The Description of Folds
Explore the process of folding and
its ability to reveal the tectonic history
of a landscape and the physical
conditions experienced by a rock
body. Use folds as a tool to indicate
ductile deformation at depth and the
location of oil, gas, and ore deposits.
Classifying folds by their mechanism
of formation and geometry helps
people to interpret fold origin.
10.1 Geometric Parts of Folds
Inflection Line: a point at which sense of
curvature changes
(i.e.
from
convex up to
convex down)
Fold train: A series of folds having
alternating senses of curvature
Antiforms: Folds that are convex upward
Synforms: Folds that are concave upward
Fold system: A set of regional folds with
similar geometry presumed to be of
common origin
Curvature: A measure of the change of
orientation per unit distance along a
surface
Hinge: The line in the folded surface
along which the curvature is the greatest
Hinge Zone: most highly curved portion
of a fold near the hinge line.
Limbs/flanks: the regions with the lowest
curvature (includes inflection lines)
Crest line: Line of highest elevation on a
folded surface
Trough line: Line of lowest elevation on a
folded surface
Fold Profile: The trace of the folded
surface seen when it is viewed parallel to
its hinge.
Fold axis: the closest approximation to a
straight line that when moved parallel to
itself, generates the form of the fold
Anticline: Fold in which the older layers
on the concave side of a bedding surface
and the younger layers are on the convex
side
Syncline: Fold in which the younger
layers are on the concave side of the
bedding surface and the older layers are
on the convex side
Culminations: where crest lines go
through maximum elevation
Depression: where trough lines go
through minimum elevation
Cylindrical fold: fold axis can be moved
along the folded surface without losing
contact with it at any point. All the poles
to its bedding should lie on a great circle.
Conical fold: whose surface is everywhere
at a constant nonzero angle to a line of
fixed orientation. All the poles to its
bedding should lie on a small circle
Axial surface trace: the intersection of the
axial surface with a surface of exposure is
a linear feature called the axial surface
trace
Hinge surface/axial surface/axial plan: the
surface joining all hinge lines in a
particular nested set of folds
10.2 Fold Scale and Attitude
Enveloping Surfaces: The two surfaces
that bound the fold train developed in a
single folded surface
Median Surface: The surface that includes
all inflection lines of a fold train in a
single surface (See Twiss & Moore figure
10.11)
Amplitude: the distance from the median
surface to either of the enveloping
surfaces, measured parallel to the fold
axis
Wavelength: the distance measured
parallel to the median surface, between
one point on a fold and the geometrically
similar point on a neighboring fold
Upright: term used to describe a fold in
which the dip of the axial surface is close
to vertical
Recumbent: term used to describe a fold
in which the dip of the axial surface is
close to horizontal
Reclined fold: A fold whose hinge
plunges down the dip of the axial surface
Overturned fold: An inclined or
recumbent fold in which one limb is
rotated 90 degrees from its original
horizontal position
Monocline: a fold pair that has two long
horizontal limbs connected by a relatively
short inclined limb
Structural terrace: fold pair with two long
planar inclined limbs connected by a
relatively short horizontal limb.
10.3 The Elements of Fold Style
The style of a folded surface is defined by
certain features.
Folding angle: the angle between the
normal to the folded surface constructed
at the two inflection points of a fold
Interlimb angle: angle between the
tangents to the two fold limbs constructed
at the inflection point.
Cylindricity: The degree to which a fold
approximates the geometry of a
cylindrical fold. This is determined with a
stereographic projection.
Symmetry: Whether or not a folded
surface creates a mirror-image on both
sides of the fold hinge
Aspect Ratio (P): The ratio of the
amplitude (measured along the axial
surface) to the distance (M) measured
between adjacent inflection points that
bound the fold
Tightness: Feature defined by the folding
angle or the interlimb angle. Folding
angle increases and interlimb angle
decreases with increased degree of
folding.
Bluntness: The radius of the curvature of
the fold at its closure relative to a
reference circle tangent to the limbs
tangent to the limbs of the fold at
inflection points
EPS116 Chapter Summary
1
Alyse Briody 2011
Gia Patafio and Finn Chen 2013
b = rc / r0; for rc ≤ r0
b = 2 - (r0/rc ); for rc ≥ r0
rc= radius of curvature
r0= radius of reference circle
Ramsay’s Classification to determine the
style of a folded layer is defined by three
geometric parameters.
Dip Isogon: the line across the layer
connecting two points of equal dip on
opposite surfaces of the layer
Orthogonal Thickness: the perpendicular
distance between the two parallel tangents
Axial Trace Thickness: the distance
between the two tangents measured
parallel to the axial surface trace
10.4 The Order of Folds
Fold order: Characterizes scales of related
folds, with the largest-scale folds being
first-order, and the smaller-scale (also
known as parasitic) folds being higherorder numbers
Pumpelly’s Rule: the style and attitude of
higher-order folds are generally similar to
that of lower-order folds
10.5 Common Styles and Structural
Associations of Folding
Chevron/Kink
Folds:
cylindrical,
harmonic folds with sharp hinges.
Chevrons are symmetrical and kinks are
asymmetrical
Ptygmatic folds: disharmonic folds that
typically develop in individual layers of
sedimentary or metamorphic rock
References & Resources
Robert J. Twiss, Eldridge M. Moores,
Structural Geology 2nd edition, (W. H.
Freeman), p. 273-295, 2006
Ramsay’s classification of folded layers:
Harmonic fold: A fold that is continuous
along its axial trace for many multiples of
the half-wavelength
EPS116 Chapter Summary
2
Alyse Briody 2011
Gia Patafio and Finn Chen 2013