AGE OF THE ANDROSCOGGIN MORAINE, WESTERN MAINE AND EASTERN NEW HAMPSHIRE, FROM 10BE SURFACE EXPOSURE-AGE DATING

Fluctuations of the northeast sector of the Laurentide Ice Sheet during the last deglaciation have the potential to reveal important information on paleoclimate and ice dynamics in the North Atlantic region. However, the chronology of most of these ice-marginal oscillations remains poor in areas east of the Connecticut River Valley, particularly in Maine and eastern New Hampshire. Here, we present the first ¹⁰Be cosmogenic ages from this area that date moraines directly. The Androscoggin Moraine is a cluster of prominent ridges as much as 30 m high that straddles the Maine-New Hampshire border. These moraines were deposited by ice flowing southeastward through the Androscoggin Valley. Seven large granitic boulders on moraine crests yielded ¹⁰Be surface-exposure ages of 12.9-14.1 ka, with a mean of 13.5 ± 0.5 ka. The coherence of the dataset and lack of outliers implies that these dates afford a reliable age for the Androscoggin Moraine. Previous estimates placed the moraine age at or slightly older than 14 ka, based on correlation with ice-marginal positions farther to the west and associations with a limited number of basal radiocarbon ages from ponds, also located farther west. Additional work will confirm whether such correlations are correct. However, taken at face value, these ages suggest a stillstand or readvance of ice during late-glacial time in the Androscoggin River Valley of western Maine and adjacent New Hampshire prior to the onset of the Younger Dryas.