Origins of Mountains - Mountain Geography: Physical and Human Dimensions - page 62

had difficult rationalizing the great horizontal displacements involved. A study in 1841
near Glarus, Switzerland, revealed that for a distance of over 50 km (∼30 mi) the moun-
tain peaks are composed of rocks older than the valleys. The original explanation for
this anomalous situation was that the older rock had been transported to its present loc-
ation by two recumbent folds coming from opposite directions (Fig. 2.31a). Even this re-
latively cautious speculation was so revolutionary at the time that its author would not
allow publication of the work: “No one would believe me if I published my sections; they
would put me in an asylum” (Vanderlinth 1841, cited in Holmes 1965: 1162). In 1884,
one gigantic overthrust was postulated for the entire region (Fig. 2.31b). This eventu-
ally became the mostly widely accepted view, although some controversy has occurred.
Dome Mountains
Dome mountains can be simple igneous intrusives such as laccoliths, which are
mushroom-shaped plutonic bodies that dome up overlying sedimentary rocks. The
Henry and La Sal Mountains of Utah (Hunt 1974) are a series of simple to complex lac-
coliths and irregularly shaped igneous stocks that have uparched surrounding strata.
Complex domal uplifts, such as the Black Hills of South Dakota or the Ozarks of Mis-
souri, involve a wide variety of tectonically uplifted igneous and metamorphic rock with
overlying arches of sedimentary rock (Thornbury 1965; Hunt 1974). Purely sedimentary
rock domes of small to large size can result from various compressive or uplifting rock
stresses. Salt-dome mountains form because rock salt is highly mobile when loaded with
other sedimentary rock; it bulges upward, in some cases piercing the rock overhead and
flowing slowly out of the ground. But because salt is so soluble in water, only desert
areas, such as the Zagros Mountains in southern Iran, retain surficial salt mountains.
Topographic Inversion
Rock structure deep underground and landforms on the surface are generally mutually
independent. Close to the surface, rock structure may or may not be reflected in the
landform. Anticlinal mountain and normal fault scarps are examples of the convergence
of rock structure with topography. Topographic inversion is an example of differential
erosion, whereby the attitude of the rock structure and the topography have opposite
character, that is, an anticlinal valley or an obsequent fault-line scarp. Proper interpret-
ation of the correct sequence of geologic events that led to the existing structure and
topography of the mountain system in question is essential.
Conclusion
Geologic interpretation of mountain structure requires not only careful study and map-
ping of rock strata, but also an ability to visualize the larger picture with enough ima-
gination. The importance of these qualities can be seen in the progression of ideas con-
cerning the origin of mountains; the theory of plate tectonics is a fantastic and spectac-
ular culmination of human ingenuity. Although much remains to be learned, especially
in the most complex continental areas, it has provided a logical framework into which