Crosscutting Relationships (Charles Lyell, 1797-1875). Any event that cuts across an existing rock unit is younger than that unit. This law is self-evident. Clearly, the older rock unit must be in place before something can happen to it. Common events that can cut across existing rock units are unconformities, intrusions and faults.
A) Unconformities. Unconformities are surfaces of nondeposition or surfaces that have been subject to erosion. There are three basic types.
Nonconformities are erosional surfaces between older crystalline rocks and younger sedimentary rocks. In this case there will be erosional fragments of the older crystalline rock in the younger sedimentary beds (components).
Angular unconformities are erosional surfaces that separate older tilted or folded beds the younger flat lying beds. This type is the easiest to recognize since the lower beds are tilted and the erosional surface cuts across multiple beds.
Disconformities are erosional surfaces between parallel sedimentary beds. This is the hardest to recognize. Geologists look for the erosional material of the older lower bed in the younger upper bed (components). They also look for cut and fill structures and an irregular erosional surface.
B) Igneous intrusions. Igneous intrusions are called dikes if the intrusion cuts across the bedding plane of the existing rock or sills if the intrusion is parallel to the bedding plane of the existing rock. Both types of intrusions can be recognized by a zone of contact metamorphism (aureole) surrounding the intrusion in the preexisting rock.
C) Faults. Faults are stress (tension, compression or shear) induced fractures in the Earth’s crust. The crust then moves in opposite directions along and parallel to the fault plane.
Normal faults result from tensional stresses and are recognized be the downward movement of the hangingwall block in relation to the footwall block. The hangingwall block is the block above the fault plane and the footwall block is the block below the fault plane.
Reverse faults result from compressive stresses and are recognized by the upward movement of the hangingwall in relation to the footwall block.
Thrust faults are low angle reverse faults.
Strike-slip faults are faults in which the relative movement of one block is lateral to the other in the horizontal plane.