in opposite directions and lengthens a section of crust. The third type of stress is
compressional stress that shortens a section of crust. In reacting to stress, rocks
might undergo elastic deformation, which is reversible if the stress is removed.
However, with the long-lasting stresses experienced within the crust the rocks
commonly undergo plastic deformation in which they bend or crack and are so
changed permanently. It is the interplay between rock type and amount and direc-
tion of the stresses that governs the result and the stress is applied slowly enough
not to break the rock.
Several different types of folds have been distinguished. Sedimentary rock is
always laid down horizontally, and it is crustal stress that brings it out of horizontal
and this angular displacement is easily observed in nature. The stresses folding
rocks are compressional. Individual folds can vary in scale from tens of kilometers
down to a very few centimeters.
A simple, common fold connecting horizontal layers is a monocline. A second
type of fold is a syncline. It is a concave compressional downfold having symmet-
ric limbs extending upward from the center. Sedimentary rock layers are progres-
sively younger with distance through the limbs toward the surface. An anticline is
a third type of fold. It is a convex compressional upfold of rock with a horizontal
geometry similar to an arch. Older rocks are found with increasing depth through
the limbs and anticlines served as traps for enormous reservoirs of oil and natural
gas. A fourth kind of fold is an overturned fold, where the axis has been pushed
past horizontal because of greater compression stress in one horizontal direction
than the other. A fifth kind of fold is a recumbent fold, where the forces have
pushed the center of the fold from vertical to horizontal. A fifth type of fold is an
overthrust fold, which is much like a recumbent fold except the extreme folding
has caused the rocks to fault and one side rides up over the other side, sometimes
for long distances. This produces the situation where one could drill a well and go
through a sequence of rock, pass through the fault, and penetrate the same
sequence of rock further down. Notable overthrusts are located in Glacier National
Park USA (80 km of overthrust), the Glarus region of eastern Switzerland (35 km),
and in the Himalayas (100 km).
There are myriad examples of major folding around the world. The ridges and
valleys of the Allegheny Mountains in the eastern United States have hundreds
of thousands of square kilometers of landscape created by folding. Other famous
examples occur in the Andes, Himalayas, and Urals. Far from having simple, sym-
metrical folds, many areas have incredible complexity with several types of folds
and faults found in association with each other.
When the stresses are greater than the internal strength of the rock, the rock
fractures and these fractures are knownas faults. A fault is the displacement of
rocks that were connected to one another. The displacement is along a fault plane
Folding and Faulting 123