On their first two albums, Led Zeppelin unleashed a relentless barrage of heavy blues and rockabilly riffs, but Led Zeppelin III provided the band with the necessary room to grow musically. While there are still a handful of metallic rockers, III is built on a folky, acoustic foundation that gives the music extra depth. And even the rockers aren't as straightforward as before: the galloping "Immigrant Song" is powered by Robert Plant's banshee wail, "Celebration Day" turns blues-rock inside out with a warped slide guitar riff, and "Out on the Tiles" lumbers along with a tricky, multi-part riff. Nevertheless, the heart of the album lies on the second side, when the band delve deeply into English folk. "Gallows Pole" updates a traditional tune with a menacing flair, and "Bron-Y-Aur Stomp" is an infectious acoustic romp, while "That's the Way" and "Tangerine" are shimmering songs with graceful country flourishes. The band hasn't left the blues behind, but the twisted bottleneck blues of "Hats off to (Roy) Harper" actually outstrips the epic "Since I've Been Loving You," which is the only time Zeppelin sound a bit set in their ways.