Snow-avalanche debris, talus, and other slope deposits derived from a Tertiary pluton and surrounding Cretaceous and Tertiary sedimentary rock supply rock fragments to 23 rock glaciers on Mount Mestas, Colorado. One prominent ice-cemented rock-glacier complex studied since 1910 has a previously unreported extensive slump and flow in shale in part of its source region. Movement of this rock-glacier complex since the 15th century is indicated by the 283 trees sampled. Data derived from tree-rings show periods of movement in two directions, one to the northwest down the "slump and flow," and the other southwest down the ice-cemented rock glacier. Movement was caused both by loading at the head of the rock-glacier complex and steepening of the toe by removal of mass during recent excavation.