A fault is a break in Earth's crust where the broken blocks of rock move with respect to one another. This movement normally generates earthquakes. The amount of movement is the fault displacement.
Faults are classified based on their relative movement. In normal faults (pictured right),
the hanging wall moves down relative to the footwall. The Sangre de Cristo fault in the San Luis Valley is a normal fault with nearly four miles of displacement. In a reverse fault, the hanging wall moves up relative to the footwall. A thrust fault, such as the Williams Range fault bounding the west side of the Front Range, is a special case of a reverse fault where the dip of the fault plane is thirty degrees or less.
Fault block diagrams showing relative movement of the footwall block (FB) and the hanging-wall block (HB) in a normal, reverse and thrust fault (from left to right).
Horsts and grabens are blocks bounded by normal faults. In a graben (left top), the block has dropped down along two, inward-dipping normal faults. In a horst (left bottom), the block has moven up along two outward-dipping normal faults. Horsts and grabens can be quite large, such as the San Luis Valley (graben) and the Sangre de Cristo Range (horst).
The graben shown to the left below is in the Paradox basin. The horst on the right is located along Highway 550 north of Durango.