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

TALUSES

Taluses are loose accumulations of coarse rock fragments that lie beneath cliffs (fig. 10). Their surfaces slope at angles ranging from about 30° to 35° Rock fragments in a talus are generally fresh and angular and range in size from pieces a few inches across to blocks that are several tens of feet in maximum diameter. Some taluses have bulbous toes that suggest movement by flowage, perhaps caused by ice within the rock debris. Taluses that are wholly forested are not shown on the geologic map.

BLOCK-FIELD DEPOSITS

Block-field deposits consist of angular rock fragments that lie on both sloping and horizontal surfaces. The rock fragments are pried from the underlying solid rock formations by the repeated freezing and expansion of moisture in cracks. Some block fields resemble taluses in that they have slopes of as much as 35° but they differ in that they are not littered with very large blocks and do not have a cliff at their head. Downslope-trending bands of vegetated and unvegetated rock debris a few feet to a few tens of feet wide give some sloping block fields a distinctly striped appearance. Other fields end downslope in elongate lobes of rock debris a few feet to several tens of feet wide and 1—4 feet thick. The shape of these lobes suggests that they originated from flowage of the rock debris, perhaps caused by saturation from rain or melting snow. Block fields are not common where rock debris produced by frost action consists mostly of fragments smaller than 3 inches in diameter. Some block fields probably date from the last major glaciation, and others may be of more recent age.

Especially well developed and easily accessible block fields are crossed by the trail to the Mount Fremont fire lookout (fig. 4, locality 26).

LANDSLIDE DEPOSITS

Landslides consist of chaotic mixtures of loose rock debris, or several very large blocks of rock, that have broken away from the bedrock and have moved downslope. They include those which seem to be inactive and those in which there was evidence of movement during the period of field study (1960—67). An example of an inactive slide is one that originated in west-dipping bedrock just north of Ohanapecosh campground (fig. 4, locality 27). Cuts in this landslide along State Highway 143 expose a jumbled mixture more than 100 feet thick of rock fragments of many sizes in which some layers or zones have a plastic purplish-gray clay matrix. The surface of the slide deposit is hummocky, includes closed depressions, and is strewn with angular blocks as much as 15 feet across. Another inactive slide is crossed by the Mather Memorial Parkway (U.S. Highway 410) half a mile north of Cayuse Pass (fig. 4, locality 28). The slide originated in west-sloping bedrock and descended into the valley of Klickitat Creek, where it now impounds Ghost Lake. Highway cuts through the slide deposit show a loose mixture of rock fragments of many sizes, some as large as 20 feet across.

An active landslide is slowly moving down the west slope of Backbone Ridge (fig. 4, locality 29). It consists of a jumbled mass of sandstone and claystone, probably only about 25 feet thick. From 1962 to 1966 the slide moved at an average rate of about 6 inches per year.

An active slide on the west side of the Nisqually River valley west of Paradise Park involves lava flows from the volcano as well as the underlying bedrock. The slide block seems to be dropping eastward toward the center of the valley and rotating, so that the north side of the block is moving more than the south side. The slide is broken into several smaller blocks by cracks that parallel the 1,000-foot-long main crack at the slide's head. Photographs taken in 1912 show the fresh scarp at the head of the slide, so it evidently began to move at some earlier time. The slide can be seen from various viewpoints along the trail from the Paradise visitor center to Panorama Point.