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P. E. Carrara

Publications and source records attributed to P. E. Carrara.

27 records · Page 2Linked to original sources
USGS1987

Holocene and latest Pleistocene glacial chronology, Glacier National Park, Montana.

Moraines of two different age groups have been identified fronting the present-day glaciers and snowfields in Glacier National Park, Montana. The subdued, vegetated moraines of the older group have been found at 25 sites, mainly in the central part of the Lewis Range. These older moraines are in places overlain by the Mazama ash. Although the exact age of the moraines has not been determined by radiocarbon dating, vegetative evidence and correlation with other pre-altithermal age moraines in the Rocky Mountains suggest that these older moraines date from 10 000 BP or earlier. Whether these moraines are the product of a separate advance after the end of the Wisconsin glaciation or are simply the product of the last advance or stillstand of Wisconsin glaciers before final deglaciation is not known.Moraines of the younger group, consisting of fresh bouldery rubble, are common throughout Glacier Park. Tree-ring analyses indicate that some of these younger moraines were deposited by advances that culminated during the mid-19th century. At that time there were more than 150 glaciers in Glacier Park. This episode of mid-19th century climatic cooling resulted in the most extensive glacial advance in this region since the end of the Wisconsin glaciation.Present-day glaciers have shrunk drastically from their mid-19th century positions; more than half the glaciers present during that time no longer exist. Much of this retreat occurred between 1920 and the mid-1940's, corresponding to a period of above-average summer temperatures and below-average annual precipitation in this region. Between 1966 and 1979, several of the larger glaciers in the Mount Jackson area of Glacier Park advanced as much as 100 m.

Montana↗
USGS1986

Deglaciation of the mountainous region of northwestern Montana, U.S.A., as indicated by late Pleistocene ashes

During the late Wisconsin glaciation, the mountainous regions of northwestern Montana were covered by glacial ice. Marias Pass, on the Continental Divide immediately south of Glacier National Park, was covered by a local ice field. This ice and that from other glaciers to the north and south flowed eastward onto the plains of Montana to form the Two Medicine Glacier, a large piedmont glacier that extended 55 km beyond the mountain front. The presence of the Glacier Peak G ash and the underlying St. Helens Jy ash in laminated lake sediments near Marias Pass indicates that in this region the Continental Divide was ice free before about 11,400 BP. Macrofossils, pollen, and spores in these same sediments indicate establishment of shrubs, herbs, and scattered conifers by that time. At Sun River Canyon, about 90 km south of Marias Pass, glaciers also flowed beyond the mountain front onto the plains to form the Sun River Glacier, another large piedmont glacier that extended beyond the mountain front for 25 km. The presence of the Glacier Peak G ash in a postglacial alluvial fan indicates that glacial ice had receded upvalley from the canyon mouth and that the Sun River Glacier no longer existed by 11,200 BP.

Montana↗
USGS1981

The late-Neoglacial histories of the Agassiz and Jackson glaciers, Glacier National Park, Montana.

Twenty-one tree-ring stations, totaling 116 trees, were sampled at various localities within the forest trimlines fronting the Agassiz and Jackson glaciers, Glacier National Park, Montana. Tree ages within these zones became progressively younger from the region of the maximum late-Neoglacial position to the bases of the bedrock slopes on which these glaciers are now confined. The age of the oldest tree plus 15 yr was used to estimate the date of glacier withdrawal from a given station. It was found that both the Agassiz and Jackson glaciers began to retreat from their maximum late-Neoglacial positions about 1860. Hence, Matthes's (1940) estimate of glacial advances culminating about 1850 to 1855 for many glaciers in the western United States seems reasonable for the Glacier National Park region. Retreat rates, derived from the tree-ring data, appear to have been modest (<7 m yr -1 ) until about 1910 when they increased reaching more than 40 m yr -1 for the Agassiz Glacier between 1917 and 1926. Retreat rates after the late 1920s could not be monitored by tree-ring analysis as both glaciers had retreated onto bare bedrock dip slopes. However, from various literature descriptions and National Park Service records, both glaciers experienced rapid retreat (>100 m yr -1 ) from this time until 1932. In addition, while the Agassiz Glacier was monitored by the National Park Service (1932 to 1942) retreat continued at a rapid rate (>90 m yr -1 ). This period of rapid retreat corresponds with a period of above-average summer temperatures and decreased precipitation in the climatic record of the region. Since the mid-1940s the retreat rate of both glaciers has slowed markedly.

Montana↗