FIG. F-17. Folding. (a) The plunge of an anticline is the angle that the axial line makes with the surface. The b-axis is the direction of the axial line, the a-axis is the plane containing the axial line and the axis of the fold, and the c-axis is perpendicular to this plane. (b) Folding, cleavage, or flow may result from stresses. The stress duration often is involved as well as the stress magnitude. (From deSitter, 1956.) (c) Faulting is often combined with folding. (After Hobbs et al., 1976.) (d) In concentric or flexural-slip folding, competent beds tend to slip along the bedding planes and retain constant thickness perpendicular to the bedding, whereas incompetent beds fold and fault and separate the fold from deeper structure. (e) In similar folding, beds tend to retain their vertical thickness t, thinning on the sides of the fold.