Sherwin glaciation
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| Sherwin glaciation | |
|---|---|
| Name | Sherwin glaciation |
| Period | Pleistocene |
| Region | Sierra Nevada, California |
| Named for | Sherwin Range |
| Type | glacial stage |
Sherwin glaciation The Sherwin glaciation denotes a major Pleistocene glacial episode in the eastern Sierra Nevada of California associated with widespread alpine and valley glaciers that sculpted the modern topography of the Eastern Sierra, Yosemite National Park, and adjacent basins. It is framed within regional stratigraphic schemes developed by workers from institutions such as the U.S. Geological Survey, the University of California, Berkeley, and the California Division of Mines and Geology, and is correlated with marine and continental records including evidence from the Great Basin, Mojave Desert, and North Pacific deep-sea cores. Interpretations of timing, extent, and climatic forcing link the Sherwin event to global Pleistocene fluctuations observed in records from Greenland Ice Sheet Project (GISP), the Vostok ice core, and North Atlantic sites like the North Icelandic shelf.
Overview and nomenclature
The name derives from the Sherwin Range usage by early 20th-century field geologists operating near the Inyo County–Mono County boundary and formalized in maps by the USGS and state geologic surveys; contemporaries included researchers affiliated with Stanford University, Scripps Institution of Oceanography, and the Smithsonian Institution. Regional workers contrasted Sherwin deposits with local units termed the Tahoe and Tioga glaciations used across the Sierra Nevada and the Lake Lahontan chronology devised by scholars from the Nevada Bureau of Mines and Geology. Debates over nomenclature have involved participants from the Geological Society of America and the Quaternary Research Association and have intersected with international correlations promoted at meetings of the International Union for Quaternary Research (INQUA).
Chronology and dating
Absolute and relative dating tie the Sherwin interval largely to the early-to-middle Pleistocene, with radiometric constraints provided by K–Ar and 40Ar/39Ar dating of volcanic ash layers interbedded with till and by cosmogenic exposure dating from boulder surfaces measured at laboratories at Lamont–Doherty Earth Observatory and Lawrence Livermore National Laboratory. Correlations associate Sherwin glaciation with marine isotope stages inferred from benthic foraminifera records in cores archived at institutions like the Scripps Institution of Oceanography and synchronized with oxygen isotope chronologies generated by Mediterranean Sea and North Atlantic studies. Published age estimates span several hundred thousand to over a million years ago in some interpretations, prompting comparisons with glacial maxima recognized in Scandinavian sequences curated by the Geological Survey of Sweden and the British Geological Survey.
Extent and geomorphology
Glacial landforms attributed to the Sherwin episode include overdeepened valleys, U-shaped troughs, cirques, moraines, and extensive drift mapped across the Yosemite Valley, John Muir Wilderness, Inyo Mountains, and the western margins of the Basin and Range Province. Morainic complexes in the Mono Basin, Owens Valley, and the Tuolumne River headwaters show sedimentologic affinities discussed in reports from the National Park Service, the California Geological Survey, and university field programs at University of California, Los Angeles. Glacial geomorphologists referencing work by pioneers affiliated with Harvard University and California Institute of Technology have compared Sherwin sculpting to alpine glaciation in the European Alps and the Southern Alps (New Zealand) for process analogues.
Climate and glacial dynamics
Interpretations of paleoclimate during Sherwin rely on isotope-based reconstructions, paleobotanical assemblages recovered near till margins, and modeling efforts conducted by centers such as Potsdam Institute for Climate Impact Research and the National Center for Atmospheric Research. Proposed drivers include shifts in Pacific sea-surface temperatures linked to stadial–interstadial variability documented in California Current records, orbital forcing described by concepts developed by Milutin Milanković and advanced in modeling at Princeton University, and changes in precipitation routed from the North Pacific High influences reconstructed by paleoclimatologists at Oregon State University. Glacier dynamics inferred for Sherwin emphasize cyclic advance–retreat behavior, surge-like episodes, and mass-balance sensitivity to winter precipitation and summer insolation, compared to modern dynamics observed by teams at the USGS and University of Alaska Fairbanks.
Evidence and stratigraphy
Primary evidence comprises tills, diamictons, sorted outwash, and lacustrine sequences with tephra layers correlated to known eruptions cataloged by the Smithsonian Institution’s Global Volcanism Program. Stratigraphic frameworks use lithostratigraphic and chronostratigraphic markers developed in publications by researchers from University of California, Davis, California State University, and the USGS. Palynological samples and macrofossils recovered near moraines have been analyzed in laboratories linked to the Natural History Museum of Los Angeles County and the Museum of Vertebrate Zoology at UC Berkeley to reconstruct vegetation shifts synchronous with glacial deposition.
Environmental and ecological impacts
Sherwin glaciation profoundly reconfigured drainage networks feeding into closed basins such as Mono Lake and Owens Lake, created habitat mosaics that influenced megafaunal distributions documented in sites investigated by teams from the La Brea Tar Pits program and the University of Utah, and set the stage for post-glacial soils and vegetation succession studied by researchers at Yale University and the University of Washington. Human prehistory implications have been discussed in light of regional archaeological syntheses held by the Museum of Natural History, Los Angeles County and the Peabody Museum.
Research history and controversies
Scholarly debate has centered on correlation of Sherwin deposits with global isotope stages, differences in age estimates generated by competing dating methods at institutions like Los Alamos National Laboratory and Caltech, and the appropriate regional nomenclature promoted in panels convened by the Geological Society of America. Controversies also involve interpretations of glacier extent proposed in classic monographs by researchers affiliated with Stanford University versus later syntheses from the USGS and international reassessments presented at meetings of INQUA and the American Quaternary Association.