A fault is a crack, or fracture, in the Earth's crust where movement has occurred. Faults are classified according to how the rocks on each side of the fault move past each other.
Strike-slip faults occur when rocks move past each other horizontally. The San Andreas Fault in California is an example.
Dip-slip faults occur when rocks move parallel to the dip in the fault. Owens Valley in California is an example.
Reverse faults are a type of dip-slip fault that result from compression or pushing together of rocks. The Sierra Madre in southern California is an example.
Thrust faults are a type of reverse fault characterized by a gentle dip. The Klamath Mountains in northern California and the Pennsylvania Crab Orchard Mountain Group in Tennessee are examples.
Normal faults are a type of dip-slip fault that result from extension or pulling apart of rocks. The Cambrian Kinsers Formation in the Thomasville Quarry in Pennsylvania is an example.