Geography Reference
In-Depth Information
are problematic because they do not fit nicely into the category of either plain or moun-
tain.
Mountains are usually envisaged as being both elevated and dissected landscapes.
The land surface is predominantly inclined, and the slopes are steeper than those in low-
lands. Although this is true as a generalization, the actual amount of steeply dissected
land may be rather limited. Much depends upon geological structure and landscape his-
tory. In mountains such as the Alps or Himalayas, steep and serrated landforms are the
dominant features; in other regions, these features may be more confined. The southern
and middle Rocky Mountains display extensive broad and gentle summit uplands, and
similar conditions exist in the Oregon Cascades. It is the young Pleistocene volcanoes
sticking above the upland surface that give distinctiveness to the Cascades. The Sierra
Nevada of California contains many strongly glaciated and spectacular features, but
there are also large upland areas of only moderate relief. Yosemite Valley is carved into
this undulating surface, and most of the impressive relief in this region derives from the
occurrence of deep valleys rather than from the ruggedness of the upland topography.
The world of mountains is basically one of verticality: Although slope angles of 10 to
30 degrees are characteristic, it is the intermittent cliffs, precipices, and ridges that
give the impression of great steepness. Nevertheless, the horizontal distances between
ridges and valleys, which establish the texture and framework for slope steepness, are
just as fundamental to the delineation of mountains as the vertical distances that estab-
lish the relief.
Mountains may be delimited by geologic criteria—in particular, faulted or folded
strata, metamorphosed rocks, and granitic batholiths (Hunt 1958; Oilier and Pain 2000).
Most of the major mountain chains have these features, and they are also important in
identifying former mountains. Good examples are found along the south shore of Lake
Superior in Michigan and throughout much of southeastern Canada, where all of these
characteristics are present, but erosion has long since removed the ancient peaks that
once were mountains. Implicit in this definition is the idea that mountains are features
of construction, built and produced by some internal force. This is certainly true of the
major ranges, but mountainous terrain may also result from destructive processes, i.e.,
erosion. For example, a strongly dissected plateau may take on a mountainous charac-
ter even though it contains none of the listed geologic characteristics. Certain areas of
the southwestern United States do in fact display such dissection. Curiously, these land-
scapes are often perceived very differently from those of constructional origin. They are
viewed as ruins, pathetic features, rather than as initial expressions of grand nature.
They evoke “the sentiment of melancholy” (Tuan 1964).
Another basis for defining mountains is their climatic and vegetational characterist-
ics. An essential difference between hills and mountains is that mountains have signific-
antly different climates at successive levels (Barry 2008). This climatic variation is usu-
ally reflected in the vegetation, giving mountains a vertical change in plant communit-
ies, or biodimatic belts, from bottom to top that hills lack (Jeník 1997; Körner 2003;
Körner et al. 2011). It is argued that 600 m (2,000 ft) of local relief in most parts of the
world suffices to bring about a distinct vegetation change. This is not always evident,
because some plants, such as sagebrush (Artemesia spp.) in the western United States
or heather (Calluna vulgaris) in Scotland, have great altitudi-nal range and may cover
entire mountains. However, even if the vegetation is homogeneous, there are measur-
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