Relationship To Folds
Cleavages display a measurable geometric relationship with the axial plane of folds developed during deformation and is referred to as axial planar foliations. The foliations are symmetrically arranged with respect to the axial plan depending on the composition and competency of a rock. For example, when mixed sandstone and mudstone sequences are folded during very-low to low grade metamorphism, cleavage forms parallel to the fold axial plane, particularly in the clay-rich parts of the sequence. In folded alternations of sandstone and mudstone the cleavage has a fan-like arrangement, divergent in the mudstone layers and convergent in the sandstones. This is thought to be because the folding is controlled by buckling of the stronger sandstone beds with the weaker mudstones deforming to fill the intervening gaps. The result is a feature referred to as foliation fanning.
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