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

Ohanapecosh Glacier lies at the head of a beautiful isolated valley high on the east side of Mount Rainier (pl. 1, fig. 12). The trail entering the valley from the southeast provides several vantage points below which the valley is laid out in full view (fig. 11). A bare amphitheater holding several perennial snow banks, and an immobile icefield that was once Ohanapecosh Glacier are seen near the head of the valley. The glacier remnant is a flat, sloping shelf of ice above the bedrock cliff. From these vantage points three to four curved moraines can be seen arcing down the north valley wall toward the river and partly encircling the lower end of the amphitheater (fig. 12).

FIGURE 11.—The bare amphitheater in the center holds immobile ice that was once part of Ohanapecosh Glacier. Today, Ohanapecosh (OP) Glacier is restricted to the sloping, partly snow-covered surface above the bedrock cliff. The glacier could not have been beyond area 3, the youngest dated moraine, after 1878, beyond area 2 after 1846, nor beyond area 1 after 1741. Mount Rainier summit is the snow-clad mound forming the apparent horizon at the left. View northwestward. Sept. 27, 1967.

FIGURE 12.—Oblique aerial photograph of Ohanapecosh River valley near Ohanapecosh Glacier. An immobile remnant of the glacier is marked OP. Numbers refer to study areas. Aug. 29, 1969.

The ages of trees growing on moraines formed in the recent past by Ohanapecosh Glacier tell us nothing more than the minimum ages of the moraines. At lower elevations the maximum ages of trees on a moraine, or any other young surface for that matter, closely match the ages of the surfaces, but the moraines at Ohanapecosh Glacier are close to timberline, 5,500 to 6,000 feet here. Environmental conditions affecting establishment of seedlings and growth of trees are adverse near timberline, and it is not known how long after a glacier recedes before seedlings will survive on the moraines at this altitude. The time interval may be 100 years or more on bouldery moraines in which there is no silt or fine sand in which tree seed can germinate (Sigafoos and Hendricks, 1969, p. B93). No trees or seedlings grow in the bouldery moraine shown on the right side of figure 13, and it is older than the moraine upon which the trees in the middle ground are growing. The oldest sampled tree was 3 inches tall in 1878 (table 9).

TABLE 9.—Ohanapecosh Glacier: ages of trees sampled from periglacial features

[Periglacial features: moraines]

The three moraines upon which trees were sampled are many years older than the oldest tree. The oldest moraine (area 1) was formed before A.D. 1741; the middle moraine (area 2) before A.D. 1846; and the youngest (area 3) before A.D. 1878. At Emmons Glacier a pumice is present on moraines older than 148 years, and it is not present on moraines younger than 114 years, suggesting that the pumice was deposited between A.D. 1821 and 1854 (Mullineaux and others, 1969, p. B15-B18). Because the pumice is present at area 3 below Ohanapecosh Glacier, this moraine must have been formed before A.D. 1820. The oldest tree, however, was only 3 inches tall in A.D. 1878; thus, a significant time interval, as much as 50 years, elapsed before the tree started to grow (Sigafoos and Hendricks, 1969, p. B92).

FIGURE 13.—These trees are the closest to Ohanapecosh Glacier, and the oldest was 3 inches tall in 1878. Pumice erupted by Mount Rainier between 115 and 150 years ago is present on the surface here; so the moraine is older. The bare bouldery surface on the right is older than the surface upon which the trees are growing and does not support trees, presumably because no fine soil is present in which seed can germinate. Ohanapecosh Glacier is out of view to the left. Sept. 26, 1967.