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Cleavage refraction occurs in layered rocks, when there is appreciable rheologic difference in response to stress. The smaller cleavage-hedding angle is always characteristic of the less competent layers, the greater angle is found in the more competent layers. The numerical

difference in value of the angle is a function of competence contrast and of the location in the fold. If the geometry of the fold is associated with higher strains on the overturned limb than on the normal limh (as is frequent), the cleavage refraction is more marked on the normal limb than on the overturned limb. Generally the axial surjace of the fold lies with an orientation between the two directions of the cleavage developed in competent and incompetent layers.

The relationships between refracted cleavage and layering at the fold crest, indicate that the crest and hinge lines are not coincident.

The geometric relationships described above hold true irrespective whether the fold is an upward or downward facing structure whether this fold is an antiformal anticline or antiformal syncline etc. definitions . If the polarity of the layers in a fold can be determined (e.g. with sedimentation structures such as cross bedding or lithological grading) then these observations can be combined with cleavage bedding relationships to determine the jacing direction of a old. Once this facing direction has been established then the cleavage -bedding relationships at individual outcrops can be used to determine the stratigraphic polarity of the beds.

Clockwise from top: Refraction in Martinburg formation; refraction on the western limb of Rhoscolyn anticline; refraction in turbidite sequence at Barmhan (Mahakoshal group) and interpretation of cleavage refraction as regards the finite strain state.