Now the question as to how slaty cleavage originates must be addressed.
|PRELIMINARY OBSERVATIONS AND HYPOTHESES|
|A diagram of a vertical cliff face that displays slaty cleavage is shown on the left. Three copies of the diagram are provided to facilitate reference to the commentary.|
Observations: A series of five parallel rock layers (1 through 5) run from left to right across the area. In the right-hand quarter of the area, the layers are unfolded. In the remainder of the area, the layers are folded, with the folds increasing in tightness as a vertical intrusive rock body is approached. As the folds tighten, slaty cleavage is more and more strongly displayed.
Since the cliff face is free of debris and vegetation, great confidence may be had that the diagram accurately represents the rocks. Where the five parallel layers are unfolded or gently folded, they show many primary structures, such as ripple marks, mudcracks and fossils. These primary structures (shown diagrammatically as jagged yellow lines within rectangular black boxs) parallel the boundaries of the layers. As the fold become tighter, these structures become increasingly deformed and hard to decipher.
Layers 2, 3 and 4 grade from shale to slate as they approach the cross-cutting rock body. Where the layers are unfolded or gently folded, the planes along which the rock tends to split are parallel to the boundaries of the layers. As the folds tighten, the orientation of the 'splitting planes' changes to become parallel to the sides of the cross-cutting body. This change is indicated on the diagram by the changes in orientation and lengths of black lines within the layers.
Hypotheses: The presence and orientations of the primary structures listed above indicates that layers 1 through five originated as flat-lying sedimentary rocks. The folds and slaty cleavage were superimposed on the layers after their formation.
|FURTHER OBSERVATIONS AND HYPOTHESES|