RATE AND TIMING OF DEGLACIATION USING 10Be COSMOGENIC NUCLIDE SURFACE EXPOSURE DATING, MT. MASSIVE WILDERNESS, COLORADO, USA

New ¹⁰Be surface exposure ages were obtained from boulders on glacial moraine crests and from glacially-scoured bedrock exposed along the Rock Creek moraine complex on the northeast flanks of Mount Massive, Sawatch Range, Colorado. A transect of exposure ages was used to constrain: (1) the timing of the last glacial maximum (LGM), (2) glacial still-stand activity, and (3) timing of upper cirque deglaciation. A boulder on the lowest recessional moraine crest (2975 m) yielded an age of 19.5 ± 1.8 ka, a boulder from another recessional moraine crest (3340 m) yielded an age of 17 ± 1.6 ka, and two ages from scoured bedrock in the upper cirque basin (3755 m) yielded ages of 12.6 ± 1.2 and 13.4 ± 1.2 ka. These ages indicate that the local timing and recession rate of the LGM are as follows: (1) the glacial system reached its maximum extent at least 19.5 k.y. ago, (2) a glacial still-stand occurred during deglaciation at approximately 17 k.y. ago, and (3) complete deglaciation of the upper cirque basin occurred by approximately 13 k.y. ago. Deglaciation on the slopes of Mount Massive took place over approximately 7.6 ka, and occurred at an average rate of approximately 1 m/yr. Timing of LGM and complete deglaciation reported here are consistent with previously reported ¹⁰Be surface exposure ages from terminal moraines and scoured bedrock at other localities in the Arkansas River Valley, the Animas River Valley and San Juan Mountains ice cap.