Chronology of Neoglacial Deposits in the Northern Sawatch Range, Colorado

Abstract

Lichenometry and geological evidence permit establishment of a chronology of rock glacier development during Neoglaciation in the northern Sawatch Range. Portions of rock glaciers were assigned approximate absolute ages based on maximum thallus diameters of Rhizocarpon geographicum, Lecanora thomsonii, and Lecidea atrobrunnea. Values for the total percentage of lichen cover also aided in separating younger from older deposits. Although lichen growth-rate curves could not be constructed for the study area, a growth-rate curve for Rhizocarpon geographicum, developed in 1967 by Benedict for the Colorado Front Range, appears to be useful for dating rock surfaces in the northern Sawatch Range.

Available data indicate that rock glacier development in four cirques was nearly synchronous during the last several thousand years. An early period of rock glacier development, here termed Temple Lake I, began more than 4,000 years ago and lasted for an unknown length of time. Temple Lake II deposits began to form between 3,750 and 3,500 years BP and continued to accumulate until about 2,500 years ago. Audubon rock glaciers began forming about 1,900 years BP and continued to develop until 1,000 to 900 years ago. Relative sizes of Temple Lake and Audubon rock glaciers indicate that processes and conditions favoring rock glacier formation during the Temple Lake stade were more intense and/or lasted longer than during the Audubon stade. The Gannett Peak stade of Neoglaciation is represented in the study area only by talus.

This study supports earlier conclusions that Neoglaciation in the Rocky Mountains was characterized by three main intervals of glacier advance and/or rock glacier development. The Neoglacial record, however, is incomplete in the study area because no evidence was found of glacier or rock glacier activity during the Gannett Peak stade.