ROCK GLACIERS IN CENTRAL COLORADO AS INDICATORS OF LATE-HOLOCENE CLIMATE CHANGE

We used measurements of the lichen Rhizocarpon subgenus Rhizocarpon to date the deposition of 48 individual lobes on 18 rock glaciers or rock glacier complexes in the Elk Mountains and Sawatch Range of central Colorado. Cumulative probability distribution and K-means clustering analyses were used to separate lichen thalli measurements separate groups, each representing a discrete interval of debris deposition on the rock glaciers. Using a growth curve developed in the nearby Front Range for Rhizocarpon geographicum, the three most prominent depositional events have ages of approximately 3100, 2100, and 1200 calendar years before present. Debris deposition ended earlier in the eastern end of the study area, where both modern and past climate is warmer and drier. Elevation had no significant effect on the timing of debris deposition on rock glaciers.

The depositional events with ages of 3100, 2100, and 1200 calendar years before present can be correlated to periods of climatic deterioration and late-Holocene glacier activity in the broader region of the central and southern Rocky Mountains. The oldest depositional event corresponds to an interval of rock glacier formation in the northern Sawatch Range, a period of neoglaciation (Triple Lakes Stade) in the Front Range, and a periglacial event in the Sangre de Christo Mountains in northern New Mexico. The middle depositional event corresponds to the onset of Audubon Neoglacial activity in the Front Range. The most recent depositional event corresponds to rock glacier formation in the northern Sawatch Range, the end of the Audubon Neoglaciation in the Front Range, and neoglacial cooling in the Wind River Range of Wyoming.