FINITE NEUTRAL POINT
If homogeneous layer shortening precedes the buckling by tangential longitudinal strain, then no neutral surface can exist in such a fold since the material was already strained by layer shortening prior to buckling. In these layer shortened strata, the effect of tangential longitudinal strain is to reduce the principal strain ratio at the outer arc and increase it at the inner arc. The internal deformation in this case is partly by slip and partly by tangential longitudinal strain. The strain at the outer arc being extensional, it slightly compensates for the earlier compressional effect of layer Shortening.
Cleavage fans are produced which are convergent in competent layers and divergent in less competent ones. Just outside the outer are, the strain in the matrix becomes zero at one point enveloped on all sides by traces of cleavage. This point is known as the finite neutral point (see fig). Thus the combination of homogeneous layer shortening followed by a folding mechanism which is a combination of flexural slip and tangential longitudinal strain gives rise to neutral points instead of neutral surfaces and alternate convergent and divergent cleavage fans. In a multilayer complex (see Fig.) of alternating competent and less competent layers, cleavage refraction is a normal phenomenon that is seen. Most folds of cuspate geometry originate by a combination of layer parallel strain and tangential longitudinal strain.