Late Pinedale Environments of the Front Range

Paleoclimatic reconstructions for the Rocky Mountain region indicate that the Colorado Front Range received less moisture than ranges to the north (in the Yellow stone region) and the south (the San Juan Mountains). Consequently, the mountain glaciers of the Front Range region were small, and glaciers from most drainages did not coalesce to form larger glaciers or ice sheets. Paleoclimatic reconstructions based on fossil insect assemblages from the Front Range region (Elias 1986, 1996b) indicate that mean July temperatures were as much as 10-11°C colder than modern temperatures as late as 14,500 yr b.p., and mean January temperatures were depressed by as much as 26°C, compared with modern climate. The fossil insect data, therefore, suggests that the temperature regime during the last glaciation was cold enough to foster the growth of glacial ice. The lack of substantive glaciers in the Front Range appears to have been caused by a lack of sufficient winter precipitation to develop the necessary alpine snow pack.

The oldest Pinedale site in the region that has yielded paleoenvironmental data is the Mary Jane site, near Winter Park. Pollen in lake sediments laid down during an interstadial interval before the last major Pinedale ice advance (circa 30,000 yr b.p.) records a sequence of vegetation beginning with open spruce-fir forest with herbs and shrubs, adjacent to the lake. This was followed by a colder phase, in which alpine tundra replaced the subalpine forest (Short and Elias 1987). The youngest (uppermost) sediments in this lake bed reflect climatic amelioration, as indicated by the return of spruce forest to the vicinity before the advance of late Pinedale ice. The Mary Jane site is at an elevation of 2882 m, in the lower part of the modern subalpine forest. The existence of alpine tundra at this site in mid-Pinedale times translates into a depression of tree line by more than 500 m. This, in turn, corresponds to a climatic cooling of at least 3°C from average modern summer temperatures.

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