New age constraints on Late Pleistocene glacial outwash deposits near Ridgway, Colorado, northern San Juan Mountains
— D. Jarrin, A. Aslan, S. Mahan, and P.R. Hanson

Optically stimulated luminescence (OSL) and post-infra-red stimulated luminescence (post-IR IRSL) geochronology provides the first numerical age estimates for the timing of glaciation in the Uncompahgre River valley of the northern San Juan Mountains of Colorado. A quartz OSL date of 23.0±1.6 ka from the youngest outwash terrace located north of Ridgway, Colorado fits well with previously documented timing on Pinedale glaciation and marine-isotope-stage (MIS) 2. However, there is no correlative terminal moraine located in the study area, which shows that Pinedale glaciers did not advance as far north as Ridgway during the Last Glacial Maximum (LGM).

Post-IR IRSL ages of 48.2±2.9 ka and 68.1±4.4 ka, and OSL ages ranging from 36.4±2.1 ka to >76 ka for the next oldest outwash terrace near Ridgway, correspond to MIS 4/3. The terrace tread of this older outwash terrace can be traced directly to a series of hills that form a small, sinuous terminal moraine, which is cross-cut by the LGM terrace. Two sets of older and larger moraines, and corresponding outwash terraces, are present slightly farther downvalley from the small, sinuous moraine. These older moraines remain undated, but based on the new luminescence data and their larger size, it is plausible that they represent Bull Lake glacial advances.

While past workers have assumed that the Ridgway moraines correlate with Pinedale glaciation, the new luminescence age estimates, along with relative age relationships, indicate that all the moraines are older than recognized previously. Lastly, the recognition that ca. 70-40 ka Uncompahgre River valley ice advanced almost as far downvalley as presumed Bull Lake moraines raises the possibility that the effects of glaciation during MIS 4/3 were greater than generally recognized in the San Juan Mountains.