8.4: Folds
- Page ID
- 28267
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Geologic folds are layers of rock that are curved or bent by ductile deformation. Folds are most commonly formed by compressional forces at depth, where hotter temperatures and higher pressures allow ductile deformation to occur.
Folds are described by the orientation of their axes, axial planes, and limbs. The plane that splits the fold into two halves is known as the axial plane. The fold axis is the line along which the bending occurs and is where the axial plane intersects the folded strata. The hinge line follows the line of greatest bend in a fold. The two sides of the fold are the fold limbs.
Symmetrical folds have a vertical axial plane and limbs have equal but opposite dips. Asymmetrical folds have dipping, non-vertical axial planes, where the limbs dip at different angles. When the axis of the fold plunges into the ground, the fold is called a plunging fold. Folds are classified into several categories. The two most common are anticlines and synclines.
Anticline
Anticlines are arch-like, or A-shaped folds that are convex-upward in shape. They have downward curving limbs and beds that dip down and away from the central fold axis. In anticlines, the oldest rock strata are in the center of the fold, along the axis, and the younger beds are on the outside. Since geologic maps show the intersection of surface topography with underlying geologic structures, an anticline on a geologic map can be identified by both the attitude of the strata forming the fold and the older age of the rocks inside the fold.
Syncline
Figure \(\PageIndex{1}\): Syncline in Rainbow Basin, Mojave Desert, California.
Synclines are trough-like, or U shaped, folds that are concave-downward in shape. They have beds that dip down and in toward the central fold axis. In synclines, older rock is on the outside of the fold and the youngest rock is inside of the fold axis.