ROCK GLACIER AGES IN THE SAWATCH RANGE, COLORADO

We measured the thalli diameters of the 50 largest lichens (Rhizocarpon, subgenus Rhizocarpon) on 15 individual lobes of 7 rock glaciers or rock glacier complexes in the Swatch Range of central Colorado. The relative age of each lobe was characterized by the mean of the five largest lichens measured. Means varied from about 70 to 140 cm, with 12 of the 15 clustering around 125 cm. Some variation in lichen size can be attributed to differences in boulder lithologies, however these variations appear to be more evident in the total sample populations and less so in the means of the five largest lichens. Absolute ages of rock glacier lobes were assigned using a calibrated lichen growth curve developed by Benedict (1993) in the nearby Front Range. Lichen ages are interpreted as intervals of cooler (periglacial) climate and subsequent rock glacier activity. The ages thus determined range from 1760 to 4050 yr BP; the mean age of the 12 lobes is 3080 ± 350 yr BP. The latter interval is identical to one identified by Refsnider and Brugger (2007) in the study area and is correlative to an interval of cooling documented by other proxies of Late Holocene climate within the region.