SURFICIAL DEPOSITS SHOWN ON THE GEOLOGIC MAP
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).
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.
Last Updated: 01-Mar-2005