Glenn Cornick, who has died aged 67 after suffering from heart failure, can take much of the credit for the memorable music created by Jethro Tull on their first three albums, when they were a tightly knit band and not merely accompanists for the increasingly dominant singer/flautist/composer, Ian Anderson.
Although the band achieved its biggest commercial breakthroughs after Cornick, the original bass player, left the group, with ambitious concept albums such as Aqualung (1971) and Thick As a Brick (1972), he and the drummer, Clive Bunker, were critical in forming the band's sound, which skipped deftly, and uniquely, between blues, folk and rock, with an added flavour of jazz.
Jethro Tull named themselves after the 18th-century agriculturist who invented the seed drill, and on the sleeve of their debut album This Was (1968), they appeared surrounded by dogs and dressed in quaint woodsmen's clothing, with Cornick in yellow waistcoat and orange bowler hat. However, their music was restless and forward-looking, and their first single, A Song for Jeffrey, was original enough to avoid easy pigeonholing. The album even made room for a version of Serenade to a Cuckoo, written by the jazz saxophonist Rahsaan Roland Kirk.
As the group progressed, Cornick acquired longer hair and an Indian headband, while Tull's music continued to explore the fertile soil of the post-psychedelic landscape. Their second album, Stand Up (1969), was not only distinguished by its unusual woodcut sleeve artwork, but also by such musical feats as Bouree (a jazzy version of JS Bach's Bourrée in E Minor, propelled by Cornick's lithe bassline) and the haunting We Used to Know, the chord sequence of which later reappeared in the Eagles' song Hotel California, the Eagles having toured with Jethro Tull early in their career.
"We still thought we were a blues band," Cornick commented. "We were all shocked when the album was released and people said, 'Oh, they're not playing blues any more.'" Stand Up topped the UK charts, and Tull also started to make inroads into the US, where they toured with Vanilla Fudge and Led Zeppelin.
Their third album, Benefit (1970), continued the band's impressive upward progress, though by now they were advancing into the realms of what became known as "progressive rock", with its complex textures and time signatures. Benefit reached number four in the UK charts and number 10 in the US, opening the door to arena-sized gigs.
In August 1970 the band appeared at the Isle of Wight festival, and then departed for a US tour. After the tour Cornick left the band, apparently not entirely of his own volition. As Jethro Tull's then-guitarist Martin Barre recently put it: "If something's not working, it's got to be fixed. Whether you say 'Look I've got to go,' or somebody says 'I think you ought to go,' is neither here nor there. There were bad feelings on many occasions and I think everybody knew what needed to be done." Cornick commented: "At the end of 1970 I was fired. No reason has ever been given."
Cornick was born in Barrow-in-Furness in Cumbria and later moved to Blackpool. He found his way to Jethro Tull via early groups including John Evan's Smash, in which Anderson also played. Cornick abandoned a civil service job to become a professional musician with the group, who headed for the south of England to look for more work. Smash got as far as Luton before splitting up, following which Cornick and Anderson joined up with Bunker and guitarist Mick Abrahams to form the first version of Jethro Tull.
After leaving the band he formed a new group, Wild Turkey, again signed to Chrysalis and for a time featuring former Deep Purple guitarist Ritchie Blackmore. Cornick was by now based in Los Angeles and later moved to Hawaii.
He is survived by his wife, Brigitte Martinez-Cornick, their daughter Molly and sons Drew and Alex.
• Glenn Douglas Barnard Cornick, musician, born 23 April 1947; died 28 August 2014
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