You have considered an argument that slaty cleavage is a secondary structure imposed on the primary rock 'shale'.

Now the question as to how slaty cleavage originates must be addressed.


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.


A: Comment. When a layer is compressed, it may, under the right circumstances, form folds. (See diagram A.)


B: Comment. For folds to develop, rather than faults, the material has to be plastic. Plastic behavior is favored by elevated temperatures.

Hypothesis: If the cross-cutting body was forcibly emplaced, it could exert the necessary lateral pressure to fold the layers. (See diagram B.) The black arrows in the cross-cutting body are meant to suggest forcible emplacement and lateral pressure.) If the cross-cutting body was hot, it could provide the elevated temperatures necessary for the nearby sedimentary rock layers to behave plastically.


C: Observation. There is a difference in the perfection of parallelism of flaky minerals in shale and slate.

Hypothesis: The composition of shale and presence in it of features like mud cracks suggest that shale is lithified mud. An important component of mud is the flaky mineral clay. When mud settles out through water due to the force of gravity, the clay flakes come to rest in a roughly horizontal orientation, more or less parallel to one another and perpendicular to the gravitational pull. The orientation of the clay flakes in mud is preserved when it turns to shale and gives the shale its tendency to split into roughly parallel slabs. The parallelism of the flaky grains in the shale is a primary texture (occurred during the process of lithification).


D: Observation. The flaky minerals in slate have a greater parallelism than do those that occur in shale. The orientation of flaky grains in the shale is horizontal whereas the orientation of the flaky grains in the slate is vertical. Parallelism of flaky grains can be brought about by compression. The flakes orient themselves perpendicular to the compressive force. The greater the compressive force, the greater the parallelism.

Hypothesis. The compressive force that created the slaty cleavage was stronger than the force that created the alignment of flaky grains in the shale. Moreover, since the slaty cleavage is oriented vertically, the compressive force that created it was oriented horizontally. Parallelism of flaky grains in the slate is greater than that in the shale.


E: Observation: The slaty cleavage coincides with tight folding.

Hypothesis. The force that created the folds also produced the slaty cleavage. The orientation of the folds and the slaty cleavage are both compatible with a horizontal force. The compressive force imposed by the emplacement of the cross-cutting body was horizontal. Gravity is a comparitively weak force compared to the force necessary to displace rock. The emplacement of the cross-cutting body created both the folding and the slaty cleavage.

  • The forcible emplacement of a hot, cross-cutting body imposed slaty cleavage on the sedimentary rock 'shale'.
  • The change from shale to slate required changes in temperature and pressure sufficient to call the process 'metamorphic'.
  • 'Slate' is the metamorphic equivalent of shale.

    The next question to address is: How did the cross-cutting body originate?

© 2001, David J. Leveson