Rib marks (Figure 1 and Figure 2) are common ornaments on a joint surface. They are elliptical or circular curvilinear ridges or furrows orientated at right angles to hackle marks and plume axes. Rib marks represent the positions of the joint front at certain times and it is believed that they represent temporal gaps in joint propagation. Comparison of the distances between rib marks records the relative propagation velocity. Therefore, they are sometimes referred to as hesitation marks and conchoidal structures.

Figure 1. Joint surface morphology (top) showing several elliptical rib marks indicating mode I-II propagation. Here the emphasis is on a series of sharp fringe joints (bottom) indicating a breakdown of the parent joint surface in mode I-III propagation. From Aydin et al. (2006).

Figure 2. Rib marks. A. Line drawing of concentric rib marks as well as hackle marks from Bankwitz (1965). B. Closely spaced rib marks with approximately rounded profiles in sandstone. C. Rib marks with sharp profiles in sandstone. From Pollard and Aydin (1988).