TWO DISTINCT ESTIMATES OF LAST GLACIAL MAXIMUM CLIMATE ON THE PIKES PEAK MASSIF, FRONT RANGE, CENTRAL COLORADO, USING NUMERICALLY MODELED 2D PALEOGLACIER RECONSTRUCTIONS

The Pikes Peak Massif, located at the southern end of the Colorado Front Range, contains extensive geomorphic evidence of Pleistocene glaciation. Down valley ice limits are recorded by well-preserved moraines in valleys radiating from the 4000m-high summit ridge of the massif. Valleys on NW-to-NE aspects of the massif hosted five individual valley glaciers up to 3.7 km long. On the SW- and S-facing slopes there is evidence for a more extensive Pleistocene transection glacier system terminating in three valleys, with a maximum glacier length of 8.6 km. To better understand the chronology of glaciation of the massif, and investigate the climate responsible, we used a dual component 2-D energy/mass balance and ice flow model developed by Phillips and Plummer (2003). Initial modeling was undertaken varying only year-round temperatures from the modern, not changing precipitation or seasonality. We were unable to fit the best-preserved and most conspicuous moraines in each valley with a single temperature depression, but were able to match all of these moraines and less conspicuous moraine sets in several valleys with one-of-two temperature depressions. A temperature depression of 5.5^oC from modern fits the distinct terminal moraines in North French Creek on the NE side of the massif and in the East and West Forks of Beaver Creek on the SW side. A temperature depression of 4.25^oC fits the most distinct moraine in the Bottomless Pit immediately N of the summit of Pike Peak as well a smaller unnamed glacial valley on the NW side of the massif, as well as smaller moraine complexes up valley from the main moraines in the French and Beaver Creek valleys. We have found no clear evidence of moraines that might have been associated with a 5.5^oC temperature depression in the three valleys where the main moraines were well fit by the lesser temperature depression. It appears, then, that there is moraine evidence for two distinct advances or standstills of ice on the massif associated with different temperature depressions. No numerical data is available for the moraines and as a result, we are uncertain whether the two represent separate glaciations (Bull Lake and Pinedale) or whether they date from a single glacial period (Pinedale).