Tree-Ring Analysis of Movement of a Rock-Glacier Complex on Mount Mestas, Colorado, U.S.A.

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 icecemented 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.