Use of Relative Age-Dating Methods in A Stratigraphic Study of Rock Glacier Deposits, Mt. Sopris, Colorado

Abstract

Data from several relative age-dating methods were collected on rock glacier debris mantles to test the usefulness of each in assigning a relative age to various parts of the mantles. Lichens, rock weathering, and soils provide the best information on age. No single method is adequate to differentiate all the deposits, most of which have been formed within the past 20,000 to 30,000 years. Each method has an ages to the Neoglacial deposits. The Neoglacial-of each method differs. Lichen data, specifically Rhizocarpon geographicum s.l. size, percentage of total lichen cover, and species composition provide the best information for assigning ages to the Neoglacial deposits. The Neoglacial-Pinedale break is recognized from a much thicker surface deposit of loess, thicker and more common rock weathering rinds, and greater soil development on Pinedale deposits relative to early Neoglacial deposits. The Pinedale-Bull Lake break was not satisfactorily recognized in many places. Where exposures are present, however, the granitic rocks in the weathered zone in Pinedale deposits are fresh whereas a substantial number of those in Bull Lake deposits are grusified.