Geological Survey Bulletin 1288 Surficial Geology of Mount Rainier National Park Washington

BURROUGHS MOUNTAIN DRIFT

The Burroughs Mountain Drift was formed during a period of glacier expansion that occurred between about 2,500 and 3,000 years ago. Although the glaciers of the park became only a little larger than they are today, this was a significant change, for during the time since the end of the last major glaciation they had been smaller. Burroughs Mountain moraines can be recognized by the fact that they are older than pumice layer C (fig. 14), and younger than layer Y.

Till in the Burroughs Mountain moraines is gray, generally loose, and not appreciably weathered. The moraines below timberline are densely forested with trees several generations younger than the first forest that became established on the moraines after their formation. Most of the Burroughs Mountain moraines are only a few yards beyond moraines formed within the last 700 or 800 years, although those of Winthrop Glacier represent a glacier 300 feet thicker than at any subsequent time. The largest Burroughs Mountain moraines occur in the areas adjacent to Winthrop Glacier and between Winthrop and Carbon Glaciers, and smaller ones have been identified at a few other places in the park. The Wonderland Trail crosses a Burroughs Mountain moraine of Winthrop Glacier on the west slope of Burroughs Mountain (fig. 4, locality 20).

PUMICE LAYER C forms a thin brown veneer on the drift in the foreground and on the Burroughs Mountain moraine in the center of the photograph. This pumice was erupted by Mount Rainier between 2,150 and 2,500 years ago. The Garda moraine of Fryingpan Glacier at the left has no pumice on it, and thus is younger. The large boulder in the middle foreground is about 6 feet long. This locality is at an altitude of 7,500 feet about a mile southwest of Panhandle Gap. (Fig. 14)

GARDA DRIFT

Moraines formed since the eruption of pumice layer C are included in the Garda Drift (fig. 14); contemporaneous glacial melt-water deposits, however, are represented on the geologic map only as alluvium. The till consists of a gray unsorted mixture of pebbles, cobbles, and boulders in a silt and sand matrix and is unweathered. The presence or absence of pumice layer W divides Garda moraines into groups that are older or younger, respectively, than about 450 years. Trees growing on some Garda moraines are at least 750 years old. Moraines younger than layer W include stable, forested deposits decades or a few centuries old as well as subsiding masses of rock debris that still bury blocks of slowly melting ice. Younger Garda moraines are especially unstable on steep valley sides in areas from which glaciers have receded within the last half century. Boulders in the moraines become dislodged from time to time and roll and bound down to the valley floor at high speed. During periods of heavy rainfall or rapid snowmelt, saturated masses of drift sometimes move from the moraines onto the valley floors as mudflows.

Moraines of Garda Drift range in size from ridges only a few feet high and a few tens of feet wide to massive complex end moraines such as the one that was formed in the White River valley by Emmons Glacier between the 17th century and about 1910 (fig. 15; fig. 4, locality 21). Dates shown on individual moraines on the geologic map are based on the earliest growth ring of the oldest known living tree on the moraine; the moraine is older than the tree by an unknown number of years, perhaps as much as several decades. One of the most readily accessible Garda moraines in the park lies at the west end of the bridge across the Nisqually River (fig. 4, locality 22), 3.5 miles northeast of Longmire. This moraine is conspicuous because trees on it are much younger and smaller than those in the adjacent older forest. When this moraine was formed in 1840, Nisqually Glacier covered the site of the present highway and reached to a point about 750 feet downvalley from the bridge.

END MORAINE of Garda Drift formed by Emmons Glacier is a hummocky accumulation of bare rock debris in the center of the White River valley. When the ice front stood at the moraine, the upper surface of the glacier coincided with the conspicuous sloping trimline in the trees on the opposite valley wall. The exposure in the center fore ground is that shown in figure 13. (Fig. 15)