Egesen moraines throughout the Alps mark a glacial advance that has been correlated with the Younger Dryas cold period. Using the surface exposure dating method, in particular the measurement of the cosmogenic nuclide Be-10 in rock Surfaces, we attained four ages for boulders on a prominent Egesen moraine of Great Aletsch Glacier, in the western Swiss Alps. The Be-10 dates range from 10460 +/- 1100 to 9040 +/- 1020 yr ago. Three Be-10 dates between 9630 +/- 810 and 9040 +/- 1020 yr ago are based upon samples from the surfaces of granite boulders. Two Be-10 dates, 10 460 +/- 1100 and 9910 +/- 970 yr ago, are based upon a sample from a quartz vein at the surface of a schist boulder. In consideration of the numerous factors that can influence apparently young Be-10 dates and the scatter within the data, we interpret the weighted mean of four boulder ages, 9640 +/- 430 yr (including the weighted mean of two Be-10 dates of the quartz vein), as a minimum age of deposition of the moraine.All Be-10 dates from the Great Aletsch Glacier Egesen moraine are consistent with radiocarbon dates of nearby bog-bottom organic sediments, which provide minimum ages of deglaciation from the moraine. The Be-10 dates from boulders on the Great Aletsch Glacier Egesen moraine also are similar to Be-10 dates from Egesen moraines of Vadret Lagrev Glacier on Juliet Pass, in the eastern Swiss Alps. Both the morphology of the Great Aletsch Glacier Egesen moraine and the comparison with Be-10 dates from the inner Vadret Lagrev Egesen moraine support the hypothesis that the climatic cooling that occurred during the Younger Dryas cold episode influenced the glacial advance that deposited the Great Aletsch Glacier Egesen moraine. Because of the large size and slow response time of Great Aletsch Glacier, we suggest that the Great Aletsch Glacier Egesen moraine was formed during the last glacial advance of the multiphased Egesen cold period, the Kromer stage, during the Preboreal chron. Copyright (C) 2004 John Wiley Sons, Ltd.
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