most successful British rock acts by presenting an apparently unsophisticated style of boogie-based rock music; and Van Morrison gained international critical acclaim through a blend of rock, jazz and blues styles. Some well-established British bands that began their careers in the British Invasion, notably The Rolling Stones, The Who and The Kinks, also developed their own particular styles and expanded their international fan base during that period, but would be joined by new acts in new styles and subgenres. New subgenres in the 1970s British folk rock British folk rock developed in Britain during the mid to late 1960s by the bands Fairport Convention, and Pentangle which built on elements of American folk rock, and on the second British folk revival. Using traditional English music as its basis, these bands drew heavily on the Child Ballads, ballads of the British Isles from the medieval period until the 19th century. An early success was Fairport Convention's 1969 album Liege and Lief, but it became more significant in the 1970s, when it was taken up by groups such as Pentangle, Steeleye Span and the Albion Band. It was rapidly adopted and developed in the surrounding Celtic cultures of Brittany, where it was pioneered by Alan Stivell and bands like Malicorne; in Ireland by groups such as Horslips; and also in Scotland, Wales and the Isle of Man and Cornwall, to produce Celtic rock and its derivatives. It was also influential in those parts of the world with close cultural connections to the UK, such as the US and Canada and gave rise to the subgenre of Medieval folk rock and the fusion genres of folk punk and folk metal. By the end of the 1970s the genre was in steep decline in popularity, as other forms of music, including punk and electronic began to be established. Progressive rock Progressive or prog rock developed out of late 1960s blues-rock and psychedelic rock. Dominated by British bands, it was part of an attempt to elevate rock music to new levels of artistic credibility. Progressive rock bands attempted to push the technical and compositional boundaries of rock by going beyond the standard verse-chorus-based song structures. The arrangements often incorporated elements drawn from classical, jazz, and international sources later called "world music". Instrumentals were common, while songs with lyrics were sometimes conceptual, abstract, or based in fantasy. Progressive rock bands sometimes used concept albums that made