Geology Reference
In-Depth Information
Figure 6.2 Some typical fold styles. A. Folds with variable profile shape in interlayered slates and
sandstones; the sandstone beds show near-parallel geometry, whereas the slates show chevron
geometry. B. Buckle folded sandstone beds with near-parallel geometry; note the extension fractures
around the hinges. C. chevron folding in thin sandstone beds with slate interlayers; the cliff face is
about 20 m in height. D. Close-up of chevron folding in thinly layered slates; note the straight limbs
and sharp hinges; 50p coin for scale. E. Similar folding; note how the profile shape is maintained
through many layers. F. Disharmonic folding: the wavelength of the upper parallel-folded thick layers
is much longer than that of the thinly-layered material below.
A
6
46
47
E
F
B
Fold orientation
The orientation of a fold is usually
recorded by the attitude of its hinge
and axial plane. The axial plane
(Figure 6.3B,C) is an imaginary plane
at equal distance from each limb, and
can be described by its trend ( strike )
and inclination ( dip ). The line along
which the axial plane intersects the
fold surface is known as the fold axis ,
and is a more geometrically precise
measure than the hinge (Figure 6.3C).
A more useful method for describ-
ing folds where the axial planes of the
C
hinge
limbs
fold
angle
anticline
A
syncline
a
b
axial
plane
B
axial
plane
fold
axis
amplitude x2
D
wavelength
C
D
Figure 6.3 Fold description. A. folds consist of two limbs separated by a hinge. B, C , axial plane, fold
axis and fold angle; the axial plane is equidistant from the limbs (a = b). D. wavelength (width) and
amplitude; note that the amplitude = half the height.
 
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