The heavier rock of the 60s/70s is sometimes called proto-metal. Some of the stuff made by bands like led zepp, deep purple, Steppenwolf, etc. Hell, even the Beatles' Helter skelter is considered proto metal by some.

Also, many metal albums used to have a ballad or two, even bands like Testament, Metallica, etc so it's not that unusual to hear acoustic songs alongside heavier ones.

Not sure if that answers your questions, feel free to ask more specific ones or whatever. It's one if my favourite subjects xD