Abstract

Here we call attention to a weakly developed pencil cleavage as an indicator of the pervasiveness of lateral shortening within the sedimentary sequence of the Appalachian Plateau. The pencil cleavage appears in Upper Devonian shales as a set of closely spaced vertical partings, and it gives the shale a crude lineation similar to the pencil structures in slate districts. The strike of the shale pencil structures parallels the strike of stylolitic solution cleavage in limestones and is normal to the direction of lateral shortening indicated by deformed fossils. These deformation markers generally conform with the variation in trend of the Appalachian Plateau folds. Where the strike of the pencil cleavage diverges from the fold axes, the relation between cleavage and folding suggests that the cleavage formed first.

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