Mountain Landforms and Geomorphic Processes - Mountain Geography: Physical and Human Dimensions - page 277

valleys have been left without a stream, either because the stream could not keep up
with the rapid uplift or because water was diverted to some other part of the drainage
system. Such features, known as wind gaps, are also important as mountain passes. One
of these is the celebrated Cumberland Gap in the southern Appalachians, through which
early settlers passed on their way to the interior.
Eolian (Wind) Processes
Elevated, exposed peaks and ridges in middle latitudes are among the windiest of envir-
onments. The geomorphic effect of wind in mountains is still not completely understood,
however. The persistence of high winds on exposed slopes, with relatively little snow
and vegetation to bind the soil, suggest considerable potential for erosion. However,
the effects of wind must be placed in their proper perspective. For example, wind was
formerly believed to be the primary agent of erosion in many desert areas, but it is
now known that running water, even though infrequent, is the major factor in landscape
sculpture in most deserts. A similar condition exists in mountains. Wind becomes more
important with increasing elevation and exposure, but its role as a denudational agent
is still less important than the other processes we have discussed.
Wind Erosion
Wind erodes by picking up small particles, transporting them, and using them to create
a sandblast effect. The size of the particles that can be picked up depends on the wind
velocity and the shape and density of the particles. The greatest work is accomplished
during fierce gusts under turbulent conditions (Fig. 5.22). During high winds, sand
grains can be lifted up to 1-2 m and carried for much longer distances. Silt- and clay-
sized material can be lifted far into the atmosphere and carried for great distances.
Wind can also push and slide rocks across ice-glazed surfaces (Schumm 1956).
Several features result from wind erosion in mountains. Among the most distinctive
are ventifacts—stones that have been polished and faceted by the abrasion of windcar-
ried particles. Ventifacts commonly have one or more flat faces or facets divided by
sharp angles. On larger stones that remain relatively stationary for long periods, one
side may show considerable abrasion and cutting by the wind, providing evidence of
the prevailing wind direction (Rudberg 1968). Under certain windy conditions, snow,
which increases in hardness with lower temperatures, can abrade rock surfaces (Fris-
trup 1953).
Wind has been deemed responsible for the cavernously weathered, fluted, honey-
combed, and deeply pitted rock walls of some mountainous areas. Related features are
weathering pits, circular depressions occurring on more or less horizontal rock sur-
faces. These are common in high mountains; although they are principally formed by
chemical and physical weathering, wind removes the fine debris from the depressions
and exposes fresh surfaces.
Wind Deposition