The area where I live was completely glaciated. This has caused natural rock outcrops to be covered by glacial till. Very little habitat remains that supports plants which would be suitable for a rock garden. The habitat that remains is either alvar (SW of my area), end moraine, or rock canyons that occur South of my area. I am posting a link that contains lots of photos. This website was created by the people who volunteer to maintain this Nature Preserve. The photos on this website should given you an idea of the plants that can grow on rocks (technically gravel) in my area. If you visit this area please do not disturb it. Everything is protected by law. Most of these plant can easily be purchased from a number of nurseries if you are interested in cultivation.
James McGeeNE Illinois