joint

In geology, a brittle fracture surface in rocks along which little or no displacement has occurred. Present in nearly all surface rocks, joints extend in various directions, generally more vertical than horizontal. Joints may have smooth, clean surfaces, or they may be scarred by slickensides, or striations. Jointing does not extend very far into the Earth's crust, because at about 7.5 mi (12 km) even rigid rocks tend to flow plastically in response to stress.