Geological Survey Professional Paper 387—B Recent Activity of Glaciers of Mount Rainier, Washington

In a dense forest low on the west slope of Burroughs Mountain (fig. 1 on pl. 6), Wonderland Trail crosses a conspicuous pumice-covered lateral moraine of Winthrop Glacier marking an advance that occurred near the end of the Fraser Glaciation, probably between 11,000 and 12,000 years ago (D. R. Crandell, U.S. Geological Survey, written commun., 1972).

Farther downslope the trail parallels, then crosses, younger moraines (area 4, fig. 5 and map on pl. 6, table 7) which show many features different from those on the higher slope. The trees are smaller and younger, the forest floor is rocky, humus is thin, and coarse pumice is not present. Where the trail enters the young moraines, the oldest tree found started to grow around 1670. A short distance upvalley, at area 7, trees started to grow about 1655. These trees are among the first to grow after Winthrop Glacier started to recede from a modern advance delimited by these moraines.

TABLE 7.—Winthrop Glacier: ages of trees sampled from periglacial features

[Periglacial features: M, moraine; OS, old surface]

West of Winthrop Creek, Wonderland Trail (shows faintly on photograph, fig. 5 on pl. 6) crosses nearly barren, rocky, morainal debris, most of which was deposited during the last century (fig. 3 on pl. 6). A short distance northward, downvalley from the trail, a low ridge (area 3) marks the position of a probable readvance of the ice front which started to recede early in the last century. Another 400 feet northward, about 500 feet from the trail, the bouldery surface ends abruptly at the forest margin (fig. 2 on pl. 6). This dense forest is growing on an older surface underlain by alluvium and mudflow deposits. The steeply sloping ridge here (area 1) is the terminal moraine of Winthrop Glacier from which the glacier started to recede early in the 18th century. The trail turns southwest from area 3 and is perched on the crest of the A.D. 1810 moraine until it enters the closed forest of small trees near the West Fork of White River about 750 feet north of area 9.

Two moraines, older than the A.D. 1810 moraine and younger than the 1670 lateral moraine crossed by Wonderland Trail east of Winthrop Creek, form prominent ridges in the valley but far from the trails. Both are present on the west side of the valley, whereas only one has been identified on the east side. A nearly continuous ridge skirts the east side of the low forested hill on the west side of the valley in the forest between areas 9 and 11 (fig. 5 and map on pl. 6). Ice started to recede from this moraine about A.D. 1710-20, probably contemporaneously with recession from the terminal moraine. A moraine of this age was not found on the east side of the valley, but another one from which ice started to recede about A.D. 1755-65 is present there (areas 6, 7a, and 8) (fig. 4 on pl. 6).

West of area 3, Wonderland Trail (fig. 5 and map on pl. 6) is perched on a bouldery ridge approximately 75 to 100 feet wide that protrudes about 350 feet westward across a flat, smooth plain (labeled "flat area" on the map on pl. 6). The west end of the bouldery ridge is cut by an abandoned melt-water channel. The plain is underlain by material deposited in a pond when melt-water was temporarily dammed by the 1710-20 moraine. The pond was soon drained, however, when its level rose above the moraine and the water broke through, flowing into West Fork White River. The melt-water channel through the A.D. 1810 moraine was formed after the ice had receded southward farther upvalley.

The ice, seen from Wonderland Trail where it turns downvalley on the east side of Burroughs Mountain and starts to descend the inner face of the tree- and shrub-covered moraine (figs. 4 and 5 on pl. 6), is the stagnant part of Winthrop Glacier. From 1960 to 1966 the shape of the stagnant front has changed from convex to concave (based on photographic and visual evidence). Photographs (figs. 4 and 5 on pl. 6) show that Winthrop Creek flows under the stagnant ice.

The active part of Winthrop Glacier is upvalley (fig. 4 on pl. 6) opposite and south of area 6. One part of it advanced approximately 25 feet between 1960 and 1966.