Folds, descriptive
Goals: 1) To understand the basic geometric elements of folds and 2) to
classify folds based on their shape.
1) Anticlines and Synclines
a) Monocline: Fold pair with two long, horizontal limbs connected by a
short inclined limb
b) Antiforms: Convex upwards folds
 Anticline: Fold that is convex in the direction of the youngest beds
 Antiformal syncline: Antiform with oldest beds on top
b) Synforms: Convex downwards folds
 Syncline: Fold that is convex in the direction of the oldest beds
 Synformal anticline: Synform with oldest beds on top
2) Basic geometric elements of a fold
 Fold profile plane: Plane normal to folding axis
 Fold hinge: Line of maximum curvature
 Hinge zone: Zone of maximum curvature around hinge
 Fold limb: Flanks of folded layer
 Inflection point: Point where limb segments change convexity
between folds
 Axial plane:
 Axial trace:
 Axial-planar foliation: Fold axial surfaces commonly approximate the
maximum shortening directions.
3) Fold classification based on the orientation of hinge/axial surface
Display diagram in ppt. and draw on board.
 Axial surface: upright, steeply inclined, moderately inclined, gently
inclined, recumbent
 Hinge line: subhorizontal, gently plunging, moderately plunging,
steeply plunging, reclined
 Doubly plunging fold: Fold plunges in two directions
 Overturned fold: One limb rotated through vertical
 Show block diagram models of folds and have students classify.
Show pictures of folds in the field and have students classify (at least
hinge-line orientation)
4) Classification based on interlimb angle
 Define interlimb angle: Draw on board and write down
 Classifications: Gentle 180–170°, Open 170–90°, Tight 90–10°,
Isoclinal 10–0°
 Show pictures of folds in the field and have students classify. Outcrop
and regional scale. ASSUMES YOU ARE LOOKING AT PROFILE
PLANE!!!
 Pass around isoclinal, recumbent fold.
5) Classification based on shape
 Cylindrical folds: Introduce poles-to-plane. Poles to surface in
cylindrical fold form half circle
 Chevron or kink folds
 Cuspate folds
 Box folds
 Symmetrical vs. Asymmetrical folds
 Show pictures of folds in the field and have students classify
6) Classification based on layer thickness
 Parallel or concentric folds: Constant layer thickness — Because of
space issues in the hinge-zones, these folds must die-out.
 Similar folds: Each folded surface has the same shape — Can be
propagated ad infinitum
 Show pictures of folds in the field and have students classify
 Ramsay’s dip-isogon classification: Define dip isogons — Class-1A,
Class-1B (parallel), Class-1C, Class-2 (similar), and Class-3
7) Additional terms and ideas
 Parasitic folds: Small folds that occur systematically along the limbs
and in the hinge zone of larger folds
 Disharmonic folds: Die out within a couple of half wavelengths
 Mechanisms of maintaining strain compatibility in non-similar folds
8) Map symbols for folds
Go over map symbols for anticlines, synclines, over-turned anticlines
and synclines, and plunging folds
Notes
 Bring fold hand samples