HEAVY: Blood Mountain makes clear what sort of crossover potential the major-label suits saw in Mastodon.

The big story in heavy music right now is mammal metal, a blood-and-guts strain of post-thrash brutality lorded over at the moment by Lamb of God and Mastodon, two loud-and-proud Southern bands with new major-label discs out. Mammal metal is about size: crushing volume, weighty riffs, hefty imagery — heavy-metal staples that have been sacrificed lately at the altar of style and accessibility by young screamo dudes intent more on breakthroughs than on breakdowns. It’s also about a self-conscious return to roots, an embrace of metal’s enraged-nerd legacy. Mammal-metal makers aren’t good-looking guys; unlike their media-savvy successors, they don’t appear comfortable in photographs or give pithy soundbites.

What they do is rock. When news broke last year of Mastodon’s signing to Reprise, outsiders wondered why. The Atlanta-based quartet’s last album was a complicated homage to Moby-Dick called Leviathan (Relapse) — not exactly the sort of thing Hot Topic shoppers are known to pump through their iPods while browsing at the temporary-tattoo counter. Insiders, meanwhile, feared a change of direction, a softening of Mastodon’s attack.

Blood Mountain, the band’s Reprise debut, negotiates those two concerns. On one hand, it makes clear what sort of crossover potential the major-label suits saw in Mastodon. Although none of the songs here stays in one place for long — in many ways the requirement of a successful single, given the quick-cut nature of radio and MTV — the riffs are the most immediate in the band’s catalogue. “Circle of Cysquatch” squeals like a European ambulance siren; “Sleeping Giant” lumbers à la “Black Album”–era Metallica; “Crystal Skull” actually swings. With a sly groove that opens up into a lush, tuneful chorus embroidered with acoustic guitars, “Colony of Birchmen” could be by Queens of the Stone Age.

On the other hand, Blood Mountain has song titles like “Circle of Cysquatch” and “Colony of Birchmen” — hardly a concession to the kids who thrilled to “I’m Not Okay” by My Chemical Romance. Despite its cleaned-up production and intermittent melody, this is still unapologetically aggressive music designed for an audience familiar with the genre. Mastodon might be at their best when they toy with that audience’s expectations. “Bladecatcher” starts out with a sweetly plucked electric-guitar figure that tricks you into thinking the band are heading toward Zeppelin-ballad territory, a commercial move if one ever existed. Then drummer Brann Dailor kicks in and the song morphs into a furious prog-metal tantrum. It’s a great gotcha moment.

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Lamb of God’s Sacrament (Epic) is less complex than Blood Mountain, and that’s its virtue. This Virginia band play supertight, superspeedy metal with no room for frills or fake-outs; they’re a little like Slayer minus that outfit’s theatricality. “This is a mean and cold town,” growls frontman Randall Blythe in “Forgotten (Lost Angels).” “I hate this fucking place” — a pretty handy illustration of the band’s world view. You don’t get any of the sweetening that could attract new fans to Mastodon, but you do get simplicity and straightforwardness, and their allure can’t be overstated in a world where listeners’ attention spans have been winnowed down to seconds. It doesn’t take much longer than that to appreciate what Lamb of God do on Sacrament; their mammal-metal girth is at once apparent.