The Lamb Lies Down on Broadway is Genesis' best-known enigma, a project almost universally respected if seldom understood.

Often impenetrable and frankly overlong, this double disc effort also features Peter Gabriel's most approachable band song in "Carpet Crawlers." The album, to that point, was Genesis' biggest seller in its native U.K., and eventually the death knell for the group's initial phase.

By the time it was over, Gabriel had left for a celebrated solo career, and Genesis had been reshaped forever. Even now, The Lamb Lies Down on Broadway – which was released in November 1974 – is a bundle of contradictions, mysteries, narrative twists and real-life turns.

Just like its principal creator, Gabriel.

Genesis put forward a tough character named Rael in a difficult situation – and it had more in common with the Who's rock operas than with anything the sometimes twee Genesis had done before. And, from the start, Tony Banks felt they'd gone too far.

"It’s okay," he told the Telegraph in 2014. "But it’s not the greatest piece we’ve ever done." Gabriel counters: "[Tony] just didn’t like me getting away with too much, or getting into a controlling position. He wanted to keep check on my power." Still, Gabriel has consistently said, “If you really want to define a world, you have to let one person paint it. There weren’t many novels created by committee.”

Gabriel's novel, dense and full of strange imagery, found a half-Puerto Rican street tough named Rael roaming through a hellish New York City, trying to rescue a lost sibling. That couldn't have had less in common with an early suggestion from fellow bandmate Mike Rutherford that the group base its next album on the classic The Little Prince, a coming-of-age fable by Antoine Saint-Exupery.

"This was 1974," Gabriel pointed out in Without Frontiers: The Life and Music of Peter Gabriel. "It was pre-punk, but I still thought we needed to base the story around a contemporary figure, rather than a fantasy creation. We were beginning to get into the era of the big, fat supergroups of the '70s, and I thought, 'I don't want to go down with the Titanic.'"

Listen to Genesis Perform 'The Lamb Lies Down on Broadway'

Still, for everything it did to push the envelope, The Lamb Lies Down on Broadway will always have a valedictory feel. This is the album that precipitated Gabriel's departure from the Rock and Roll Hall of Fame group that he had co-founded. Sorting out just why has proven every bit as interesting as anything found in Rael's timeline.

In one sense, Gabriel left Genesis because he had done all he could do creatively. The Lamb Lies Down on Broadway was as monumental, as complex, as triumphal and as mainstream as the group had ever been. So Gabriel released a resignation letter to the music press, declaring that his departure was meant to facilitate a focus on "other literary and experimental interests outside of music."

Phil Collins later admitted that he and the others "were not stunned by Peter's departure, because we had known about it for quite a while."

Gabriel continued touring with Genesis for six months after telling them he was leaving. "There was no enormous schism; there was no affair with another band member’s wife; he was not out of it on drink or drugs," Daryl Easlea, author of Without Frontierssaid in 2013. "He’d just had enough and knew that if he wanted to get on, it had to be outside of the group."

Gabriel nevertheless seemed to be of two minds as The Lamb Lies Down on Broadway progressed, resigning at least before once only to return. He'd disappear during the sessions but refused to relinquish his central role in the project.

He was torn, on one hand, by an offer from film director William Friedkin, who'd won an Oscar for The French Connection and had just released The Exorcist. The idea of a film version of The Lamb Lies Down on Broadway was batted around. Gabriel had also been in discussions about becoming a part of Friedkin's creative team.

The remaining members were left to create their own separate dynamic. When Gabriel first quit, Collins said the group always considered going forward alone, because so much instrumental music had already been written during Gabriel's absences.

Listen to Genesis Perform 'The Carpet Crawlers'

But their leader returned, and the album was eventually cobbled together. "You could see that the cracks had started," Collins told the Telegraph. "And when he came back, it was like papering over those cracks – and the cracks reappeared." But not before The Lamb Lies Down on Broadway appeared on shelves, a final testament to a group falling apart. "It was a type of Pilgrim's Progress, but with this street character in leather jacket and jeans," Gabriel said. "Rael would have been called a punk at that time, without all of the post-'76 connotations." Ultimately, Gabriel described it as "a traditional concept album," which "was looking toward West Side Story as a starting point."

The Lamb Lies Down on Broadway went gold in the U.S., in France and (a first) in the U.K., reaching the Top 10 at home and almost making the Top 40 in the States. But that didn't change things for Gabriel.

Turns out, there was something more personal going on, too. Gabriel's daughter had a troubled birth and infancy, and in 1974, that seemed far more important to him than a career – with or without Genesis. It had become a sticking point throughout the album's sessions, which were held at the remote Headley Grange in Hampshire. Gabriel was often distracted and itching to return to his family.

The rest of the band, childless and in their 20s, couldn't understand. "Jill and Peter were the first ones to have a baby," Rutherford says in Without Frontiers. "When Angie and I had a baby, and Tony and Margaret had theirs later, we realized it was life-changing. Pete's came very early on, and we were not good at change. We were very unsympathetic towards him. That was a big part of the problem, really. It all came to a head with The Lamb Lies Down on Broadway.

"As you talk it through now, you can see it pretty clearly. We'd done a double album that was always rated by the fans but was not much fun to make for lots of reasons. A double album is a lovely idea, but it's hard to make on a good day – without having all the problems and the worries of what all went down."

Listen to Genesis Perform 'In the Cage'

In the end, Gabriel didn't just quit Genesis, he disappeared into parenthood. Genesis had become so dysfunctional at this point, Collins told the Telegraph, that Gabriel's family emergency was "unbeknown to me, because I didn’t ask. That’s how selfish we all were in those days. His first wife was having a difficult pregnancy. No one knew."

Gabriel eventually disappeared again, this time into world music – finally emerging a couple of years later as a fully formed solo artist, almost completely opposite of the figure he had been before. So, in the end, The Lamb Lies Down on Broadway isn't just his final album with Genesis; it also closes the book on a chapter of the restless artist's life.

The question that remained for Genesis – Who would take over as frontman? – seemed insurmountable at the time, even though history tells us that Collins made an easy transition from the drum chair to the microphone. But in this moment, as Gabriel was preparing to exit, he stopped to encourage Collins, a prospective singer who would one day hurtle Genesis to the top of the pop charts.

It's an irony not lost on Gabriel today. "We'd be in the dressing room after soundcheck working on song ideas at the piano," Gabriel told the Quietus in 2011, "and I remember encouraging him and saying, 'You could make it as a singer if you wanted.' Ha, ha, ha!"

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