the MC5 and The Stooges, who employed a much more aggressive style. These bands began to be labelled punk rock and are now often seen as proto-punk or proto-hard rock. Blues rock In America blues rock had been pioneered in the early 1960s by guitarist Lonnie Mack, but the genre began to take off in the US the mid-60s as acts developed a sound similar to British blues musicians. Key acts included Paul Butterfield (whose band acted like Mayall's Bluesbreakers in Britain as a starting point for many successful musicians), Canned Heat, the early Jefferson Airplane, Janis Joplin, Johnny Winter, The J. Geils Band and Jimi Hendrix with his power trios, the Jimi Hendrix Experience and Band of Gypsys, whose guitar virtuosity and showmanship would be among the most emulated of the decade. Early blues rock bands often emulated jazz, playing long, involved improvisations, which would later be a major element of progressive rock. From about 1967 bands like Cream had begun to move away from purely blues-based music into psychedelia. By the 1970s blues rock had become heavier and more riff-based, exemplified by the work of British bands Led Zeppelin and Deep Purple, and the lines between blues rock and hard rock "were barely visible", as bands began recording rock-style albums. The genre was continued in the 1970s by figures such as George Thorogood, but bands became focused on heavy metal innovation, and blues rock began to slip out of the mainstream. Folk rock By the 1960s, the scene that had developed out of the American folk music revival had grown to a major movement, using traditional music and new compositions in a traditional style, usually on acoustic instruments. In America the genre was pioneered by figures such as Woody Guthrie and Pete Seeger and often identified with progressive or labor politics. In the early sixties figures such as Joan Baez and Bob Dylan had come to the fore in this movement as singer-songwriters. Dylan had begun to reach a mainstream audience with hits including "Blowin' in the Wind" (1963) and "Masters of War" (1963), which brought "protest songs" to a wider public, but, although beginning to influence each other, rock and folk music had remained largely separate genres, often with mutually exclusive audiences. The folk rock movement is usually thought to have taken off with The Byrds' recording of Dylan's "Mr. Tambourine Man" which topped the charts in