THE PACE OF DEGLACIATION AND POSTGLACIAL LANDSCAPE RESPONSE IN YOSEMITE VALLEY, CALIFORNIA

The landforms and sediments of Yosemite Valley record the evolution of the Merced River catchment since the Last Glacial Maximum (LGM). The Yosemite Valley glacier likely began an early and slow retreat, given a new 25.5 ± 3.4 ka luminescence age (post-IR IRSL) for glacial outwash terraces between the Bridalveil LGM end moraine and the El Capitan recessional moraine. Near-maximum glacial deposition continued as late as 20.4 ± 3.0 ka based on the mean ¹⁰Be exposure age for boulders in the El Capitan recessional moraine. Glacial occupation at the valley’s east end occurred as late as 19.8 ± 1.8 ka based on subglacial stony silt exposed to light by ice retreat. A sequence of varve-like silt and sand, emanating from Tenaya Canyon, stands ≥ 3 m above outwash terraces along the Merced River and indicates proglacial ponding, likely in a kettle or behind an ice or a now-eroded sediment dam, between ~20 and 16 ka (post-IR IRSL). Downstream along the Merced River, inclined rhythmic layers of sand and silt, deposited on outwash gravel and till, are exposed in the active channel bed by incision and are consistent with deposition in proglacial ponds fed by braided streams as late as 16.2 ± 1.5 ka (post-IR IRSL).

The Merced River catchment likely shifted from proglacial aggradation to fluvial incision after ~15–13 ka given the ¹⁰Be exposure age of 16.1 ± 0.8 ka for the Royal Arches Meadow rock avalanche and luminescence ages of 16.1 ± 1.8 ka and 15.2 ± 2.2 ka for fluvial gravels deposited on top of outwash. This fluvial activity infilled glacier-recessional topography and produced inset sequences up to ~4 m thick with luminescence and ¹⁴C charcoal ages of ~13 ka to 10 cal kyr BP. Subsequent fluvial incision from reduced sediment loads stranded fill terraces dated at ~9–6 cal kyr BP and reworked fluvial gravels on the valley floor. The widespread meadow-forming terraces flanking the modern river consist of basal gravels capped by bedded sand representing the fine fraction of debris flows and rockfalls from the valley walls, as well as floodplain deposits of the river. These meadow deposits contain abundant charcoal as old as ~1700 cal yr BP. Stranded terraces and narrow floodplains in the western half of the valley may reflect further channel lowering over the past two centuries from human modifications to the El Capitan moraine and widespread removal of woody debris.