The mechanisms of folding that can operate within a sequence of layered rocks are dependent on the relative ductilities and on the nature of inherent anisotropy in the rocks. Flexural mechanisms require the presence of mechanical anisotropy, and folding is effected by slip between layers, by flow within layers, or by a combination of the two. Flexural folding represents a true bending of layers. Passive mechanisms operate only in rock sequences that are mechanically isotropic. This condition may exist either because of the absence of effective planar anisotropy or because of the ineffectiveness of anisotropy resulting from high ductility. The geometry of flow in passive folding reflects only the stress field and velocity gradients existing during deformation. Flow occurs cross the layering, and the layering serves only to record the relative displacements. A gradational mechanism, by which certain layers are flexed in response to passive behavior in the associated rocks, causes quasi-flexural folding. This mechanism operates primarily in layered sequences characterized by high-ductility contrast, and produces strongly disharmonic folding.

Because changing environmental conditions can alter both the ductilities of the rocks involved and the effectiveness of planar anisotropy, the mechanism that initiates folding may be superceded by one or more other mechanisms during the course of deformation. The effect of layering in the folding process decreases with increasing ductility, and every gradation exists between ideally flexural and ideally passive folding.

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