Extreme southwestern margin of late Quaternary glaciation in North America: Timing and controls

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Abstract
Well-preserved latero-frontal moraines in the eastern San Bernardino Mountains of southern California provide evidence for several glacial advances during the late Quater- nary and mark the southwesternmost limit of glaciation in the Western Cordillera. Using geomorphology and 10Be cosmogenic radionuclide dating, a succession of moraines from three glaciated valleys is dated to 18-20 ka (Last Glacial Maximum), 15-16 ka (Heinrich Event 1), 12-13 ka (Younger Dryas Stade), and 5-9 ka (early-middle Holocene). These ages substantiate the view that glaciation throughout the American Cordilleras was syn- chronous during the late Quaternary. Furthermore, these data show that glacial advances in southern California occur when a significant decrease in summer temperature is cou- pled with an increase in moisture flux producing high winter snowfall. This allows for perennial snow accumulation that may, under appropriate conditions, persist to form glacial ice.