Late Quaternary Glacial and Vegetative History of the Glacier National Park Region, Montana

Carrara, Paul

Deglaciation was at least 90 percent complete in the Glacier National Park region by the time the Glacier Peak G (11 ,200 yr. BP) and Mount St. Helens Jy (11 ,400 yr. BP) ashes were deposited. The presence of these two ashes at Marias Pass, on the Continental Divide, indicates that the Marias Pass area was deglaciated before 11,400 yr. BP. The fact that the Two Medicine Glacier, a large piedmont glacier on the plains of Montana, was supplied by ice that flowed through Marias Pass indicates that this glacier retreated from the plains of Montana long before 11,400 yr. BP. Hence, by that time, former tributary glaciers along the eastern side of the Lewis Range in Glacier National Park no longer flowed east beyond the mountain front but were confined to their respective mountain valleys. On the west side of Glacier National Park, the large trunk glacier that filled the valley of the North Fork Flathead River had retreated at least 10 km north of the site of the present-day town of Polebridge by the time the Glacier Peak G and Mount St. Helens Jy ashes were deposited. Similarly, the valley of the Middle Fork Flathead River was ice-free by the time the Glacier Peak Gash was deposited. Furthermore, stratigraphic evidence in both valleys suggests that the large trunk glaciers that occupied these valleys may have retreated from these areas several hundred to several thousand years before the deposition of the Glacier Peak G and Mount St. Helens Jy ashes. By 11,200 yr. BP, glaciers in the Lewis, Livingston, Whitefish, and Flathead Ranges that were formerly tributary to the North and Middle Fork trunk glaciers were confined to their local mountain valleys. This conclusion is indicated by a radiocarbon age of 11,150±90 yr. BP (W-5043) from a bog on a moraine at the lower end of Bowman Lake and by the presence of the Glacier Peak Gash in a bog on a moraine deposited by the former McDonald glacier about 1 km southeast of the site of the present-day town of West Glacier. By 11,000 to 10,000 yr. BP, remaining glaciers in Glacier National Park were probably confined to the same cirques and well-shaded niches where present-day glaciers and snowfields lie. Although evidence was not found in the park, data from other areas in the Rocky Mountains suggest Manuscript approved for publication, June 30, 1989. that by as early as 11 ,000 yr. BP remaining glaciers may have been similar in size to present-day glaciers. Radiocarbon ages near present-day glaciers in Banff and Jasper National Parks, Alberta, and Yoho National Park, British Columbia, indicate that by 10,000 yr. BP, late Wisconsin glaciers had receded to positions close to those of present-day glaciers. A similar amount of deglaciation by these times is inferred for the Glacier National Park region. Initial revegetation of the Glacier National Park region had also occurred by the time the Glacier Peak G and Mount St. Helens Jy ashes were deposited. At Marias Pass, a conifer needle and an alder strobile were found beneath the Glacier Peak G ash, and several small willow fragments were found beneath the Mount St.

Late Quaternary Glacial and Vegetative History of the Glacier National Park Region, Montana

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