Mountain Landforms and Geomorphic Processes - Mountain Geography: Physical and Human Dimensions - page 252

Mass wasting is the downslope movement of material because of gravity. Many different
processes are included, from slow creep and solifluction, to more rapid mudflows and
slumping, to spectacular rockfalls, debris avalanches, and landslides. Mass wasting oc-
curs with the greatest intensity in periglacial regions because of surface movement
through frost creep; high moisture content, which lubricates the soil and produces high
pore water pressures; and because glaciers have oversteepened slopes (French 1996).
At high altitudes, frost action is the chief agent of rock disintegration, and mass wast-
ing is the chief agent of transport. With the exception of glacial sculpture, frost action
and mass wasting account for most of the characteristic features of high mountain land-
scapes. Movement of coarse debris on alpine hillslopes is largely due to mass wasting
(Caine 1986). Slow mass wasting accounts for only about 15 percent of the geomorph-
ic work done in most mountains; its importance decreases with increasing basin size
(Barsch and Caine 1984).
Forms of mass wasting have long been difficult to delineate. A mass of rock breaking
loose from a mountain may slide, fall, and flow during various segments of its journey,
resulting in very different forms. Processes and forms are transitional, and so are their
identification and classification. The literature is filled with different names applied to
similar features. Without becoming entangled in such problems, we will discuss several
of the more distinct processes.
Creep
Creep, the slow downslope movement of surface material, is usually only detectable
through long-term observations. It is a process common to all environments and can
be caused by wetting and drying, heating and cooling, freezing and thawing, disturb-
ance of the soil by organisms, or simply the effect of shear stress on slopes. The rate of
creep is so slow in most lowlands that its effect can only be seen over decades or cen-
turies. In mountains, however, the effect of these processes is greatly intensified. This
is especially true of frost creep. Frost creep produces a ratchet-like motion in which a
soil, during a freeze-thaw cycle, is displaced normal to its surface. The displaced soil is
then moved downslope by gravity and settles parallel to the hillslope (Benedict 1970).
Soil creep reveals a convex-upward velocity profile near the surface (Roering 2004),
and there may be a retrograde motion (upslope) because of cohesion and attraction
of finegrained particles (Benedict 1976). Frost creep is primarily controlled by the fre-
quency of freeze-thaw cycles, the angle of the slope, the moisture available for heave,
the frequency of variation around 0°C, and the texture and frost susceptibility of the soil
(Troll 1958; Benedict 1970, 1976; French 1996; Matsuoka 2005). The potential for frost
creep is greatest in autumn when soils are saturated, and at snow-free sites within the
discontinuous permafrost zone where seasonal freezing is deep (Benedict 1976). Two-
sided freezing, or freezing from the permafrost upward, is important for inducing creep
during the thaw period. Water can percolate through the soil, reach the freezing front
created from the permafrost, freeze, and cause a summer heave (Lewkowicz 1988). On
the crest slopes, frost creep occurs diurnally in areas that lack vegetation and have a
thin debris mantle; seasonal frost heave can induce deeper movement (Matsuoka 2005).
Although frost creep is a clearly distinguishable process, its measurement and isol-
ation are difficult because other processes are also involved, particularly when the soil