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

ROCK-GLACIER DEPOSITS

Rock-glacier deposits are thick flat-lying or gently sloping accumulations of coarse angular rock debris that have a lobate form and a surface marked by pits and by ridges and furrows. Angular blocks several tens of feet in maximum dimension are generally strewn over the surface. A depression separates the upslope margin of most rock-glacier deposits from nearby talus (fig. 10). The fronts of rock glacier deposits are abrupt, steep, and from 25 to 100 feet high. Rock-glacier deposits owe their shape and size to flowage of accumulations of rock debris that contain ice. Active rock glaciers differ from true glaciers chiefly in having a very high proportion of rock debris to ice. All of those in the park were formed during a late part of the most recent major glaciation and are inactive now.

Most rock-glacier deposits occupy north- or east-facing cirques within an altitude range of 5,500 to 6,600 feet. Many are closely associated with McNeeley Drift, and in some cirques they merge with McNeeley moraines. All are about the same age as the McNeeley Drift.

A trail just east of Mount Fremont crosses a representative rock-glacier deposit (fig. 4, locality 12), and one of the largest in the park occupies an east-facing cirque between the Palisades and Hidden Lake (fig. 10; fig. 4, locality 13).

ROCK-GLACIER DEPOSIT in the foreground is an accumulation of light-colored rock debris derived from the cliffs of The Palisades in the northeastern section of the park. The deposit covers an area of about 100 acres and is 100—300 feet thick. The steeply sloping deposit of rock debris just below the cliffs is a talus. (Fig. 10)

PROTALUS RAMPARTS

Protalus ramparts are ridges of loose angular rock fragments that lie a few tens of feet beyond the toes of taluses (fig. 11). Blocks larger than 3 feet in diameter are rare, and most ramparts seem to consist chiefly of fragments 6 inches to 2 feet in diameter. The ridges are generally 10—30 feet wide and 5—15 feet high on the side next to the talus. In ground plan they range from straight to sinuous; many are arcuate and are convex in a direction away from the talus. Their distribution, shape, and nearness to taluses indicate that they were formed at some time when thick persistent snowbanks buried the toes of some cliffs and parts of taluses. Rock debris falling from a cliff and sliding down the snowbank accumulated at the toe of the snowbank as a ridge or rampart; a subsequent change in climatic conditions prevented such a large snowbank from forming, so that the falling rock debris then became part of the main talus and building of the rampart ceased.

Most protalus ramparts are the same age as the McNeeley Drift. The close association of protalus ramparts with McNeeley moraines in some cirques suggests that the glacier responsible for the moraine dwindled until only a perennial snowbank remained, at the toe of which the rampart was formed.

The trail to White River Park crosses a typical and readily accessible protalus rampart at the base of Sunrise Ridge (fig. 4, locality 14), and there are several other ramparts associated with McNeeley moraines along the north side of Sunrise Ridge a little to the west.

PROTALUS RAMPART on the north side of Sunrise Ridge is separated from the partly vegetated talus on the left by a depression 5—6 feet deep and 20—30 feet wide. A thick wedge-shaped snowbank blanketed the talus and the depression when the arcuate rampart was formed. (Fig. 11)