Aside from the genre's punk and alternative rock roots, many grunge bands were equally influenced by heavy metal of the early 1970s. Clinton Heylin, author of "Babylon's Burning: From Punk to Grunge", cited Black Sabbath as "perhaps the most ubiquitous pre-punk influence on the northwest scene". Black Sabbath played a role in shaping the grunge sound, through their own records and the records they inspired. Musicologist Bob Gulla asserted that Black Sabbath's sound "shows up in virtually all of grunge's most popular bands, including Nirvana, Soundgarden, and Alice in Chains". The influence of Led Zeppelin is also evident, particularly in the work of Soundgarden, whom "Q" magazine noted were "in thrall to '70s rock, but contemptuous of the genre's overt sexism and machismo". Jon Wiederhorn of "Guitar World" wrote: "So what exactly is grunge? ... Picture a supergroup made up of Creedence Clearwater Revival, Black Sabbath and the Stooges, and you're pretty close." Catherine Strong states that grunge's strongest metal influence was thrash metal, which had a tradition of "equality with the audience", based on the notion that "anyone could start a band" (a way of thinking also shared by US hardcore punk, which Strong also cites as an influence on grunge) which was also taken up by grunge bands. Strong states that grunge musicians were opposed to the then-popular "hair metal" bands.