I would say it's possible it's natural but has the definite look of being man made. I just don't think it was a maul which would have been swung hard and should show serious wear on either end. As well as the narrow groove wouldn't facilitate a handle stout enough to not break when used. What its purpose was I have no idea. It may help if the finder told us which state it was found in.Newman you seem to be a person of knowledge . This is not a negative question. How would a stone like that be formed ?