It sounds like you are in some fairly soft shale.
One way to rip it loose would be to get a 3 pt tool bar and a couple of ripper teeth to mount on it and a weight bracket.
With a single ripper shank mounted to the uphill side as you back in drop the ripper and pull away.
Shale always has one direction that it will rip easier then any others, if the layers have the edges facing your ripper
it can and will dig it an tear it up, if the seams (edges) are going in your direction many times a ripper will ride up the smooth
face and not penetrate to be able to rip. In that case turn a ripper shank around and back into it, carefully.