If it's anything like this old red clay here in WNC it's actually full of nutrients. It's just so hard packed that plants can't get their roots to penetrate it. The bad part is, the more you till clay, the more it breaks down and the harder packed it becomes.
You have to add something like compost and sand to loosen it up and you should have a good garden.
As KennyV suggested... get a soil test and amend what you have.
Good luck,
Rip