Martinsburg slate near Delaware Water Gap contains two sets of precleavage calcite veins, both probably related to brittle failure during early flexural folding. Laminated veins that lie along bedding-plane faults of small displacement are locally folded, whereas veins that were originally perpendicular to bedding have boudinage structure. The latter have been used to define the strain ellipsoid (axial ratio 2.08:1:0.48) for the deformation accompanying cleavage formation. Clastic dikes are at various angles to cleavage and bedding and do not provide evidence for cleavage having formed prior to lithification. Slaty cleavage in the Martinsburg formation at this classic locality formed in lithified pelitic flysch and is unrelated to dewatering.