NEW CONSTRAINTS ON LATE-GLACIAL ADVANCES AT MT. BAKER, WASHINGTON: ANOTHER FEATHER IN JERRY’S CAP

Recently published lidar data that covers the entire volcanic edifice of Mt. Baker, WA, allows new constraints on the maximum extents of latest Pleistocene alpine glaciers on the mountain following the retreat of the Cordilleran Ice Sheet. Previous interpretations of late-glacial ice extents on the mountain vary widely; Kovanen and Easterbrook (2001) concluded that alpine glaciers reached maxima 25-45 km beyond the mountain between ca. 15-12 cal kyr BP, reaching altitudes as low as 100 m asl, with advances both pre-dating and coincident with the Younger Dryas period (YD, 12.8-11.5 cal kyr BP). Conversely, Osborn et al. (2012) concluded that moraines on the west side of Mt. Baker indicate that glaciers there only extended a few km beyond their Little Ice Age (LIA) maxima during the late glacial period, with ¹⁴C dates indicating the advances were restricted to the YD. Lidar data published in 2017 provides new support for the interpretations of Osborn et al., indicating that late-glacial advances around the mountain remained within a few km of the LIA maxima for the same valleys. Equilibrium-line altitude (ELA) estimates based on highest occurrences of lateral moraines (particularly where lateral moraines of adjoining lobes merge) indicate that late-glacial ELAs varied from ~1500-1700 m asl around the mountain, and that they remain within 100-300 m of ELAs associated with the LIA maxima in those valleys. The similar extents and morphologies of the moraines and consistent associated ELAs, combined with the absence of other alpine moraines further down valley, all indicate that late-glacial advances on the mountain were relatively restricted in space and time, and likely represent the local response of glaciers to the YD event.