US Geological Survey


Date of this Version



Quaternary Science Reviews 23 (2004) 193–206


Eight uncorrected 36Cl ages for Pinedale boulders in north-central Colorado fall in the range 16.5 to 20.9 kyr. 10Be age determinations on four of five boulders are in close agreement (<6% difference) with 36Cl determinations. Hypothetical corrections for snow shielding increased the 36Cl ages of Pinedale boulder surfaces by an average of ~12%. Most ages for pre-Pinedale (Bull Lake) boulders fall within marine-isotope stage (MIS) 5, a time when continental and Sierran ice accumulations were small or nonexistent. Under the assumption that these boulders were deposited on moraines that formed before the end of MIS 6 (~140 kyr BP), calculations indicated that rock-surface erosion rates would have had to range from5.9 to 10.7mmkyr‒1 to produce the observed 36Cl values. When compared to rates that have been documented for the past 20 kyr, these erosion rates are extremely high. Snow shielding accounts for 0–48% of the additional years needed to shift pre-Pinedale dates to MIS 6. This suggests that some combination of snow shielding, sediment shielding, or 36Cl leakage has greatly decreased the apparent ages of most pre-Pinedale boulders. Inability to account for the effects of these processes seriously hinders the use of cosmogenic ages of pre-Pinedale boulders as estimators of the timing of alpine glaciation.