I found this on a gravel bar in se Kansas the surface rocks are Pennsylvanian age. it looks to me like fossilized mud cracks, first I would like to know if you guys agree. second I'm trying to figure out how this was formed, the cracks curve around the edge of the rock and show up on the bottom edge. my guess is that it was pressed up on the one side exposing it to the air, maybe from something stepping in the mud? anybody have a better explanation?