Mud cast

This was an interesting find. It was a hard clod of mud with a crack that revealed what looked like an edge of a shell. I gently pried it apart, and it contained a perfect casting of a shell imprint. In other words, a shell was there once, then went away, leaving an imprint of its outside. Mud came along and buried the imprint. Everything hardened up until I found it. The half on the left is convex, the half on the right is concave. How old it? Maybe a year, or maybe ten thousand. It was a fun find, even though it is not rock.