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The neutral surface can be incremental or infinitesimal when established soon after the inception of folding process and may change its orientation as the folding Progresses. Thus through a series of surfaces of no incremental longitudinal surfaces

F the neutral or finite neutral surface or surface of no finite longitudinal strain is formed. Along the neutral surface the original length of the layer prior to folding is maintained. Most of the folds formed by tangential longitudinal strain depict either subclass 1A or 1B geometry. Normally tangential longitudinal strain does not operate alone but most buckle folds are formed by a combination of this process and flexural or interlayer slip. Since the outer arcs suffer extension, tensile cracks are commonly formed at the outer arcs (see Fig). Since the inner arcs are compressed, thrusts are formed in the initial stage of buckling which give rise to high angle reverse faults in advanced stages of folding.

Normally it is found that in naturally developed folds the neutral surface shifts towards the inner are boundary. This implies that progressive fold development is accompanied by greater addition of strain at the outer than at the inner arc. Needless to say, if the position of neutral surface in the natural fold can be exactly located, the original length before deformation can be easily computed. But this requires detailed strain analysis through the buckled layer.

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.