Origins of Mountains - Mountain Geography: Physical and Human Dimensions - page 59

(Thornbury 1965). Chief Mountain, mentioned above as an example of an overthrust
remnant, may actually have been formed by gravity sliding. Another of the most famous
examples of displacement from gravity sliding is the Rock of Gibraltar in the Mediter-
ranean; it consists of older limestones and shales overlying younger sedimentary rocks.
This huge monolith was carved from a larger mass that moved laterally from the east to
its present location (Garner 1974: 194).
FIGURE 2.27 Graphic representation of gravity sliding. Rock beds slide slowly downslope and deform
under their own weight to produce local folding and faulting.
At its simplest, in folded rocks, a number of common mountain shapes can form in
high mountains of sedimentary rocks (Cruden 2003). Castellate and Matterhorn-type
mountains occur in subhorizontal to gently inclined beds, cuestas form in gently dip-
ping beds up to about 10° dip, monoclinal ridges form between ∼10° and 45° of dip,
and hogback ranges form in steeply dipping beds >45°. Cataclinal slopes on mountains
in sedimentary rocks have the dip in the same direction as the slope, whereas anacli-nal
slopes have a dip in the opposite direction.
The most spectacular feature associated with folded mountains is the nappe fold
(from the French for “tablecloth”; Fig. 2.30). Nappes are enormous slab-like masses of
rock, bulbous protrusions that have been extruded into and over other rock strata for
considerable distances, in some cases up to 100 km (∼60 mi). Closely associated with
overthrust faults, or breakthrusts where the nappe's lower overturned strata fracture
and thrust forward, nappes are generally attributed to overturned and recumbent fold-
ing, wherein the axis of maximum curvature of the anticline is strongly asymmetrical
and is commonly forced laterally over the adjacent strata for some distance. Nappes are
formed as a result of several possible mechanisms, including horizontal compression,
or gravity glide and gravitational collapse. In horizontal compression, nappes are em-
placed by a force applied to the rear of the nappe. This so-called “push from behind”
involves ductile shearing of a nappe over its base by a combination of simple shear and
shortening parallel to the basal fault. In gravity models, the nappe deforms under grav-
ity by gliding down a gently inclined base.
Nappes, which occur in most mountain ranges, are an integral part of orogenesis.
The entire superstructure of the European Alps is dominated by a series of overlapping
nappes that were displaced from their point of origin near present-day Italy as Africa
collided with Europe, causing the creation of these mountains from the sediments of
the ancient Tethys Seaway (Strahler 1998; De Graciansky et al. 2010). Millions of years
of erosion have removed many of the displaced rock sheets entirely, leaving only rem-
nants. The famous Swiss Matterhorn in the Alps is a remnant of one of these nappes. It
consists of older, more resistant rocks overlying younger rocks.