I was talking with some homies about this and we all agreed that there was a group of rock bands who got sick of rock around 1967 and started making "post-rock," before that was even a term people used.
I'm talking about bands like the Doors, Jethro Tull, King Crimson, Greenslade, Jade Warrior, Yes, MC5, Iggy Pop.
That's all part of metal, too, isn't it, but less the hard rock kind than the underground metal kind.