Taskey, R. D.; Harward, M. E.; Youngberg, C. T. 1978. Relationship of clay mineralogy to landscape stability. In: Youngberg, Chester T., ed. Forest soils and land use: proceedings of the 5th North American forest soils conference; 1978 August; Fort Collins, CO. Fort Collins, CO: Department of Forest and Wood Sciences, Colorado State University: 140-164.
Erosion by mass wasting is the major problem facing land managers workingwith the volcanically derived forest soils of the northwestern United States.Slope failures are common on the undisturbed mountainous lands, and theiroccurrence and severity are greatly increased by activities such as roadbuilding and timber harvesting. Unfortunately, some of the most productiveforest lands in this region are also some of the most unstable--a fact whichpresents a significant management challenge in concentrating timber producingactivities on the most productive sites. To help determine the underlyingreasons for differences in stability from one site to another, and hence tobetter characterize unstable and stable sites, we have evaluated the natureof the clay fractions of a number of volcanic soils which are subject tovarious degrees of stability and to various types of mass movements. Anunderstanding of cause and effect relationships is requisite to evaluation ofremedial practices. The study is also of interest because it provides apredictive tool for diagnosing unstable areas.
The study was concentrated in Oregon's Western Cascade Range, which ischaracterized by adolescent, rapidly developing stream drainage systems andan overall hummocky appearance. Steep slopes, high annual precipitation anda history of seismic activity are combined with clayey soils formed predomi-nantly from low-strength pyroclastic tuffs and breccias to make the mountain-sides highly unstable. To a great extent, the Cenozoic volcanic materialswhich comprise the bulk of the geology control the distribution, frequency ofoccurrence and types of land failures. The highly weathered pyroclastictuffs and breccias of the Little Butte Volcanic Series (late Eocene to earlyMiocene) are notorious for their susceptibility to mass movement (Burroughset al., 1976; Dyrness, 1967; Path et al., 1971; Pope and Anderson, 1960;Swanson and James, 1975). In many parts of the Range, the Little ButteVolcanics are capped by an overburden of harder andesite or basalt, princi-pally of the Sardine Formation (late Miocene). The rock of these lava flowsoften occurs as a colluvial deposit which has weathered to a soil mantle two to four meters thick over the altered pyroclastics. Some of the most unstableland in Oregon is situated at the peripheral contacts of these two geologicunits.