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American rock seems to have come to terms with the fact that it's now an essentially conservative force. Tom Petty and Sebadoh release singles ('Skull' and 'How It Feels') bearing an alarming melodic resemblance, testifying that they're mining the same Byrdsy/Beatlesy seam. Neil Young and Pearl Jam cross a 20-year generation gap to discover that their soundworlds and worldviews are perfectly attuned. Rancid resurrect The Clash, complete with sore-throat English accent, spraying gob and punky-reggae riddims. Guided By Voices plan a Tommy-style rock opera. Even Royal Trux pare down their avant-raunch to a Grand Funk Railroad pastiche. Reverence and referentiality are all-pervasive; parricide and canon-busting nowhere to be found. The best anyone in America seems able to hope for is that rock will abide, will carry on much the same as always.

The current moment in white American rock feels uncannily like Britain in 1979. Kurt Cobain's suicide seemed like a death-knell for rock itself, the last word (how do you top topping yourself?), just as Johnny Rotten's auto-destruct or the Pistols and Sid Vicious' OD signalled "rock is dead" back in '78. Rotten reverted to his given name, Lydon, and formed the experimental, studio-based unit, Public Image Limited, who quickly became figureheads or an anti-rockist vanguard (The Raincoats, Cabaret Voltaire, The Pop Group, Scritti Politti, Gang Of Four et al. PiL saw Lydon return to his first, true loves, dub and Can; the result was Metal Box, music that retained the emotional force of rock but expanded/exploded the form. Post-rock, in other words.

Perhaps Tortoise are the American PiL, and their second full-length album, Millions Now Living Will Never Die, is the US underground's long overdue Metal Box: sixteen years tardy in its rapprochement with dub, maybe, but much more timely in its flirtations with avant-techno and trip hop. Grunge and lo-fi returned to the grubby garageland values of pseudo-spontaneity, but Tortoise know that if rock has any future it will involve the fusion of real-time improvisation with the studio's virtual space and time-warping tricknology. Rock must mutate and miscegenate; anything else is living death or trad-jazz style necrophilia.

Typically self-effacing, Tortoise are quick to point out that bands were doing the cross-hybridising thang 15 years ago. But that just proves the 1979 thesis; after Green Day and Rancid's Anglo-pop-punk revivalism, it's time for the post-punk experimental resurrection: The Pop Group, This Heat, A Certain Ratio, Pere Ubu...

"So we're retro, too!" says Tortoise's drummer, John Herndon.

Yes, but in a weird, back-to-the-future way, a future that was prematurely abandoned.

"No, man: we're forward-to-the-past," quips bassist Dan Birney.

Then again, even those 1979 anti-rockists were really picking up the baton dropped by Krautrockers Can, Neu and Faust, by the Canterbury crew (Henry Cow, Robert Wyatt, etc.), and by Tim Buckley, Beefheart, etc. Almost as soon as "rock" consolidated its identity in the late Sixties, there's been post-or avant-rock; for decades, a post-rock continuum has been running in parallel with the mainstream. And when the mainstream gets dull, people's attention starts to drift to the outerzone.

As if in preparation for the soon-to-arrive droves of discontented, the American underground is currently quivering with the activity or brainy bands trying to think their way out of the retro-obscurantist dead end that lo-fi quickly proved itself. Escape routes range from nouveau kosmik/kraut rock (Sabbalon Glitz, Space Needle, Cul de Sac) to proto-ambient dronescapes (Stars Of The Lid, Jessamine, Labradford), from neo-Neu! analog-synth-propelled trance (Trans Am, Rome) to avant-funk groove science (Ui, Run On). Tortoise touch on all these avenues of possibility, but they also throw in some studio-sorcery inspired by dub and hip hop producers like The Scientist and Hank Shocklee.

Already prime movers in the new American post-rock network, Tortoise are well on their way towards the kind of pivotal role that Pavement had a few years ago vis-a-vis irony-drenched lo-fi.

If Tortoise's innovatory music is deceptively low-key, Tortoise as people are just low-key. They may have an un-American awareness of dub and ambient, but like almost all US rock bands they are annoyingly averse to auto-hype, manifesto-mongering, or any kind of categorical/colourful utterance. Chronically non-committal, they downplay their significance and originality at every opportunity. Let me tell you, there were times during this interview when I'd have gladly traded my left leg to be interviewing any old bunch of Camden-based media-whore chancers, just for some rentaquote action.

Tortoise didn't so much form as coalesce, gradually accumulating members who were frustrated with the limits of their previous, guitar-centric bands (which include Tar Babies, Eleventh Dream Day, Poster Children, Bastro and Slint). The germ of the band was the duo of Herndon and bassist Doug McCombs, "a fantasy of being this rhythm section like Sly & Robbie that could operate alone or work with other groups, and was more about creating super-minimal interludes than actual songs." Signalling their post-rock intentions, Tortoise was initially formulated as a guitar-free zone.

Tortoise have since relaxed the no-guitar rule, but rarely. Even the twangy, spy-movie guitar motif on the new LP's tres Portishead-like 'Along The Banks Of Rivers' is actually a six-string, piccolo bass. Some songs feature three or four basses. Other elements in their textural arsenal include marimba, Rhodes electric piano, strange African percussion and antique analog synths.

American rock criticism is totally fixated on the notion of the song-as-story, as autobiographical confession or pseudo-literary vignette. "Resonance" is the big buzzword; music that has no text to interpret (whether sociological narrative, or the singer's individual neurosis) simply does not compute. Which is why American's current crop of wholly or partially instrumental bands – Tortoise, Ui, Labradford et al – are so unusual, so refreshing, and so ignored in their native land. This is music that can't be analysed using Lit Crit or Social Studies techniques; metaphors from the pictorial arts or cinema seem more appropriate for these audio-sculptures, soundscapes, mood-mosaics and soundtracks-to-imaginary-movies.

On Millions Now Living, Tortoise flit effortlessly from modern exotica (a chiming and tinkling gamelan dinner music), to Ennio Morricone/Ry Cooder themes unmoored from their celluloid settings; from crystalline jazz-punk a la Slint's Spiderland, to Mouse On Mars-style art-tekno enchantment. On the astonishing opener, 'Djed', a 21-minute musaic of seamlessly linked segments, Tortoise are all of the above and more. I'd assumed that 'Djed' signified "dee-jayed", and referred to the way the track is the missing link between the collage aesthetic of The Faust Tapes and today's DJ cut'n'mix and remixology techniques. But, relentlessly, oblique as ever, Tortoise apparently intended 'Djed' to be a person's name, like Jed Clampett. The track does proceed by "remixing" itself, with one element from the preceding segment surviving the transition. But, two thirds of the way through, evolution gives way to catastrophe: the track seems to shake off its skin like a chrysalis, and rebirth itself as a totally different, Aphex-like dub-tekno reverbscape.

It turns out that John McEntire, the band's drummer and engineer (he's also produced much of the new Stereolab LP), had gotten fed up trying to find a good edit for a transition. Exasperated, he grabbed "all these little bits of tape I'd cut off, 40 or 50 tiny scraps on the floor, and I said, 'Fuck it!'" Drawing on the tape-editing skills he gleaned when studying Fifties musique concrete at Oberlin college, McEntire pasted them arbitrarily together; hence the jolting, tape drop-out effect of the transition. The first time I heard this "bridge", I leapt up to check the cassette deck wasn't chewing up the advance tape. It still never fails to startle.

"Every person I've played that for so far has thought the CD was breaking," says Doug McCombs. "It's just more of the genius of Tortoise – always keep 'em jumping!"

© Simon Reynolds, 1996

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Tortoise play on the Quietus Village Mentality stage at Field Day, Victoria Park, this Saturday 2nd June. They join a bill that also includes the likes of Mazzy Star, R. Stevie Moore, Julia Holter, Grimes, Laurel Halo, Kassem Mosse, Franz Ferdinand, Andrew Bird, Toy and more. For more information and tickets head to the Field Day site.