You can't leave out Bob Dylan's influence with introducing the band to marijuana, as well as Jimi Hendrix not making it big in the US, and being "shipped" to the UK to start the Jimi Hendrix Experience. That didn't just have a big effect on The Beatles, but acid rock/psychedelia as a whole. You also had Black Sabbath taking blues music and downtuning it, and playing it slower, all due to Tony Iommi's work accident where he lost his finger tips. Modern metal music owes a TON to Black Sabbath, especially styles that play slower, like doom metal and all its various sub-genres. There was a lot of sharing between countries, and I think that is why the music is so closely tied together (along with the entire psychedelic movement).