>> Brenden, you could ask your local extension office, they should have more idea.
Here's MI extension contact info by county:
http://msue.anr.msu.edu/county
I think you would need a finer map than just "county" to find what the deeper soil horizons are, right where you live.
But if that gray layer is clay, it doesn't matter very much to gardeners what's underneath it. Since water won't penetrate it very fast, rain water will run off according to the slope of the permeable layers above the clay.
Since oxygen can't penetrate it very well, it will have little soil life and no roots growing down through it. So it could be bedrock, sand or green cheese, as far as your plants are concerned.
