A few more nails might help solve that problem. Apparently the rock just wasn't up to it.
When we first started breeding german shepherds, I had a couple who would scale 6 foot dog wire and would even jump it. After we got through stringing electric wire in, over and around their pen, it looked like a concentration camp. Now they have the run of the back yard with 6 foot wooden privacy fencing(still have the electric wire though), and they seem to be happy with the larger area. Dogs resent small inclosures and will constantly try to escape. A large running area makes for happier animals. Besides, would you want to be locked up in your own bathroom with no ability to flush?