FAULT, in geology, any rock-crack or fis sure with dislocation of the strata so that one wall of the fracture has slipped past the other wall. The rock movement may accompany the formation of the crack, or may come later and the total movement may vary from a small fraction of a foot to many thousands of feet. Faults are produced by the same strains and stresses which make folds, the rocks breaking instead of bending; a brittle stratum may be faulted, while softer strata above or below may be only bent. The dip of a fault is the amount of inclination of the plane of fracture from the horizontal; the bade or slope is the complement of the dip, being the amount of inclination of the plane from the vertical. Thus the hade of a fault with dip of 60 degrees would be 30 de grees. The rock-strata relatively dropped from the downthrow side; the upthrow side is oppo site. In the case of a fault plane that is not verticle, the side of the fracture that overlies the other is the hanging wall; the underlying side is the foot-wall. The trace of the fault plane on the horizontal is its strike. The throw of a fault is the total vertical dis placement, the heave is the total horizontal dis placement, both measured in a plane normal to the strike. If a fault dips 45 degrees the heave and throw are equal.
Faults are classified as normal and reversed. In a normal fault the hanging wall is on the downthrow side. Such faults are usually pro duced by gravity, the hanging wall having slipped downward along the fracture. In such faults, the fracture is usually steep and they are often known as gravity faults. In a re versed fault the hanging wall is on the upthrow side, that is, has been thrust up by compression, the beds on one side of the fracture being thrust past those on the other. As a rule thrust faults have lowei dips than normal faults, since foi the hanging wall to ride up over the other side, the plane of breaking must usually be in clined at a low angle. The best examples of
great normal faults in the United States are in the Arizona-Colorado Plateau. Some of these such as the Grand Wash and Hurricane Faults, have vertical displacements of several thousand feet. A few are so recent that the upthrown side stands as a great wall, overlooking the downthrown side. Such a wall is known as a fault escarpment, or more briefly, as a fault scarp. Erosion, of course, ultimately removes all traces of a scarp, but even after that the structure is still called a fault. Horizontal movements also occur in faulting, offsetting roads and fences, as in the famous San Fran cisco earthquake, the tremor of which was pro duced by a fault movement. Thrust faults are common in the southern Appalachian Moun tains, or in other regions of close compression and intense folding. Such faults are known, in which the plane is nearly horizontal and in which older rocks have been thrust out over younger beds for distances of 15 miles or more. The Lewis overthrust of Montana and the Hart Mountain overthrust of Wyoming are striking examples. Generally a fault-fissure is filled with more or less ground-up material, called breccia, formed from the dislocated strata and the hanging and foot wall faces are grooved and polished, giving the appeirance called slickensided.
It is obvious that faults are essentially sur face phenomena; for with increasing depth and increasing pressure from above, rocks bend rather than break, and we can easily imagine a depth at which all rocks, while not truly molten, are plastic. See ESCARPMENT; GEOLOGit ; MOUN TAIN ; ORE DEPOSIT.