 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.