But we have only been measuring CO2 for a very short slice of the planet's history. When tropical conditions existed over much of the planet, long before human interference, CO2 levels were undoubtedly higher than they are now. Coral reefs in Michigan (Petoskey stone) are a good indicator of previous global warmth cycles.

As for the thin layer of sand over your shells, Jim, the glaciers had a hand in that. We have two pieces of property that are glacial end-morianes, the dumping grounds when glaciers retreated. Downstate we have a ridge of 10' of pure sand over clay substrate. Up north we have about 15' of pure sand. Oil/gas deposits are at about 4500' down up north, and as the source of those deposits was decaying organic matter, the terrain was obviously much different in a previous global era.