she has been trying to get out of what was essentially, a prison was designed to absorb magic. With the golem as a warden.