Mount Hood Glaciations

The last major advance of glaciers in Washington and British Columbia occurred during the Fraser Glaciation. This glaciation began some time before about 29,000 years ago and ended about 10,000 years ago (Armstrong and others, 1965; Armstrong and Clague, 1977). By comparison with glaciers in western Washington and British Columbia, those at Mount Hood probably reached their maximum downvalley extents by 18,000 years ago and then generally retreated until about 11,000 years ago. Glaciers in the mountains probably were not significantly larger by that time than they are today.

Deposits of the Fraser Glaciation at Mount Hood can be recognized by yellowish-brown soil oxidation and by a lack of appreciable weathering of stones in soil profiles. The tickness of the oxidized zone on till measured at 17 localities ranged from 35 to 80 centimeters and averaged 63 centimeters. These characteristics are similar to those of deposits of the first eruptive period (Polallie) described in this report. Some Polallie deposits, in fact, resemble those of glacial origin because of their coarse, poorly sorted, or unsorted texture. For example, deposits in roadcuts along U.S.Highway 26 about 3 kilometers west of Government Camp resemble till, but they were formed by volcanic mudflows (figure 3 -- (Web note: not online) -- ). Similar mudflow deposits on the northeast side of Mount Hood have obscured the extent of glacial deposits of Fraser age, thus, the extent of glaciers is not known in that area.

The average altitude of north-facing cirques near Mount Hood is about 1,030 meter (3,400 feet) in the areas north and northwest of the volcano, about 1,250 meters (4,100 feet) south of the volcano, and about 1,370 meters (4,500 feet) to the east. These cirque floors provide a crude measure of the altitude of areas of ice accumulation during the last glaciation; however, the lower limits of accumulation in areas outside cirques must have been higher because ice was not as protected from the sun there as in the cirques.

When Fraser glaciers were at their maximum extents, the northern slopes of Mount Hood probably were largely covered by ice at altitudes about 1,370 meters (4,500 feet), and the southern slopes above perhaps 1,525-1,675 meters (5,000-5,500 feet). Most north-facing glaciers today terminate at altitudes of 1,830-1,980 meters (6,000-6,500 feet), and the lower limits of perennial snow on the south slope of the volcano seem to be at about 2,150 meters (7,000 feet).

Deposits of one or more older glaciations have also been recognized at scattered localities in the Mount Hood area. One such deposit is till that forms a terminal moraine in the Sandy river valley near Brightwood (figure 2 -- (Web note: not online) -- ); this moraine probably represents the farthest downvalley extent of a glacier during the glaciation that immediately preceded the Fraser Glaciation. Yellowish-brown soil oxidation extends to a depth of 1.5-2 meters in the till, and stones in the soil profile have weathered rinds 1-2 millimeters thick. Glacial deposits in the Mount Rainier area of Washington that have similar weathering characteristics were assigned to the Hayden Creek Drift of the Salmon Springs Glaciation (Crandell and Miller, 1974).

Till thought to be of Hayden Creek age elsewhere in the Sandy River valley underlies a deposit of distinctive pumice that was erupted at Mount St. Helens in southern Washington between about 35,000 and 40,000 years ago (D.R.Mullineaux, oral commun., 1976). The best outcrop at which the relation of the till to the pumice can be seen is along a road to a rock quarry south of U.S.Highway 26 (appendix B, measured section1).

At Bennett Pass, on the southeast side of Mount Hood, a large cut along State Highway 35 exposes three tills separated by yellowish-brown oxidized zones that constitute buried soils. The uppermost till forms a lateral moraine of Fraser age; the ages of the underlying tills are not known.