FLOW OF THE SPRUCE CREEK ROCK GLACIER, TEN MILE RANGE, COLORADO, U.S.A., OVER ANNUAL TO MILLENIAL TIMESCALES: PALEOCLIMATIC IMPLICATIONS

Flow rates of the Spruce Creek Rock Glacier in the Ten Mile Range of central Colorado are examined over three time scales. In 1985, three cross-glacier transects were surveyed and marked. Five resurveys conducted between 1987 and 2000 provide a record of short-term flow rates and their spatial and temporal variations. Intermediate-term flow rates are assessed photogrametrically, using aerial photography dating back to 1938. Long-term flow variations are evaluated through measurement of lichens (Rhizocarpon s.l.) on the rock glacier surface, which provide a record of flow spanning the last 2000-3000 years. Lichen data are calibrated using Benedict's (1993) growth curve for the Colorado Front Range.

Survey data indicate that 1985-2000 mean centerline velocities ranged from 5.9 to 10.2 cm/yr on the three transects. Centerline velocities measured over shorter (2-5 year) intervals ranged from 2.8 cm/yr to 13.3 cm/yr. There was no clear trend in velocity through time during the 15-year survey interval. Photogrametric measurements indicate that centerline flow velocities over the interval between 1938 and 1990 ranged from 18 cm/yr to 23 cm/yr, two-to-three times the rates surveyed between 1985 and 2000. Lichenometric data indicate mean flow rates of about 35 cm/yr over the past 500-600 years and 4-5 cm/yr during the preceding ca. 1800 years. Lichen data indicate that approximately 2/3 of the rock glacier formed in the last ca. 2500 years and suggest that the entire glacier is Neoglacial in age.

These observations strongly suggest a climatic control on both formation and flow rate variations of the rock glacier. The glacier as a whole appears to be a product of late Holocene (Neoglacial) cooling. Flow rates were relatively low through much of the Neoglacial interval, increased during the Little Ice Age interval of the last several hundred years, and then decreased progressively with post-Little Ice Age warming of the last several decades.