Fig. 2. On meeting a contact an extension fracture of any type (here a dyke in a lava pile or a sedimentary basin) may respond in four different ways. The fracture may (a) become arrested, (b) penetrate the contact, (c) become singly deflected, or (d) doubly deflected. Layers A and B are stiffer than, whereas layer C has the same stiffness as, layer D. In cases c and d, the dyke forms a sill.