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Coarse angular rock fragments produced by crushing during fault slip, commonly in a finer matrix. Fine-grained, poorly cohesive comminuted material is fault gouge; cohesive material formed by similar grain-size reduction can be called cataclasite.

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.

Within a shallow fault zone undergoing brittle fracture, repeated sliding crushes and grinds grains. Expect fault gouge, potentially rich in fine clay minerals, and coarser angular fault breccia; a cohesive, cemented crushed rock is a cataclasite. Slickensides or displaced grains can record the sliding direction.

An opening geological joint need not contain any fault rock: it may be an open crack. If fluids circulated, it can contain a mineral vein, for example calcite or quartz, grown from the fracture walls. Thus comminuted fault rock supports shear, while open space or a crack-filling mineral vein supports opening. Mineral filling is conditional on fluid chemistry and timing; its precise composition cannot be inferred from the photograph.