Fold is a structural form seen in the rocks of Earth’s crust. Stratified rock originally lay flat, but folding caused the rock to bend or curve, and the folds remain visible today as undulations on Earth’s surface, often covering kilometres in size. Folds have several important characteristics that distinguish them from each other: their hinge points, axial planes, and fold axes.
What is Fold and Example?
The axial surface of a folded rock is a plane that connects the inflection lines of the stacked layers of a rock. The axial surface can be either planar or curved Wash and fold, and it is defined by the angles (the vergence) of the fold limbs with the original unfolded surface. The angles are described as strike and dip. The limbs of a fold can be long or short, and when they are the same length they are called symmetrical.
When they are different they are called asymmetrical. Folds whose limbs are long-short-long have S symmetry; those with a sequence of long-short-long limbs have Z symmetry. Those with equal length limbs are called isoclinal folds.
A fold may also be described as either antiform or synform. Anticline folds have layers that are convex upward, and the oldest layer is found in the fold core. Syncline folds have layers that are concave downward, and the youngest layer is found in the fold core.
Those that are neither antiform nor synform are called neutral folds. Folds can be described as tight, open, or loose. The more tightly folded a rock is, the smaller its hinge lines are, and the more angular it is; for example, a chevron fold has an inter-limb angle of around 60°.
