Rock glaciers in central Colorado, U.S.A., as indicators of holocene climate change

We measured thalli diameters of the lichen Rhizocarpon subgenus Rhizocarpon on 48 individual lobes of 18 rock glaciers and 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 into statistically distinct groups, each interpreted as representing a discrete episode of rock glacier activity driven by an interval of cooler climate. Lichen ages for these episodes were assigned using a growth curve developed for Rhizocarpon geographicum in the nearby Front Range. An early Neoglacial episode, ca. 3080 yr BP, is correlative to other glacial and periglacial activity in the southern Rocky Mountains and surrounding areas and broadly corresponds to an interval of climatic deterioration evident in several other proxies of Holocene climate. The younger two episodes, ca. 2070 and 1150 yr BP, are also coeval with regional (Audubon) glacial and periglacial activity but are thus far not widely recognized in other climate proxies.