During compaction of mudstones, clay particles become aligned parallel to bedding planes to form the parting that occurs in shalesThe rocks are flattened parallel to beddingDuring deformation that produces shortening parallel to bedding, the clay particles to rotate so that their planes are at a high angle to shortening directionThe combination of the early compaction strain and the tectonic shortening result in a finite linear strain that produces the pencil fragments.With continued tectonic shortening, the tectonic strain overcomes the compaction to form slaty cleavage.