Everything is scribed and wedged into the rocks, you gotta take into account the weight of the structure as well. I'm not sure of your specific problem but maybe try renting a cordless hammer drill and using drop in anchors or even dropping some rebar into the holes and doing a stacked rock/concrete/dirt lip.

 what do you use to anchor wooden features into rock i have a lip i need to build on the top of a large rock face for a 15ft drop and im not certain how to anchor a wooden frame for the lip. any tips?