The Southern Alps are formed mainly of hard grey sandstone called greywacke (left), which is the most common rock type in New Zealand. When deeply buried and heated, greywacke is converted to a layered rock called schist (right). The western edge of the Southern Alps has been uplifted many kilometres, exposing schist. The boulders in the main rivers that begin in the Southern Alps are mainly greywacke, with a smaller amount of schist.