In my experience the rock hard dirt is just the top layer of mud/poo/goo from the rainy season that has dried into a hard crust. If you dig a pitchfork under it it will flake off in sections and there is normal dirt underneath. The good thing is, the chickens can't dig holes all over through the hard stuff. The bad news is... once it gets wet again, you have extra inches of goo that are going to mix in with the new goo and make a deeper mess next time wet weather hits, stink twice as much and take three times as long to dry out...