LATEST PLEISTOCENE DEGLACIATION OF THE MERCED AND TUOLUMNE RIVER LANDSCAPES, YOSEMITE NATIONAL PARK

The Merced and Tuolumne River landscapes in Yosemite National Park record the extent and retreat of glaciers that flowed from the Sierra Nevada crest during Last Glacial Maximum (LGM). The terminus of the LGM glacier that occupied the Merced River canyon and delivered sediment to Yosemite Valley was tentatively identified as the Bridalveil moraine, between Roosevelt and Bridalveil meadows (e.g., Wahrhaftig et al, 2019). New cosmogenic ¹⁰Be-exposure ages (n = 2) for glacial erratics in Roosevelt Meadow, however, indicate glacial deposition beyond the Bridalveil moraine at 23.1 ± 3.6 ka. Ice retreat began early and gradually given an infrared-stimulated luminescence (IRSL) age of 21.2 ± 3.3 ka for the glacial outwash between the Bridalveil and El Capitan recessional moraines and exposure ages (n = 2) at 19.7 ± 0.6 ka for erratics in the outwash terrace tread. ¹⁰Be-exposure ages (n = 3) for boulders in the El Capitan moraine overlap with this result, implying continued glacial deposition in western Yosemite Valley as late as 20.4 ± 3.0 ka. ¹⁰Be-exposure ages (n = 2) for glacial erratics ~2.9 km up valley suggest ice retreat to middle Yosemite Valley by 19.8 ± 0.9 ka. Luminescence ages for subglacial stony silt deposits (16.4 ± 1.5 ka) overlain by Upper Pines glacial outwash (16.0 ± 1.1 ka) and exposure ages for the Royal Arches Meadow rock avalanche at 16.3 ± 0.8 ka together indicate deglaciation of eastern Yosemite Valley by ~17-16 ka. Age dating of remnant fluvial terraces near Swinging Bridge (11.8 ± 1.0 ka; IRSL), Stoneman Bridge (11,760–11,290 cal yr BP; ¹⁴C), and Royal Arches (11.8 ± 1.1 ka; IRSL), previously attributed to glacial Lake Yosemite (Matthes, 1930), appear to reflect sedimentation in the early Holocene. Beyond Yosemite Valley, new exposure ages for erratics (n = 13) in subalpine Tuolumne and Dana meadows show glacial occupation as late as 16.0 ± 1.3 ka, consistent with previous studies, but ages lack a clear spatiotemporal relationship consistent with gradual ice retreat. New ¹⁰Be-exposure ages (n = 4) for erratics deposited at Tioga Pass record the loss of ice at a relatively high elevation in the drainage basin by 14.5 ± 0.7 ka. Collectively, these geochronologic data suggest relatively slow deglaciation at lower elevations from about 21 to 16 ka, followed by more rapid deglaciation of the Yosemite high country from 16 to 14 ka.