I just came across a book on the mountain man upon whom the Robert Redford film Jeremiah Johnson was supposedly based. So I bought it, being interested in life in remote places.

His name was actually John not Jeremiah and Johnston not Johnson. (AFAIK, Johnston with a "t" is a Scottish borders name, but that's by the by.)

Anyway, legends seem to have accreted around this man, John Johnston. However, one informant who knew the man was known as truthful, and was conscientious about giving exact information to a historian who interviewed him, even taking a train to follow and catch up with the historian to tell him he realized he'd misremembered some details. Details about tribes and terrain were also checked with the Bureau of American Ethnology. So presumably the story is not all tall tales.

It turns out that Johnston was not as cuddly as the character played by Redford, being pretty much "mad, bad, and dangerous to know". The informant mentioned above was actually nervous about speaking of him to the historian even though he'd been dead for many years!

Here's how the book starts:

One May morning in 1847, Crow Indians killed and scalped John Johnston's pregnant wife; for many years thereafter, he killed and scalped Crow Indians. Then he ate their livers, raw.
Talk about getting primal on someone's ass -- or rather liver!

Amazon.com: Crow Killer: The Saga of Liver-Eating Johnson (Midland Book) (9780253203120): Raymond W. Thorp Jr., Robert Bunker: Books