Well-preserved latero-frontal moraines in the eastern San Bernardino Mountains of southern California provide evidence for several glacial advances during the late Quaternary 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 synchronous during the late Quaternary. Furthermore, these data show that glacial advances in southern California occur when a significant decrease in summer temperature is coupled 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.