Oh my. My, my, my. Read the "more" of his biography here. Why, why, why...
I'd just like to add that whatever he was, Carter was an odd sort of clever and he tapped into a zeitgeist which managed to fascinate me 3 decades later.
Another book I've read suggests that the Scots-Irish in Appalachia were the last frontier between the British and the Indians and the first to fight for their indepedence in the War of Independence and possibly, the Civil War.
By creating a self-reliant grandfather, half Scottish, half Cherokee, he may have tapped into a parable of America. The surname Carter also appears to be an Irish surname.