Types of cleavage
Cleavage and folds Click here to see how to use cleavage to aid understanding of folds. Beware - you may find it hard to navigate back to this site.

Cleavage is a new fabric the develops in rocks during deformation. There are several ways in which cleavage can form. Probably the most famous type is "slatey cleavage" - so called because it is characteristic of slates. In slates the cleavage can come to dominate the rock so that you have to look really hard to find features like bedding. In these situations the cleavage is said to be "penetrative". Slatey cleavage results from the mechanical realignment or growth of platey minerals such as clays and micas, essentially so that they are flattened perpendicular to the direction of maximum compression. In folded sequence slatey cleavage is commonly found to be parallel to fold axial surfaces (this type of arrangement is termed "axial planar cleavage" and is useful for determining vergence. When rock layers with different mechanical properties are interlayered cleavage is often found to refract through the layers. Cleavage planes can also form by dissolution of material - although in these situations it is commonly spaced. This type is called "pressure solution cleavage".

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