CORRELATION OF THE EASTERN MAINE PINEO RIDGE SYSTEM TO MORAINES ON MOUNT DESERT ISLAND THROUGH LIDAR IMAGERY AND 10BE SURFACE EXPOSURE AGES
The correlation of the PRS to MDI is further supported by 10Be surface exposure dating (Koester and others, 2017; Hall and others, 2017; Braun and others, 2018). The mean 10Be age from the Hall and others (2017) boulders (14970 +/-550 yr BP) is about 500 years older than the mean 10Be age from the Koester and others (2017) boulders (14480 +/- 490 BP). Although the mean ages barely overlap at 1 standard deviation, Hall and others (2017) correctly note that the Koester and others (2017) ages are from boulders that lie a few km south of and should be older than the Hall and others (2017) boulder ages. In essence, surface exposure dating of boulders is not precise enough to distinguish ages on this time frame over such short geographic distances.
New 10Be surface exposure ages (Braun and others, 2018) from six boulders on the Jordan Pond moraines on MDI (14630 +/- 1100 BP) are on average over 1000 years older than the five exposure ages from boulders along the south shore of Jordan Pond published by Koester and others (2017), and statistically indistinguishable from the surface exposure ages on boulders at Pineo Ridge. The mean 10Be age for these six new boulders is also statistically indistinguishable from the mean 10Be age of 16 boulder and bedrock samples along two altitudinal ridgeline transects on MDI (Koester and others, 2017). The ridgeline boulder 10Be ages suggest that ice sheet surface lowering between 460 and 170 m a.s.l. occurred rapidly at about 15.2 ka, followed soon thereafter by deposition of the Jordan Pond moraines.
Previous workers on the PRS (Miller, 1986; Ashley and others, 1991) have concluded that the feature represents a period of slow retreat of the glacier rather than a significant re-advance. That slow retreat coincided with the ice front reaching MDI where the glacier became “banked” against the north side of the mountain range and formed distinct lobes in the intervening glacial troughs.