Photo: Brandon Cuicchi

Toward the end of last night's Tortoise show at Webster Hall, what had been an adventurous, enervating night of instrumental math-rock turned unexpectedly into a heavy-metal rock-a-thon. Despite our concerns about a band we'd last seen a full ten years before, hearing a combo well into their second decade of making music together had been quite satisfying. As usual, virtually the entire group played switch-hitter, moving wordlessly from guitar to drums to keys to vibraphone. Many songs played out with two drummers, often John McEntire and John Herndon, hammering away to craft a skittering, disjointed beat. And this remains the group's forte: creating the impression of improv while adhering to shifting rhythms and complex tempos.

That's why the show's most surprising moment was 2004's "Crest," which the band slowed down into a dark blast of crushing drums and wailing guitar. Dan Bitney, playing vibes, headbanged so hugely he swung his head almost down to his waist. Some of the crowd joined in while others looked on in confusion; who knew a jazz-rock fusion show could transform so quickly into Headbangers' Ball? —Brandon Cuicchi