tend to stand in vertical cliffs, whereas noncohesive materials are stable only at angles of about 30 to 40 degrees
above the horizontal.
a. Erosion by Mass Wasting. The erosion associated with mass wasting may occur either slowly or
rapidly. In any case, the most significant sign that an area has undergone erosion by mass wasting is the presence
of a prominent scar that trends downslope and contains no vegetation.
(1) Slow Erosion of Slopes. Downslope movement that is imperceptible to the eye is considered to be
slow. Therefore, in order to detect slow mass movements, it is necessary to utilize indirect methods, such as the
recording of displaced fences or tilted telephone poles. There are several different types of slow downslope
(a) Creep. This is the most widespread form of slow mass wasting. It is the imperceptible downhill
movement of soil and unconsolidated rock material. Numerous processes, such as alternate heating and cooling,
the growth of ice crystals, alternate wetting and drying, the actions of organisms, and the vibrations caused by
seismic activity, result in minor disturbances of unconsolidated silt to boulder-sized particles. The disturbed
particles are, in turn, acted upon by the force of gravity; consequently, they are transported minute distances
downslope. Several episodes of disturbance (each of which is followed by downslope movement) may occur,
resulting in transportation distances of a few millimeters to a few centimeters over a year's time.
(b) Solifluction. This is the slow downslope movement of consolidated material that has been
saturated with water. This phenomenon most often occurs in arctic or subarctic regions during the summer
months when the upper portions of the frozen ground begin to thaw, becoming saturated with water. The thawed
portion may then move downslope in the form of a lobate mass, travelling at a rate of a few millimeters to several
centimeters per year. As winter approaches, the mass refreezes, only to thaw once again with the return of
(c) Rock Glaciers. Rainwater that inundates a loose accumulation of rock material may surround
the individual component grains of the mass, filling any void spaces that are present. If the material has
accumulated at high altitudes, the water within the mass may freeze, forming a rock glacier, which is a lobate
mixture of rock and ice that, under pressure, flows downslope at a rate of up to one meter per year.
(2) Rapid Erosion of Slopes. In contrast to slow downslope movement, rapid erosion of slopes takes
place at a perceptible rate. In fact, rapid types of mass movement may occur at rates of more than 30 meters per
second. There are several types of mass wasting, and the classification of a particular event depends on the
characteristics of the movement and the type of material involved.
(a) Debris Flow. This is a viscous downslope movement of undifferentiated material including soil,
rock, vegetation, and man-made objects. Debris flows that are volumetrically composed of greater than 50
percent fine particles and also contain abundant water are termed mudflows. These types of flows, which are
capable of transporting buildings as well as large boulders, are common where slopes are moderate, precipitation
is intermittent, vegetation is sparse, and the clay or silt content is considerable.