FOLD DEFINITIONS
ASYMMETRIC FOLDS: Folds in which the limbs are of unequal length.
AXIAL SURFACE: the surface joining the hinge lines of a set of nested
folds; may also be called the axial plane if planar.
AXIAL TRACE: the line representing the intersection of the axial surface
and another surface.
CREST LINE: line on the surface of the fold at which the dip changes
direction with respect to the horizontal
CROSS SECTION: a plane perpendicular to horizontal.
CYLINDRICAL FOLD: a fold generated by a straight line moving parallel
to itself in space.
ENVELOPING SURFACE: planes drawn tangent to the crest and trough
lines.
FOLD AXIS: the straight line that generates a cylindrical fold. Unlike the
hinge line, the fold axis is not a specific line but rather a
hypothetical line defined by its attitude. Only cylindrical folds,
or cylindrical segments of folds, have fold axes.
FOLD LIMB: area of folded surface between hinge zones.
HINGE LINE: line of maximum curvature (smallest radius of curvature) on
a folded surface
INFLECTION POINT: the point on a fold where the rate of change of slope
is zero; usually chosen as the midpoint of each straight section
of the fold limb
INTERLIMB ANGLE: the angle between adjacent fold limbs.
LINE of INFLECTION: locus of inflection points of a folded surface
MEDIAN SURFACE: surface that joins the successive lines of inflection of a
folded surface.
PROFILE PLANE: a plane perpendicular to the fold axis.
SYMMETRIC FOLDS: Folds in which the median surface is planar, the
axial plane is perpendicular to the median surface, and the
folds are bilateral symmetrical about their axial planes.
TROUGH LINE: same as crest line except that the dip directions point
toward one another.
FOLD TERMS
Fold description based
on interlimb angle
Interlimb Angle Fold Type
Parasitic asymmetric
folds
asymmetric “Z”
folds dextral
axial
surface
trace
asymmetric “S”
folds
sinistral
enveloping
surface
TYPES OF FOLDS
ANTICLINE: a fold with older rocks in its core.
ANTIFORM: a fold that closes upward.
ANTIFORMAL SYNCLINE: an antiform with with younger rocks in the core.
OVERTURNED FOLD: a fold in which one limb has been tilted more than
90°, resulting in both limbs dipping in the same direction.
RECLINED FOLD: a fold whose axial surface is inclined and whose hinge
plunges down the dip of the axial surface.
RECUMBENT FOLD: a fold whose axial surface is horizontal.
SYNCLINE: a fold with younger rocks in its core.
SYNFORM: a fold that closes downward.
SYNFORMAL ANTICLINE: a synform with oldest rocks in the core.
TIGHTNESS: fold classification based on interlimb angle.
VERGENCE: the direction towards which the fold is turned (clockwise =
dextral or "z" folds, counterclockwise = sinistral, "s" folds)
viewed down plunge.
VERTICAL FOLD: a fold whose hinge line is vertical.
Fold description
based on orientation
of axial plane and
hinge line
> 120°
120 to > 30°
30 to > 10°
10 to 0°
< 0°
gentle
open
tight
isoclinal
elasticas