When the Mountain Changed its Clothing, Ruhr Triennale, review Composer/director Heiner Goebbels can make gripping music-theatre out of the most unpromising materials: political songs, Thoreau's diary of a life in the woods, a quartet of early music singers. He can do this because telling a gripping narrative is not his aim. He wants to illuminate a difficult theme through cunningly contrived assemblages of sound, movements and words. In Goebbels" latest work, unveiled in the Bochum Jahrhunderthalle as part of the hugely impressive Ruhrtriennale Festival (which he also directs), the materials seemed his least promising yet. It was a bunch of 40 or so adolescent girls. These were the singers of the Vocal Theatre Carmenica Slovenica from Maribor in Slovenia, who've gained a world-wide reputation for their powers of story-telling through choral song. At the beginning of Goebbels" new piece they edged diffidently on stage in a huddled mass, as if blind, in total silence, stumbling over chairs scattered over the stage, while over the speakers one could just make out the sound of one of Schoenberg's haunted expressionist pieces. It was a typical Goebbels moment, fraught with symbolic meaning of we knew not what. Over the next 90 minutes it became clear (sort of) that this was a meditation on the transition from girlhood to adulthood, overlaid with thoughts of other transitions - from communism to capitalism, and imperial subjection to nationhood (two themes of particular resonance in Slovenia). The choir sang all kinds of vocal music, including 13th century polyphony, political songs and Slovenian folk-songs, including one about the Mountain which changes its clothing in the spring from a white shirt to green trousers (thus the name of the show). As they sang they turned the stage into a swirl of choreographed movement. Cuddly toys and naive Balkan paintings added a minimal sense of place. In their bizarre mish-mash of multi-coloured costumes - here a pugnacious gangly one in a red dress, there a short shy one in blue tights - the girls looked like an animated flower-bed. At times individuals emerged from the mass, engaging in dialogues in English drawn from Rousseau or Gertrude Stein or Ian McEwan. The girls" power of memory in a foreign language were impressive, but these episodes were less moving than they were meant to be. One felt a certain awkwardness. It was at the joyous moments of collective energy or anarchy that these astonishing performers shone, and the piece itself really came alive.