Joints were studied in open to tightly folded Paleozoic rocks of various lithologies.
Dip, strike, and oblique joints are normal to bedding in competent, well-bedded sandstone, silt-stone, and limestone. In calcareous slate, steep dip sets predominate, accompanied by oblique sets usually perpendicular to bedding; rare strike joints are normal to cleavage. In slate, the dip joints are the predominant set sometimes accompanied by a secondary strike set perpendicular to the cleavage. The dip joints in slates bear no consistent angular relationship to variably plunging meso-fold axes. Thus, it is suggested that the joints are independent of the geometry of local folds, are not due to extension along fold axes during the main phase of folding, but are later in origin.