(b) Avalanche. This is a mass of material ranging in composition from completely ice and snow to
predominantly rock debris. Avalanches are capable of moving downslope at speeds in excess of 300 kilometers
per hour; thus, they are the most rapid and usually the most destructive form of mass wasting.
(c) Debris Slide. A cliff of unconsolidated debris that is undercut by erosion tends to collapse along
discrete shear planes, forming a debris slide. This type of mass wasting is common along major rivers during
(d) Rockslide. This is a rapid downslope movement of masses of bedrock that become detached
along bedding planes, joints, or faults. Rockslides most commonly occur where sedimentary rocks dip steeply
downslope. Because they often involve large masses of bedrock, rockslides may be very devastating.
(e) Slump. This is a form of rapid mass wasting in which a portion of unconsolidated, relatively
homogeneous material slides downward, as a single unit, along a failure surface that is concave upward. A
common cause of slumps is the removal of the toe of a slope by natural processes, such as stream erosion, or by
human-induced factors, such as road excavation.
(f) Rockfall. Many steep cliffs are composed of fragmented bedrock that periodically becomes
dislodged from the parent material. In these instances, individual rock masses ranging in size from sand grains to
huge boulders may bounce down the side of the cliff, occasionally undergoing vertical free-fall. Consequently,
there is very little interaction between the falling rock and the material remaining on the cliff face. This type of
mass wasting is called a rockfall. Rockfalls are capable of transporting weathered bedrock up to two kilometers
from its source area.
b. Deposition by Mass Wasting. Materials that have been transported downslope by mass wasting
processes eventually reach a more or less stable position, usually at the base of a hill or cliff. Colluvium is a
general term used to describe these incoherent materials, which normally occur as chaotic accumulations of
angular particles. Types of colluvial deposits include talus and boulder field.
(1) Talus. Talus accumulations are fan-shaped deposits of broken rock fragments near the base of a cliff
that have slowly accumulated as a result of numerous individual rockfalls. Talus deposits exhibit remarkably
consistent slopes; they are generally inclined at angles of 34 to 35 degrees from the horizontal.
(2) Boulder Fields. Boulders carried to the base of a cliff by creep action may accumulate as large