Folds are expressed by the warping of a reference surface. They belong to the category of continuous heterogeneous deformations and are in fact their most spectacular example. The reference to a surface is important in two respects: it is thanks to the deformation of reference surfaces that we can observe folding in sedimentary and metamorphic rocks, and secondly, the bedded or stratified nature of these rocks favours the generation of the instabilities that produce the majority of folds. It is important to be aware that folding can affect other rock types. Magmatic flow and plastic deformation of homogeneous formations can also be accompanied by folding, which is clearly visible when the medium possesses a definite layering (fig.7.2 or 7.4b), but more difficult to identify when it only has a lamination or foliation plane (fig.7.3a).
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