materials and are distinctive because they
                                                    usually occur on very gentle slopes. The fail-
                                                    ure is caused by liquefaction, the process
                                                    whereby saturated, loose, cohesionless sedi-
                                                    ments (usually sands and silts) are trans-
                                                    formed from a solid into a liquefied state; or
                                                    plastic flow of subjacent material. Failure is
                                                    usually triggered by rapid ground motion such
                                                    as that experienced during an earthquake, or
                                                    by slow chemcal changes in the pore water and
                                                    mineral constituents.
I$
          A                                                                              SLIP SURFACE
     "m

Figure 1Ob. Rotational landslide, Golden,
Colorado (photograph by Colorado
Geological Survey).

distances if conditions are right. Slide material
may range from loose unconsolidated soils to
extensive slabs of rock.

           GROUND SURFACE

                                                    Figure 12. Block slide (Colorado Geological
                                                    Survey et al., 1988).

                                                                        Flows
                                                    Creep
                                                    Creep is the imperceptibly slow, steady
                                                    downward movement of slope-forming soil or
                                                    rock. Creep is indicated by curved tree trunks,
                                                    bent fences or retaining walls, tilted poles or
                                                    fences, and small soil ripples or terracettes
                                                    (Figures 14a, b).
                                                    Debris flow
Figure 11. Translational slide (Colorado            A debris flow is a form of rapid mass movement
Geological Survey et a/., 1988).                    in which loose soils, rocks, and organic matter
                                                    combine with entrained air and water to form a
                                                    slurry that then flows downslope. Debris-flow
Block Slide. A block slide is a translational
                                                    areas are usually associated with steep gullies.
slide in which the moving mass consists of a
                                                    Individual debris-flow areas can usually be
single unit, or a few closely related units that
                                                    identified by the presence of debris fans a t the
move downslope as a single unit (Figure 12).
                                                    termini of the drainage basins (Figure 15).
               Lateral Spreads                      Debris avalanche
Lateral spreads (Figures 13a, b) are a result of    A debris avalanche is a variety of very rapid to
the nearly horizontal movement of geologic          extremely rapid debris flow.

                                                                                                        13
Earthflow                                          of snow and ice due to heat from volcanic vents;
Earthflows have a characteristic "hourglass"       or by the breakout of water from glaciers, crat-
shape (Figures 16a, b). A bowl or depression       er lakes, or lakes dammed by volcanic eruptions.
forms a t the head where the unstable material
collects and flows out. The central area is                                              SPREADING

narrow and usually becomes wider as it reach-
es the valley floor. Flows generally occur in
fine-grained materials or clay-bearing rocks on
moderate slopes and with saturated conditions.
However, dry flows of granular material are
also possible.
Mudflow
A mudflow is an earthflow that consists of
material that is wet enough to flow rapidly and
that contains at least 50 percent sand-, silt-,                                     OUT OF ALIGNMENT
and clay-sized particles.
Lahar
                                                   Figure 14a. Creep (Colorado Geological
A lahar is a mudflow or debris flow that origin-
                                                   Survey et al., 1988).
ates on the slope of a volcano. Lahars are
usually triggered by such things as heavy rain-
fall eroding volcanic deposits; sudden melting

Figure 13a. Lateral spread (Colorado
Geological Survey et al., 1988).

                                                   Figure 146. Creep, vicinity o Mt. Vernon
                                                                                f
Figure 136. Lateral spread, Cortez, Colorado.
                                                   Canyon, Jefferson County, Colorado (photo-
(Photograph by Colorado Geological
                                                   graph by Colorado Geological Survey).
Survey).

14