Truth be told, the grunge era never quite fit Sonic Youth. They may have been at the peak of their popularity, but they had traded their experimentalism for sheer, bracing noise. It may have sounded good, but ultimately Dirty didn't have the cerebral impact of Sister, largely because it was tied to an admittedly effective backbeat. Beginning with Washing Machine, Sonic Youth returned to more adventurous territory, and in 1997, they released a series of EPs that illustrated their bond with such post-rock groups as Tortoise and Gastr del Sol. Those EPs, as well as the epic Washing Machine closer, "The Diamond Sea," provide the foundation for A Thousand Leaves, the band's most challenging and satisfying record in years. The blasts of dissonance that characterized their SST masterworks have been replaced, by and large, by winding, intricate improvisations. There's a surprising warmth to the subdued guitars of Thurston Moore, Lee Ranaldo, and Kim Gordon, which keeps the lengthy songs captivating. Both Moore and Ranaldo concentrate on quiet material, which almost makes Gordon's noisy politicized rants sound a little out of place, but her best moments ("French Tickler," "Heather Angel") have unsettling, unpredictable twists and turns that greatly contribute to the success of A Thousand Leaves. It may be their most cerebral album in ages, but that only makes it all the more engaging.