Zeppelin IV (1971), which contained elements of both American folk rock and English electric folk on ‘Stairway to Heaven’ and most obviously on ‘The Battle of Evermore’, on which Sandy Denny had the distinction of being the only person ever to be invited to do guest vocals on a Led Zeppelin album. These influences would also appear on later albums, but reduced as the band returned to a hard rock sound from Presence (1976) onwards. As Led Zeppelin moved away from electric folk, another long term survivor of the British blues movement, Jethro Tull, began to move towards it. Ian Anderson had produced Steeleye Span’s album Now We Are Six in 1974 and first demonstrated a clear interest in more traditional sounds on Minstrel in the Gallery (1975), but it was in 1977 with the release of Songs from the Wood (1977) that Anderson took the band into electric folk territory. All the songs on the album focused on rural life and, in addition to the normal electronic instruments and flute of the band, used mandolin, lute and a pipe organ. Two tracks, ‘Hunting Girl’ and particularly ‘Velvet Green’ followed the form of erotic folk ballads, much suited to Anderson's song writing interests. Two more albums followed in a similar vein: Heavy Horses (1978) and Stormwatch (1979) to form a loose folk rock trilogy, before Anderson moved into more electronic territory at the beginning of the 1980s. Ironically it was at this point that Dave Pegg of Fairport Convention would be the first of several members of that band to join Jethro Tull. Electric and progressive folk Progressive folk developed in Britain in the mid-1960s partly as an attempt to elevate the artistic quality of the folk genre, but also as a response to diverse influences, often combining acoustic folk instruments with jazz, blues and world music. As a result, it was already established in Britain, albeit a difficult to define and varied subgenre, before the advent of electric folk at the end of the 1960s. It can be seen as including performers such as Donovan, the Incredible String Band, Pentangle, Strawbs, Nick Drake, Roy Harper, John Martyn and the original Tyrannosaurus Rex. Some of this, particularly the Incredible String Band, has been seen as developing into the further subgenre of psych or psychedelic folk. The advent of electric folk had profound effects on this developing strand of the folk