Wave like bends in the crustal rock due to tangential compressive force resulting from horizontal movement caused by the endogenetic force originating deep within the earth, such bends are called Fold. Folding or bending, is the most common form of deformation of layered, sedimentary rocks and may occur at any scale, from crumpling of thin beds into folds only a few centimetres long and high, to huge waves in the crust many kilometres long and hundreds of metres high. folds may result from compression in the crust, from uplift of a block beneath a sedimentary rock cover, so that the cover becomes draped over the rising block or from gravitational sliding and folding where layered rocks slide down the flanks of a rising block and crumple.
Folds are two parts like that the up folded rock strata in arc – like form are called ‘Anticlines’while the down folded rock strata is called as ‘Syncline’. The two sides of a fold are called ‘Limbs’ of the fold. The limb which is laying between an anticline and its companion syncline is called ‘MiddleLimb’. The plane which bisects the angle between the two limbs of the anticline or middle limb of the syncline is called the ‘Axis of fold’.
Read about types of Folds,