Dolly Parton has spent the past couple of decades vacillating between roles as country-music doyenne, theme-park financier, and Hollywood curiosity -- a wide range, even for a woman who can't have an article written about her that doesn't include a reference to her ample bosom (see?). But her latest incarnation, which she slips into at the end of her new Halos & Horns, has to be one nobody saw coming: banshee-voiced heavy-metal matriarch. Parton shows off the character in her stomping bluegrass reading of "Stairway to Heaven," surely the first version of the evergreen tune that actually sounds like something the scraggly old dude on the cover of Led Zeppelin's fourth album might dig; where Zep's version was scary because it felt like it might never end, Parton's taps into a more primal, fire-and-brimstone gravitas. Elsewhere on Halos (the third in a series of back-to-basics efforts Parton's recorded for the North Carolina roots label Sugar Hill), the singer remembers a bucolic childhood "sweeter than candy and cake and pie," wonders why "birds sing for everyone but me," and impersonates the straw-haired witch who probably dated Mr. Zoso back in junior high. Will this lady ever stop working?