Fractures in rock are classified according to the type of relative motion that has occurred across the fracture. Extensional fractures, also known as joints, are characterized by movement perpendicular to the fracture. The masses of rock separated by a joint moved away from each other, even if imperceptibly, when the joint was formed. Joints stand in contrast to faults, which are shear fractures across which the opposite sides slide past (rather than away from) each other. Rocks can undergo more than one episode of deformation during their existence, so it is possible for a fracture to begin as a joint and evolve into a fault as the stresses acting on the joint change through geologic time. The precise definition of a joint, however, is not universal and some geologists classify fractures as joints if there seems to have been only a small, but measurable...