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

AVALANCHE DEPOSITS FROM LITTLE TAHOMA PEAK

Seven or more rockfalls from the steep north face of Little Tahoma Peak in December 1963 resulted in a series of large avalanches that rushed as far as 4.3 miles down valley. Although these avalanches were a form of landslide, they differed from most other landslides of the park in their speed. An extremely high velocity, probably exceeding 100 miles per hour, is indicated by the height to which some avalanches rose onto the sides of obstacles in their paths.

Such a velocity is also suggested by the way some avalanches caromed from one side of the valley to the other during movement, by their distance of movement, and by the presence of rock fragments embedded in the trunks of trees along the south margin of the avalanches a quarter of a mile upvalley from White River campground.

The avalanche deposits are jumbled mixtures of angular rock debris of many sizes in a loose reddish-gray matrix of sand (fig. 19). Scattered blocks of andesite breccia in the deposits are as large as 60 by 130 by 160 feet. The surface of the avalanche deposits is hummocky in most areas, but in some places there are ridges and furrows in the debris. Some ridges and furrows parallel the trend of the valley, and others are transverse to it. The deposits have a maximum thickness of about 100 feet, and they originally covered an area of about 2 square miles, including the part on the surface of Emmons Glacier. The total volume is estimated to be at least 14 million cubic yards.

AVALANCHE DEPOSIT of rock debris in the foreground originated in rockfalls from Little Tahoma Peak. The deposit buried the former floor of the White River valley to depths as great as 100 feet. The boulder on the ridge at the left has dimensions of about 24 by 30 by 46 feet. (Fig. 19)

The long distance some avalanches moved is attributed to the development of cushions of compressed air beneath the avalanches as they hurtled off the steep front of Emmons Glacier. These air cushions buoyed up the swiftly moving avalanches for long distances and prevented them from striking the ground.