It's difficult to think of a Red Rocks headliner that's simultaneously as unlikely and as appropriate as Sigur Rós. The quartet uses even less of the standard rock lexicon than does fellow Icelander Björk, who headlined at the venue last year — and while the band's been around since 1994, Jón "Jónsi" Thor Birgisson and his compatriots waited until their most recent disc, the melodically titled Med sud i eyrum vid spilum endalaust, to cut a tune in English. Not that the recording represents a disquieting compromise. Despite the participation of Flood, a producer whose credits include work with Depeche Mode, the album retains the sweeping, elemental qualities that have always made the outfit so intriguing. Indeed, the primordial nature of the collective's sound, marked by keening, otherworldly vocals and frequently grand musical movements, correlates perfectly with the amphitheater's geography. In that sense, Sigur Rós was made for Red Rocks — or is it the other way around?