I reckon it was much earlier than that - the riffing and heaviness is evident in the Kinks and the Who respectively, and the "amps up to 11" thing is almost completely inherent in the music of Blue Cheer - who are at the root of grunge as well, IMO.
Next under consideration are the Cream, Spooky Tooth, Bakerloo, High Tide, May Blitz and a swathe of heavy blues bands around in the late 1960s-early 1970s.
Next to these heavyweights, Genesis are very much at the lighter end as a generalism - but there's no disputing that Prog Rock absorbed influences from ALL genres, and Genesis at their heaviest wrote some stonking riffs.
Also, consider that Metal and Prog Rock can have a lot more in common than hardcore fans of either genre would really like to admit.
Edited by Certif1ed