 Solution

The coarse angular fragments in the light-coloured band identify a fault breccia, probably cemented and accompanied by finer comminuted material. Repeated brittle fracture, clast rotation and crushing during fault slip produce the fragments; fluid flow can later fill pores and fractures with mineral cement. If the finer material is cohesive it can be termed cataclasite; a loose, very fine seam is fault gouge. The photograph alone does not establish all those cohesion and grain-size distinctions.

Slip is commonly concentrated on a narrow principal slip surface or fine-grained seam at a margin or inside a broader damaged band. Thus the coarsest, widest breccia is not automatically the material that accommodated the largest shear displacement. In this image the sharp, finer-looking boundaries of the coarse band are the best candidates for localized slip. Identifying the uniquely most displaced seam would require displaced markers, thin sections or additional exposure; the coarse band itself records substantial distributed damage.