A composite joint is a collection of vertically aligned but discrete joint segments, with each segment confined within an individual brittle mechanical layer. Segments in neighboring brittle layers are commonly slightly out of plane with each other; this is called stepping offset. Figure 1 shows a vertical composite joint in profile in a siltstone-shale sequence exposed in a road cut in Watkins Glen State Park, NY (Helgeson and Aydin, 1991). This perspective view is almost parallel to the strike of the joints. The most remarkable feature of the composite geometry (Figure 2) is that the segments appear to be continuous in siltstone layers, while they are discontinuous across shale layers. In spite of the discontinuities along vertical joint traces, however, they are well aligned in a vertical sense.

Figure 1. A perspective view of a vertical composite joint in the alternating siltstone and shale turbidite sequence within the Ithaca Formation of the Genesee Group on a road cut near Watkins Glen, NY. Note that the large apparently throughgoing vertical joint, which appears to be continuous, is actually discontinuous. Furthermore, the discontinuities appear to coincide with bedding interfaces made up of thin shale layers. Height of cliff face is about 6 m. From Helgeson and Aydin (1991).

Figure 2. (a) Drawing of the composite joint surface in Figure 1. The siltstone layers are generally individually jointed, as indicated by each layer having its own plumose structure. Joints in adjacent siltstones are out of plane with each other, which gives the overall vertical joint trace a discontinuous and side-stepping character. This is seen better on an enlarged cross section of a portion of the structure (b). From Helgeson and Aydin (1991).

Composite joints occur when a joint propagates incrementally across layered rock with alternating layers of brittle and ductile rocks (from one brittle layer to another). Ductile layers commonly inhibit jointing, though they may allow stress fields to communicate across if the ductile layers are relatively thin. Across thick ductile (inhibiting) layers, joints appear to be independent with a wide range of arbitrary stepping offsets. This phenomenon and a related general trend between the thickness of the shale layers and the degree of the communication among joints across shale, is discussed in the section 'Mechanisms and Mechanics of Composite Joints.'