So, go to a soils page and look up "Mexico Silt loam". It's basically 0-8" of topsoil, then it's various flavors of clay all the way to fire clay and then bedrock. Sometimes there will be a shale seam in there somewhere, just to screw up a pond dam. Under the shale is more clay. Creek bottom soils like Belknap are basically clay on top of sand. There are some better soils up in the Macon, Shelbyville area that are deeper, but they're 12-16" on top of multiple feet of clay.