Most critics view 1971's Nursery Cryme as the first classic Genesis album – the band's progressive watershed. It's a logical view: Their third LP brought drummer Phil Collins and guitarist Steve Hackett into the fold, cementing the most dynamic lineup in prog-rock history. But that hype unjustly overshadows what came before: Trespass, the crucial turning point in the band's discography.

The album, released on Oct. 23, 1970, abandoned the precious teenage twiddling of the previous year's over-produced From Genesis to Revelation. Freed from the smothering production of Jonathan King, the band dove into fanciful imagery and lush, long-form arrangements, helping cement the folk-prog sub-genre.

"From the beginning, we'd set out as songwriters," singer Peter Gabriel reflected in the album's reissue interview series. "And we were quite happy to be writing what were effectively pop songs. But I think there was always inside of us a yearning to explore, to push the boundaries and to mix styles. And as we started to get a little more adventurous, it got too out of mainstream, left-field for Jonathan and for the publisher we were working with."

The album was recorded at London's Trident Studios during June and July 1970, which guitarist Anthony Phillips calls "the summer of many drummers" – a reference to their ever-revolving drum throne. Though influenced by late-'60s psychedelia and the experimentation of Family, Procol Harum and Fairport Convention, Trespass found the band merging their disparate backgrounds – Gabriel's love of soul music, Tony Banks' classical and pop expertise and the ethereal 12-string folk of Phillips and Mike Rutherford – into an unusual new style.

"The folk sound was definitely from Ant and Mike and this 12-string combination," Gabriel noted. "I think that was really quite innovative. And I loved it and tried to encourage it from my point of view."

Listen to Genesis' 'The Knife'

Tracks like "White Mountain" were based predominantly on the pastoral twin 12-string attack, while heavier rockers like "Looking for Someone" and "The Knife" (originally titled "The Nice," as a nod to Keith Emerson's keyboard-heavy group) originated from the Banks-Gabriel camp. The most collaborative piece – and the one that Banks acknowledges as the album's peak – is the majestic, nine-minute epic "Stagnation," which combines both of these extremes. (Gabriel referred to the track as a "journey song," navigating through a "series of landscapes.")

Trespass feels like an outlier in the Genesis oeuvre because it's so damn gentle – "Dusk," in particular, breezes by without any sense of urgency. But considering the heavy input of Phillips, the de facto leader, this band is very different from the quintet that created Nursery Cryme.

Phillips, suffering from intense stage fright exacerbated by health issues, soon announced his departure – a decision that left the band scrambling. “We went back on the road and I just couldn’t do it," the guitarist told The Telegraph in 2014. "I had to tell Mike. It was difficult, but I knew I was going to hold them back.”

With its quiet reception, the album didn't thrust the band into stardom (though it did curiously land at No. 1 on the Belgian charts), and Phillips' exit only intensified their anxieties. But Genesis rebounded with renewed focus and firepower for Nursery Cryme, taking another massive creative leap.

Looking back, Trespass is more than just a crucial link to the classic era – it's the sound of Genesis discovering their songwriting strengths. "If you look at, say, Crosby, Stills, Nash, & Young, 10CC, other bands which are songwriter-based, rather than musician-based, I think you do see long life spans," Gabriel said. "It's a different sensibility. Most bands begin playing together and then find writing, whereas we wanted to write and we found playing."

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