Dave, it's not just a riff, it is the whole friggin' signature intro of the song. And that intro defines the song, if people play/ape Stairway to Heaven in a guitar shop, that is what they play, not Jimmy Page's solo at the end of the song. Those intro arpeggios have anybody who has listened to the radio in the last 40 years (especially in the US!) recognize that song immediately.
But its not just a bunch of chords, Zep chose the same mood/phrasing that it's uncanny.
I'm not saying that Page sat down with the Spirit track and decided "I'm gonna steal this from them." But I find it hard to rule out that he had heard the Spirit song before and that its progression and mood had not sat with him.
Whether that warrants a judgement today awarding zillions and or a co-write credit to Mark Andes and California's estate is another matter, but I've sure seen more frivolous cases. And Page might have graciously dealt with it sometime around the millenium by buying California's widow a house and seeing that his kids get a good education. It would have been the decent thing to do given what the song has done for Page and his compatriots.
It has nothing to do with me liking to knock Zeppelin around a bit (all of it in jest, Zep fans are generally a humorless bunch with religious zeal so they make a good target), if this was Purple I'd actually be even stricter in my views about it. With Zeppelin, I'm not expecting much, they went through the Peter Grant school of things and have always been bullies.
That said, whether they "loaned" that signature part of the song or not plays no role for Zeppelin's status in the pantheon of rock, they have other merits and songs they can be proud of. Himmel, I never even liked "Stairway ..." in the first place, for me it will forever sound pasted together and unremarkable with a jammy, but hardly brilliant ending, but music lovers worldwide beg to differ so who am I?
