Closer to Cat's answer. It is a knothole. It does have moss on it. But please do not ask me for the species of tree. All I know about the tree is that it is about four inches in diameter, and it is located right next to my bus stop. LOL.
It could be a H.R.Giger knot.. in which case it might be the knot from ALIEN.
But it actually looks like a fossile.. so I'm not sure. The Fred Flintstone and Benjamin Jacob Grimm Memorial?
-kiki2