6 Folds

Definition and origin

• A fold is a bending or wrinkling of the crust's rocks, usually due to pressure (compression) forces.
• It appears clearly in sedimentary rocks (layers vary in thickness and extension).
• It may be a single simple fold or a set of connected folds.
• Folds rarely keep their original shape; refolding makes them complex, with fractures and cracks.

Importance of folds

• They form traps and reservoirs for crude oil, groundwater and mineral ore deposits.
• They help to find the relative age of rocks (older or younger).
• They are evidence of tectonic activity and rock deformation.
• They matter in the design and construction of engineering projects.

Structural elements of a fold

• Axial plane: the imaginary plane that divides the fold into two equal, identical parts.
• Limbs: the rock blocks on the two sides of the axial plane.
• Fold axis: the imaginary line where the axial plane intersects any layer surface.

Types of folds

Point of comparison	Anticline	Syncline
Shape of the layers	Concave upward (arch-like)	Concave downward (trough-like)
Oldest layers	In the centre	On the outer sides
Youngest layers	On the outer sides	In the centre