action brought on by changes in temperature
                                                                                                    ~

(expansion and shrinkage), the freeze-thaw                                              FIRM BEDDED ROCK

cycle, and the burrowing activity of animals.
Earthquakes
Earthquakes not only trigger landslides, but,
over time, the tectonic activity causing them can
create steep and potentially unstable slopes.
Rapid sedimentation
Rivers supply very large amounts of sediment
to deltas in lakes and coastal areas. The rapid-
ly deposited sediments are frequently under-
consolidated, and have excess pore-water
pressures and low strengths. Such deltaic
sediments are often prone to underwater
delta-front landsliding, especially where the
sediments are rich in clay and/or contain gas
                                                        Figure 8a. Rockfall (Colorado Geological
from organic decomposition.
                                                        Survey et al., 1988).
Wind-generated waves
Storm waves in coastal areas are known to
trigger underwater landsliding in deltas by
cyclically loading weak bottom sediments.
Tidal or river drawdown
Rapid lowering of water level in coastal areas
or along river banks due to tides or river dis-
charge fluctuations can cause underwater land-
sliding. The process in which weak river bank
or deltaic sediments are left unsupported as
the water level drops is known as "drawdown."

          Types of Landslides
The most common types of landslides are des-            Figure 86. Rockfall on U S . Highway 6,
cribed below. These definitions are based               Colorado (photograph by Colorado
mainly on the work of Varnes (1978).                    Geological Survey).
                       Falls
Falls are abrupt movements of masses of                 separates from the main mass, falling to the
geologic materials that become detached from            slope below, and subsequently bouncing or
steep slopes or cliffs (Figures 8a, b). Movement        rolling down the slope (Figures 9a, b).
occurs by free-fall, bouncing, and rolling. De-
                                                        Slides
pending on the type of earth materials invol-
ved, the result is a rockfall, soilfall, debris fall,   Although many types of mass movement are
earth fall, boulder fall, and so on. All types of       included in the general term "landslide," the
falls are promoted by undercutting, differential        more restrictive use of the term refers to move-
weathering, excavation, or stream erosion.              ments of soil or rock along a distinct surface of
                                                        rupture which separates the slide material
                      Topple                            from more stable underlying material. The two
A topple is a block of rock that tilts or rotates       major types of landslides are rotational slides
forward on a pivot or hinge point and then              and translational slides.
                                                                                                        11
                                         Figure 9b. Topple, western Colorado (photo-
                                         graph by Colorado Geological Survey).
                                         Rotational slide
                                         A rotational slide is one in which the surface of
                                         rupture is curved concavely upward (spoon
                                         shaped) and the slide movement is more or less
                                         rotational about an axis that is parallel to the
                                         contour of the slope (FigureslOa, b). A "slump"
                                         is an example of a small rotational slide.
                                         Translational slide
                                         In a translational slide, the mass moves out, or
                                         down and outward along a relatively planar
                                         surface and has little rotational movement or
                                         backward tilting (Figure 11).The mass com-
                                         monly slides out on top of the original ground
                                         surface. Such a slide may progress over great

Figure 9a. Topple (Colorado Geological   Figure 1Oa. Rotational landslide (modified
Survey et al., 1988).                    from Varnes, 1978).

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