Best explanation from the discussion thread:
It's not carved. These are likely iron and manganese concretions that develop similar to the way a pearl develops, but in a swamp. When soil undergoes constant wetting and drying it forms super concentrations of certain minerals; iron, manganese, and calcium being some of the most common. As it formed, gaps were filled in with silt and eventually it became encased in a mudstone. It has since eroded to reveal the layers of build up.
I've encountered a few concretions, but never any as elaborate as this one.