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Deformation features that we examine in rocks are a result of strain; therefore, we can understand the general nature of strains affecting rocks by studying these deformed features. Some features, such as folds, typically form as a result of shortening strainsOther features, such as boudinage structures tend to form as result of elongation strainsStill others, such as shear zones, form in areas of concentrated shear strain. In many cases the deformation features that we see in rocks are a result of different responses of interlayered rocks to strain.It is common for interlayered rock materials to have different competencies or ductilities.Competent layers tend to undergo an "active" response to imposed strains while adjacent incompetent layers tend to behave "passively" and conform to deformation in the competent layer. Examples are less competent layers taking on a geometry close to that of the competent layers and less ductile matrix flowing in the boudin gaps or pressure shadow zones of porphyroblasts. During progressive strain or deformation, the rocks may rotate in response to shear strains. The strain ellipse may rotate because of a rotational or shear strain component. Because of this rotation, material lines (e.g. a competent rock layer) may see a complex deformation history ranging from shortening to elongation during the progress of deformation. Where a leesow competent layer makes a large angle with the direction of shortening, the result is thinning and extension of that layer. However, in an interlayered sequence of competent and less competent materials, the ductility contrast between layers results in different rates of strain in the two lithologies involved. This causes viscous instability in the competent layer.