By the end of the 1960s, many rock musicians had returned to the rootsy sources of rock and roll's origins, leading to what Barney Hoskyns called a "retrogressive, post-psychedelic music" development; he cited the country rock and blues/soul-inspired rock of the Rolling Stones, The Band, Delaney & Bonnie, Van Morrison, and Leon Russell. At the same time, a more avant-garde development came with the contingent of artists associated with Frank Zappa, including The Mothers of Invention, Captain Beefheart, Wild Man Fischer, The GTOs, and Alice Cooper. According to musicologist Frank Hoffman, post-psychedelic hard rock emerged from the varied rock scene, distinguished by more "cinematic guitar stylings and evocative lyric imagery", as in the music of Led Zeppelin, Black Sabbath, and Robin Trower. Music scholar Edward Macan notes that the "post-psychedelic hard rock/heavy metal styles" that emerged had "a weaker connection to the hippie ethos" and "strongly emphasized the blues progression". Psychedelic rock, with its distorted guitar sound, extended solos, and adventurous compositions, had been an important bridge between blues-oriented rock and the later emergence of metal. Two former guitarists with the Yardbirds, Jeff Beck and Jimmy Page, moved on to form key acts in the new blues rock-heavy metal genre, The Jeff Beck Group and Led Zeppelin, respectively. Other major pioneers of the heavy metal genre had begun as blues-based psychedelic bands, including Black Sabbath, Deep Purple, Judas Priest and UFO.