A lichenometric study of Holocene rock glaciers and neoglacial moraines, Frances Lake map area, southeastern Yukon Territory and Northwest Territories

Abstract
Different ages of rock glaciers and Neoglacial moraines in southeast Yukon are readily differentiated on the basis of the size structures of their Rhizocarpon populations and per cent lichen cover. Most rock glaciers formed ji-om talus during the middle Holocene but formation and flow continued thereafter. Perhaps conditions for formation were climatically enhanced at several times during the middle and late Holocene but the apparent climatic signal is weak and equivocal. With one exception, glacial advances date from the last four centuries. Moraines are of several ages but there is no clear correlation between glacial advances and formation of rock glacier lobes during the last four centuries. Climatic change that led to Neoglaciation triggered only minor, if any, rock glacierization of talus, and whatever conditions led to massive rock glacierization of talus during the middle Holocene failed to trigger glacier growth. Rock glacierization is probably self regulated and occurs at a threshold talus thickness and ice content. Thresholds likely vary with geotechnical properties of talus, particularly grain size, and are only slightly modulated climatically. Large Rhizocarpon lichens increase their diameters at the same rate as small ones so the growth rate is constant following the initial "great period"; hence the fundamental premise of lichenometry is correct. Rhizocarpon communities show no symptoms of senescence at 5000 years of age so the genus is useful in lichenometry beyond that.

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