tim1198 wrote:
I have the same rock problem. I plan on digging what I can, and when I
hit a rock, using a rockdrill to drill out 1/2" holes to epoxy rebar.
Then you can drop your posts on top of the rock and pour cement aound
it. Another method is to use a bigger rock drill bit (like 5" for 4x4
posts), drill into the rock and then drop your posts in the hole and
fill with cement or epoxy.

Let me know what ends up working best for you, since I have to build a
deck and fence on a rockledge.

Depth is important because it keeps the post upright. But you can also
use a large disk of concrete on the surface to achieve the same effect.
Or, you could use two long pipes, stronger than rebar, at right
angles drilled through the wooden post.