Chapter 1

marble. These changes may be minimal, only slightly altering the sediments, or extensive enough to
produce gneiss, which is almost indistinguishable from igneous rock.

Metamorphic rocks may have not only joints and bedding, but cleavage or foliation, a series of
thinly spaced cracks caused by the pressures of folding. Because of this cleavage, lower grades
of metamorphic rocks may be completely unsuitable for climbing because the rock is too rotten
for safe movement.

Higher degrees of metamorphism or metamorphism of the right rocks provide a solid climbing
surface. The Shawangunks of New York are an excellent example of high-grade conglomerate
quartzite, which offers world class climbing. The center of the Green Mountain anticline
contains heavily metamorphosed schist, which also provides solid climbing.

MOUNTAIN BUILDING
1-20. The two primary mechanisms for mountain-building are volcanic and tectonic activity. Volcanoes
are constructed from lava and ash, which begin within the earth as magma. Tectonic activity causes plates
to collide, heaving up fold mountains, and to pull apart and crack, forming fault-block mountain ranges.

PLATE TECTONICS
1-21. The massive slabs composing the outer layer are called tectonic plates. These plates are made up of
portions of lighter, granitic continental crust, and heavier, basaltic oceanic crust attached to slabs of the
rigid upper mantle. Floating slowly over the more malleable asthenosphere, their movement relative to
each other creates earthquakes, volcanoes, ocean trenches, and mountain ridge systems.

MOUNTAIN STRUCTURE
1-22. The different horizontal and vertical stresses that create mountains usually produce complex patterns.
Each type of stress produces a typical structure, and most mountains can be described in terms of these
structures.

Dome Mountains
1-23. A simple upward bulge of the crust forms dome mountains such as the Ozarks of Arkansas and
Missouri, New York’s Adirondacks, the Olympics of Washington, and the High Uintahs of Utah. They are
usually the result of the upward movement of magma and the folding of the rock layers overhead. Erosion
may strip away the overlying layers, exposing the central igneous core.

Fault-Block Mountains
1-24. Faulting, or cracking of the crust into large chunks, often accompanies upwarp, which results in
fault-block mountains. Many forms are created by the motion of these chunks along these faults.

The ranges of the desert country of California, Nevada, and Utah provide the clearest display of
faulting. The breakage extends to the surface and often during earthquakes―caused by slippage
between the blocks―fresh scarps many feet high develop.

Sometimes a block is faulted on both sides and rises or falls as a unit. More often, however, it is
faulted on one side only. The Tetons of Wyoming and the Sierra Nevada display this―along the
single zone of faults the range throws up impressive steep scarps, while on the other side the
block bends but does not break, leaving a gentler slope from the base of the range to the crest.
An example of a dropped block is California’s Death Valley, which is below sea level and could
not have been carved by erosion.

Fold Mountains
1-25. Tectonic forces, in which continental plates collide or ride over each other, have given rise to the
most common mountain form―fold mountains. Geologists call folds geosynclines. Upward folded strata
are anticlines and downward folds are synclines. When erosion strips down the overburden of rock from
folded mountain ranges, the oldest, central core is all that remains. The Alps and the Appalachians are

1-4

TC 3-97.61

26 July 2012