LATE PLEISTOCENE GLACIAL MAXIMUM AND DEGLACIATION OF THE NORTHERN SAWATCH RANGE, MOUNT MASSIVE, COLORADO

Last glacial maximum (LGM) deposits and associated terminal and lateral moraines are abundant in the Arkansas Valley, Colorado, and provide an important record of Sawatch Range glaciation. Recessional moraines inside the LGM limits are generally absent from, or markedly less prominent in, the geologic records of the largest paleoglacial systems in the region. In contrast, Mount Massive is flanked on its northeast side by a nearly continuous till mantle, including several prominent moraine ridges between the terminal moraine at ca. 2920 m on the Arkansas Valley floor and late glacial moraines near cirque floors at ca. 3660 m. We applied ¹⁰Be surface exposure dating over a 6 km transect, collecting 9 samples, including sites on the terminal moraine (2 dates at 21.5 ± 0.5 and 21.8 ± 0.6 ka), intermediate moraine ridges (ages decrease progressively up-valley; 19.0 ± 1.7, 17.0 ± 1.6, and 14.8 ± 0.4 ka) and scoured bedrock in the cirque headwall (13.2 ± 1.2 and 12.6 ± ka). Our estimation of the ELA (equilibrium line altitudes) during LGM depend on assumptions made about conditions that affect accumulation area ratios. A conventional approach (toe-to-headwall altitude ratio, 0.5) yields local ELA estimates during the LGM of about 3400 m. The latter dates imply that local cirque snowline/ELA to be above 3900 m at 12–13 ka. In contrast, local perennial snow patches during the late 20^th century occur at 4200 m.