New data for Late Pleistocene Pinedale alpine glaciation from southwestern Colorado

New cosmogenic surface-exposure ages of moraine-crest boulders from southwestern Colorado are compared with published surface-exposure ages of boulders from moraine complexes in north-central Colorado and in west-central (Fremont Lake basin) Wyoming. Be-10 data sets from the three areas were scaled to a single Be-10 production rate of 5.4 at/g/yr at sea level and high latitude (SLHL), which represents the average 1 Be production rate for two high-altitude, mid-latitude sites in the western United States (US) and Austria. Multiple nuclide ages on single boulders indicate that this 1 Be production rate yields ages comparable to those calculated with a commonly used (36)C1 production scheme. The average age and age range of moraine-crest boulders on terminal moraines at the southwestern Colorado and Wyoming sites are similar, indicating a retreat from their positions similar to16.8 Cl-36 ka (Cosmogenic ages in this paper are labeled Be-10 or Cl-36 ka or just ka when both Be-10 or Cl-36 ages are being discussed; radiocarbon ages are labeled C-14 ka, calibrated radiocarbon are labeled cal ka, and calendar ages are labeled calendar ka. Errors ( 1 (7) associated with ages are shown in tables. Radiocarbon ages were calibrated using the data of Hughen et al. (Science 303 (2004) 202). This suggests a near-synchronous retreat of Pinedale glaciers across a 470-km latitudinal range in the Middle and Southern Rocky Mountains. Hypothetical corrections for snow shielding and rock-surface erosion shifts the time of retreat to between 17.2 and 17.5 Be-10 ka at Pinedale, Wyoming, and between 16.3 and 17.3 Cl-36 ka at Hogback Mountain, Colorado. (C) 2004 Elsevier Ltd. All rights reserved.