When rocks deform by plastic deformation they can bend and fold. The process of folding occurs when rock is compressed, as it is along colliding plate boundaries. Upturned folds are called anticlines and down turned folds are called synclines . Anticlines and synclines are geologic structures, that is, they are folds in rock material. They give expression to the surface as linear ridges (anticlines) and troughs (synclines). The sides of the fold are called the limbs. Each fold has an axial plane, an imaginary plane that runs down its length and divides the fold in half.
Symmetrical or open folds with their near-vertical axial planes and gently dipping limbs of about the same angle are a product of gentle compression. Symmetrical folds are found near the margins of mountain systems where tectonic activity is relatively quiet. If the compression is more pronounced from one direction, an overturned fold may occur. Extreme directed pressure may lay the fold over with its axial plane nearly horizontal with the surface producing a recumbent fold. Sometimes the length of the folds are tilted creating plunging folds. One of the best examples of folded topography is the Appalachian mountains found in the Ridge and Valley physiographic province of North America.
When enormous stresses build and push large intact rock masses beyond their yield limit, faulting of the surface is likely to occur. A fault is a fracture along which movement occurs. The plane that extends into the earth and along which slippage occurs is called the fault plane. The fault dip is the angle from horizontal that the fault plane makes. The map direction that the fault takes is called the strike, measured east or west of true north. Generally, two walls are distinguished, the footwall and hanging wall. The hanging wall moves horizontally, vertically, or in both directions relative to the footwall. We identify the hanging and foot walls relative to the fault plane. The hanging wall is above the fault plane while the foot wall is below. The steep face of an exposed block is called the fault scarp. The fault line is the trace of the fault along the surface.
Faults can be traced for thousands of kilometers across the surface at tens of kilometers in depth. The sudden slippage of rock masses past one another results in shock waves that we feel as an earthquake. For information about earthquakes see "How Earthquakes Occur" by the USGS.