The general approach to categorizing Chapel Hill quartet Caltrop has been first to describe them as heavy metal, and then immediately to qualify that classification. “Caltrop is metal, but they really like The Allman Brothers and soul music, but not in a cheesy way,” goes one hypothetical descriptor. Another might go, “Caltrop sounds metal, except their guitars crackle more than crunch, and maybe their drummer listens to math-rock.”

That’s fair enough: From the start, Caltrop had as much to do with the steely menace of Slint as they did the stoner metal of Sleep. In some sense, they even suggested regional forebears Polvo, just with beards and brawn and an ample swill of Pipe’s toughness. Drummer John Crouch has always cared less about his own technicality and brutality than the band’s momentum and movement, while guitarists Sam Taylor and Adam Nolton together suggested Southern rock lines being unfurled into a psychedelic expanse.

Caltrop’s second LP, Ten million years and eight minutes, seems like an attempt to not only push this issue but to make it crumble at the core. The press materials for the record, for instance, only mention the word “metal” when quoting various music journalists (and even then, with qualifications), while an accompanying promotional photo depicts a scowling Murat Dirlik, the band’s bassist, looking tough while the rest of his bandmates simply laugh at himno black T-shirts and no self-serious frowns. Instead, with the band clad in work clothes and camouflage pants, the photo affirms that these are blue-collar dudes playing their and your blues, loudly and with tube amps.

The music certainly holds up its end of the deal. There are harmonies and howls, blissful eruptions and brutal tangents. As capable of being punishing (or is it punished?) as it is being peaceful, Ten million years realigns Caltrop instead with the heavier side of the last decade’s emergence of New Weird America, putting them somewhere between the stunted blues-rock of Pontiak and the wayward progressions of those first few records by Akron/Family. Ten million years comes stockpiled with the sort of transcendent moments its covera rhinoceros lifted by massive lepidopteron wingssuggests: Opener “Birdsong” builds into a righteous surge, Crouch and Dirlik pushing hard beneath an irascible chorus. “Form and Abandon” slingshots from German-engineered instrumental acrobatics to an aestival impasse, where the roar tucks itself in beneath gentle harmonies concerning existentialism and immolation. “Shadows and Substance” dissolves through a righteous jam, the guitars and rhythm section wrapping around one another in an infinite array of helices.

But that all sometimes gets tedious across these 53 minutes, the twists and turns turning into a rat race that sometimes seems based more upon Caltrop’s indecision than adventurousness. Ten million years is a collection of great moments divided by some pretty forgettable ones, too, meaning it doesn’t always work so well as a record. The 13-minute “Perihelion” offers a microcosm of that problem, if such an epic can correctly be called that. From its electric blues introduction, the track transitions into a rock ‘n’ roll ricochet, where razor-thin guitars and much thicker harmonies chase each lead vocal. But the song shifts into a drift that recalls the dubious genre of “Southern-fried frat rock.” Elsewhere, “Perihelion” sprints skyward into space rock, shoots back toward sludge metal, stabs back toward the scabrous noise-rock stammers of Unsane and ends with three minutes of plunging, propulsive guitar-and-bass interplay.

When it’s exciting, “Perihelion”like pieces of Ten million years itselfis a reminder of the energy to be found at the borders of hard rock and heavy metal. But when it’s not exciting, the memory of that reminder might be the only thing pushing you forward until the next such moment.