Slope-movement features common to many areas in the unglaciated Missouri Plateau in North Dakota, South Dakota, Montana, and Wyoming, which appeared to be earthflows, were studied at a locality in North Dakota. The features were examined at the surface and in a trench on a west-facing slope inclined at 14 deg (25 percent). The trench, 0.76 m (2.5 ft.) wide, 71 m (233 ft.) long, and 1.5-3.7 m (5-12 ft.) deep, extended from near the crest to the base of the slope and exposed the relations between colluvium, alluvium, bedrock, and sliding. The sliding is common to a variety of geological formations in the Missouri Plateau and, at the study site, is in the Sentinel Butte Member of the Paleocene Fort Union Formation. The bedrock is exposed at the surface near the crest of the slope, overlain by colluvium farther down in the upper part of the slope and colluvium and alluvium in the lower (westernmost) 27.3 m (90 ft.) part of the slope. The evidence of movement exposed by the trench consisted of cross sections of scarps and surfaces of parting. The scarps, which appear to be scattered haphazardly over the slope, are typically less than 0.5 m (1.5 ft.) high, 8-15 m (25-50 ft.) long, and crescent-shaped in plan view. The surfaces of parting occur near the top of the B horizon at the base of the root zone in the colluvium and, where colluvium is thin or absent, the parting surface is in a creep zone developed in bedrock. The upslope traces of the parting surfaces could be traced to or within 0.4-0.6 m (1.3-2 ft.) downslope from the scarps. There was no evidence of deeper sliding or flowing in the alluvium, colluvium, or bedrock, and movements that were interpreted from surface form to be earthflows are in fact thin soil slides. Movement has been restricted to thin slabs of soil sliding a short distance on the top of the relatively impermeable, clay-rich B horizon in the soil. Although these soil slides are relatively thin, they could damage structures placed on them. However, understanding the nature of these slides suggests simple control measures that can prevent damage. (Author)

  • Supplemental Notes:
    • Proceedings of the Eighteenth Annual Engineering Geology and Soils Engineering Symposium, held at the Red Lion Inn, Boise, Idaho.
  • Corporate Authors:

    Idaho Transportation Department

    3311 W State Street, P. O. Box 7129
    Boise, ID  United States  83707-1129

    University of Idaho, Moscow

    Department of Geology and Geological Engineering
    Moscow, ID  United States  83844-3025

    Boise State University

    Department of Geology and Geophysics
    Boise, ID  United States 

    Idaho State University, Pocatello

    Department of Geology
    Pocatello, ID  United States  83201

    Idaho State University, Pocatello

    Department of Geology
    Pocatello, ID  United States  83201
  • Authors:
    • Trimble, D E
    • Fleming, R W
  • Conference:
  • Publication Date: 1980-4

Media Info

  • Features: Figures; References;
  • Pagination: p. 33-34

Subject/Index Terms

Filing Info

  • Accession Number: 00334057
  • Record Type: Publication
  • Created Date: Sep 16 1981 12:00AM