Slaty Cleavage

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The origin of cleavage

Incompetent sedimentary rocks such as mudstone contain clay minerals. When these rocks are heated and compressed, the minerals begin to recrystallise. They usually recrystallise as micas or mica like minerals with a distincitve platy form. Recrystallisation causes a change in the rock so it is no longer a mudstone but altered by low grade metamorphism to form a slate.

These platy minerals are recrystallising and growing in an environment where there is a strong stress field, so they will tend to grow in the direction of the least resistance. This is at right angles to the stress field which means that all of these platy minerals form roughly parallel to each other. They are also parallel to the axial plane of the folds, which result from the applied stress.

On the normal limb of the fold, the bedding dips at a lower angle than the cleavage. On the inverted limb, the opposite is true, that is the dip of the bedding is steeper than the dip of the cleavage.

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