Just to make you feel better, about a year ago I got a real heavy post hold digger, anyway, with 10 inch auger, I tried to put some holes in a few spots, auger just spins as hits rock about 2 inch or so down, 8 in some places, so I added weight, and more weight, till I broke the digger frame, once, twice, and many more times, I think 8 inches was about the best.
I have re build the frame and added new teeth, but essentially, I have had to manually bar and dig, breaking rock or removing rock of various kind....the real hard kind, not shale or sandstone.
It makes me think of people fencing new land hundreds of years ago, with no machines, no AC, primitive tools and having to make posts from trees on own block