Late Pleistocene History of the Front Range Region

The Wisconsin Glaciation is called the Pinedale Glaciation in the Rocky Mountain region, after terminal moraines near the town of Pinedale, Wyoming. The Pinedale Glaciation began about 110,000 yr b.p. and included at least two major ice advances and retreats in most regions of the Rocky Mountains. The history of glaciation is not as well worked out for the Colorado Front Range region as it is for regions in the Central and Northern Rockies. For example, geologists have documented three separate ice advances in the Teton Range during Pinedale times (Pierce and Good 1992). In northern Colorado we know that there were earlier and later Pinedale ice advances, but we do not know when the earlier advance (or multiple advances) took place. However, based on geologic evidence (Madole and Shroba 1979), the early Pinedale glaciation was more extensive than that of the late Pinedale. Early Pinedale moraines can be seen near the western edge of the town of Estes Park, whereas late Pinedale ice formed moraines several kilometers up-valley.

Geologic evidence indicates that during the last glaciation, Pinedale glaciers flowed out of high mountain cirques, down-valley to elevations between 2440 and 2470 m, the elevation of the lower montane forests (Madole and Shroba 1979). On the western slope, Pinedale glaciers extended downslope as much as 33 km. These glaciers were fed by Pacific moisture. On the eastern slope, Pinedale ice advanced only 14-15 km downslope from the continental divide. Then as now, this region was in a rain shadow for westerly moisture. Pinedale ice may have been as much as 450 m thick near the heads of the glaciers in the Front Range (Madole and Shroba 1979).

We now have good estimates of the timing of some Pinedale glacial events, based on radiocarbon ages of organic-rich sediments at several high-elevation sites. At the Mary Jane ski area, near Winter Park, Colorado (figure 18.2), an excavation for a ski lift tower exposed a series of alternating lake sediments and glacial tills. The oldest lake bed was dated at about 30,000 radiocarbon years before present (yr b.p.) (Nelson et al. 1979). This bed was overlain by glacial till, and the next youngest lake bed yielded a radiocarbon age at the base of about 13,750 yr b.p. Based on the Mary Jane sequence, it appears that the last major ice advance of the Pinedale Glaciation took place between the time of deposition of the older and younger lake beds, so between 30,000 and 13,750 yr b.p.

At Devlins Park, near Lake Isabelle in the Indian Peaks Wilderness area (figure 18.2), Legg and Baker (1980) studied sediments from a lake that was dammed by late Pinedale ice. During the time that Glacial Lake Devlin existed (22,400-12,200 yr b.p.), ice covered the Devlins Park region. The lake drained as the ice retreated, and the youngest sediments from this lake provide a limiting age for this event. Presumably the late Pinedale glacier that advanced downslope from the continental divide west of Devlins Park area took some centuries to reach that elevation (2953 m), so the glacial advance began before 22,400 yr b.p. The terminal moraine of this glacier is located 2.3 km downslope from the study site.

The commencement of alpine peat bog growth has been used to date the retreat of late Pinedale ice from montane valleys back to the alpine zone where they originated. At La Poudre Pass (figure 18.2), Madole (reported in Elias 1983) obtained a radiocarbon date of 10,000 yr b.p. from peat that formed in a bog after Pinedale ice retreated. The pass, which is located at modern treeline, was free of ice prior to that. In the Indian Peaks Wilderness, sediments began accumulating in Long Lake, in the upper subalpine zone north of Niwot Ridge, by 12,000 yr b.p. (Short 1985). Studies in the San Juan Mountains of southern Colorado suggest that the melting of mountain glaciers began after 14,000 yr b.p. (Carrara et al. 1984), so the process of deglaciation was relatively rapid, probably because the glaciers were not very thick compared to glaciers farther north, and the relatively low latitude of the Southern Rockies (the Rocky Mountain ranges from southern Wyoming to central New Mexico) is associated with greater insolation than that of more northerly regions.

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