The past decade of modern American rock has seen the rapid rise and near explosion of genre fusion. While the increasing popularity of electronica and the universal appeal of hiphop are contributing factors, this was a historically inevitable development. The structure of rock 'n' roll is as reliable as ever, but it's logical that so many artists want to remodel the architecture and add their own annexes.

Such amalgamations can be amusing-but-forgettable (2001's temporarily revered Avalanches), new-genre defining (Flaming Lips' sugar-sweetened psychedelic tour de force The Soft Bulletin), or they can elevate the band above all the hyphenated descriptors to be recognized as simply a high-quality American rock band. It is the latter benchmark that My Morning Jacket has carved out with the graceful execution of their fourth full-length, Z.

When this Louisville, Kentucky band first bent ears with their 1999 debut, The Tennessee Fire, critics and fans had an incredibly difficult time describing their sound. "Neil Young fronting the Flaming Lips" was a typical and understandable analogy—frontman Jim James possesses a similarly haunting, weathered falsetto and a preference for celestial-sounding guitar lines—but that was only part of the picture. What distinguishes My Morning Jacket is their fusion of palpable vulnerability and euphoric, childlike optimism in their songs.

"I remember hearing [the band's] music and deciding I was going to do everything possible to be a part of it," recalls Carl Broemel, a veteran musician who got the gig as MMJ's second guitarist last year via an introduction by fellow Nashville-ite Bobby Bare Jr. "When I first heard their music, it was a very emotional reaction—like listening to Beethoven—it grows and grows slowly without any irony. It's very moving, really."

What he's referring to is immediately apparent when the pulsating, low-bass keyboard line starts pulsating on Z's opening track. Quite simply, they've merged the bravado of classic-rock foundations with an extremely engaging sense of humility and topped it with a light dusting of countrified psychedelia.

Part of what helped bring the band's already majestic-yet-modest tones to a concise crescendo was the involvement of revered British producer John Leckie, the mastermind behind such definitive opuses as George Harrison's All Things Must Pass and Pink Floyd's Dark Side of the Moon. After six weeks of rehearsals, the band hibernated in upstate New York's Allaire Studios with Leckie and began bringing the 10 songs on Z to fruition.

"We were basically snowed in for three and a half weeks in this mansion on a hill, with nothing to do but eat, sleep, and record. It's pretty much Jim's ship to drive, but any idea is fair game," says Broemel, describing their creative process.

Thanks to the extensive rehearsals and James's perfectionist tendencies, they were able to track much of the album live, with minimal overdubbing—a remarkable achievement, given the album's positively mammoth sound and glittering, extraterrestrial atmospherics. As always, James' ghostly, reverb-saturated vocals echo deeply and give the songs their dreamy, majestic sprawl, while drummer Patrick Hallahan's bedrock percussion gives the album a necessary sense of gravity and strength.

Z is the sort of release that will find new listeners 20 years from now and will be remembered as one of 2005's most legitimately lauded masterpieces—and one that left even more flamboyant genre bending by the wayside.