Mountain Landforms and Geomorphic Processes - Mountain Geography: Physical and Human Dimensions - page 224

years (Stock et al. 2009). These lethargic rates illustrate the slowness of modern de-
nudation in the Colorado Front Range (Caine 2001). Generally, as relief increases, per-
haps through rejuvenation, denudation rates increase. This illustrates the importance of
mountain systems as a source of sediment (Fig. 5.1) (Milliman and Syvitski 1992; Caine
2004).
The form, structure, and composition of mountains greatly affect the rate and type of
geomorphic processes. Horizontally oriented rock, volcanic features, as well as folded,
faulted, and domed structures with strata dipping in various directions, all present very
different surfaces and starting points for erosional processes. Because water, soil, snow,
and ice tend to follow the path of least resistance, the processes of erosion tend to rein-
force and exaggerate existing slopes and structures.
Oilier and Pain (2000) provide an excellent discussion of how mountains originate
(Fig. 5.2). The development of landscapes in different mountain regions depends on the
rate of uplift and deformation, the nature of various rock surfaces, and the intensity and
type of erosional processes. These combined circumstances result in a wide variety of
conditions and products. For example, the structural modification of rock units often
involves their compression in some areas and their stretching in others, producing al-
ternating resistant and susceptible zones. Thus, in folded rock, anticlines are areas of
extension and therefore are weak, whereas synclines are areas of compression and thus
have greater strength. As a result, valleys often develop in anticlines, where stretching
has made the strata more susceptible to breaching by erosion, and the elevated anti-
clines are worn away more rapidly than the synclines. This paradoxical situation results
from the more aggressive and prolonged erosion of topographic promontories as well
as the structural modifications associated with folding. The relationships are not the
same everywhere; in some places, streams occupy synclinal valleys and anticlines form
the ridges. Where folding is extremely rapid and the rock strata resistant, the structure
may not be modified as much by erosion, so that it is expressed directly in the form of
the land surface. Such features are the exception rather than the rule. Much depends
upon the rate of uplift. Short-term rates of uplift can be determined using high-resolu-
tion Global Positioning Satellite (GPS) measurements or satellite altimetry (Bishop et al.
2002). Long-term estimates can be derived by identifying corals, deltas, or other sea-
level features (at known elevations) or radiometric dating of minerals at known depths.
If an area has no uplift or deformation taking place, the major controls are the exist-
ing differences in bedrock, topography, and structure. However, if the area is tectonic-
ally active, a much more dynamic relationship exists between form and process because
erosion is taking place simultaneously with uplift and deformation. Under these condi-
tions, for example, streams may sometimes cut across the structural grain of the land
with indifference to the underlying structural conditions.