Origins of Mountains - Mountain Geography: Physical and Human Dimensions - page 24

FIGURE 2.4 A simplified interpretation of the sequence of events that resulted in the active-margin
crumpling of a former geoclinal or passive-margin couplet into the Appalachian Mountains. These
rocks were deposited as sediments in Paleozoic time (540-225 million years ago). (A) The sand-
stones and limestones of the miogeocline, or landward continental shelf, were folded into a series
of ridges between the Blue Ridge and the Allegheny Front. (B) The turbidites or flysch of the
deeper-water eugeocline were altered by great heat and pressure associated with metamorphic
and igneous activity as the passive margin became unstable and activated into a subduction zone.
(C) Sediments eroded from the newly rising mountains were deposited as continental or terrestri-
al molasses deposits. The orogeny also produced extensive crystalline rocks that were elevated
in a large mountain range whose crest lay to the east of the Blue Ridge, (d) Erosion continued to
take its toll; all that remains are the present-day Appalachians, the deeply eroded roots of a range
originally much like the Himalaya. (Adapted from Dietz 1972: 36.)
Along the eastern seaboard of the United States, for example, a wedge of sediments
some 250 km (155 mi) wide and 2,000 km (1,245 mi) long comprises the continental
shelf and slope (Dietz and Holden 1966). This is a “living” miogeocline on a mature
passive margin. The continental rise, which ascends to the continental slope from the
abyssal plains of the oceanic deep, is thought to be a “living” eugeocline (Fig. 2.4a).
The sediments on the continental shelf thicken seaward, attaining thicknesses of 3-5 km
(1.8-3 mi), and consisting of heterogeneous deposits resulting from muddy suspensions
of turbidity currents. These flow down the continental rise and deposit at its base great
fans of turbidites having thicknesses of up to 10 km (6 mi). The recent sediments on the
continental shelf and rise closely resemble ancient rocks occurring in the Appalachian
fold belts (Fig. 2.4d). It is reasoned, therefore, that these deposits are the stuff of which
mountains are made, and that similar examples of living geoclines exist of the coasts
of the other major continents today; research continues on mechanisms causing their
eventual deformation into mountains (Strahler 1998).
Theories of Mountain Origin