Origins of Mountains - Mountain Geography: Physical and Human Dimensions - page 56

FIGURE 2.24 Schematic representation of various types of folded rock structures. Such features can
occur in a neotectonic, uneroded fashion, as in this idealized view, where internal rock structure
and external landscape form or geomorphology are coincident or congruent. More commonly,
such rock structures are eroded into and, because the rocks in the axes of the anticlines are under
more tension than the commonly adjacent synclines, a breached anticline results. (Adapted from
Putnam 1971: 392.)
FOLDED MOUNTAINS
Folding compresses rock strata into wave-like troughs and ridges without fracturing
the rock (Fig. 2.24). This plastic deformation takes place slowly over millions of years.
Where the deforming forces are mildly compressive, the amplitude of the spacing of the
downwarped synclines and upwarped anticlines resembles a gently rippled sea, but un-
der extreme pressures the rock strata can take on the appearance of a tempest frozen
at the point of greatest fury, as in most complex mountains where the strata have been
deformed, overturned, and wrapped into the configuration of a ribbon candy. Because
folded mountains commonly stretch rocks under tension in the axes of anticlines and
compress axes of intervening synclines, the uparched anticlines are subject to greater
erosion and synclines are protected. The result can be topographic inversion, such that
rock structure and topography are incongruent and instead, anticlinal valleys and syn-
clinal ridges result (Fig. 2.25).
The major orogenic belts are folded mountains, consisting of thick accumulations of
marine sediments (geoclines) that have been altered and deformed (Fig. 2.26). This is
thought to result from large-scale compressive forces shortening sections of the Earth's
crust so that the strata become thickened and pile up as mountainous accumulations of
deformed rock. Similar effects, however, can result from local processes; if an area is
domed upward, for example, the beds on either side can gravity slide downslope under
their own weight (Fig. 2.27). Plentiful evidence supports both processes; strongly meta-
morphosed rocks indicate that folding took place at considerable depth within the Earth
under great heat and pressure; rocks with little or no metamorphism suggest that fold-
ing occurred at the surface under normal atmospheric temperatures, and may be due
to gravity sliding.