Fig. 30.—Ideal view of a complete fault.
The rock above an inclined fault, vein, or dike ([Fig. 28]) is called the hanging wall, and that below the foot wall. Now inclined faults are divided into two classes, according to the relative movements of the two walls. Usually, the hanging wall slips down and the foot wall slips up, as in [Fig. 28]. Faults on this plan are so nearly the universal rule that they are called normal faults. They indicate that the strata were in a state of tension, for their broken ends are pulled apart horizontally, so that a vertical line may cross the plane of a stratum without touching it.
A few important faults have been observed, however, in which the foot-wall[**no hyphen before] has fallen and the hanging-wall[**] has risen ([Fig. 29]). These are known as reversed faults; and they indicate that the strata were in a state of lateral compression, the broken ends of the beds having been pushed horizontally past each other, so that a vertical line or shaft may intersect the same bed twice, as has been actually demonstrated in the case of some beds of coal.
Fig. 31.—Explanation of normal faults.
The usual explanation of normal faults is given in [Fig. 31]. The inclined fractures of the earth’s crust must often be converging, bounding, or enclosing large V-shaped blocks (A, B). If now, through any cause, as the folding of the strata, they are brought into a state of tension, so that the fractures are widened, the V-shaped masses, being unsupported, settle down, the fractures bounding them becoming normal faults, as is seen by tracing the bed X through the dislocations. The single fracture below the block A is inclined, and the stretching has been accomplished by slipping along it and faulting the bed Z as well as X, the entire section to the right of this fracture being part of a much larger V-shaped block the right-hand boundary of which is not seen. But the united fracture below the block B being vertical, any horizontal movement must widen it into a fissure, which is kept open by the great wedge above and may become the seat of a dike or mineral vein. The beds below the V may, in this case, escape dislocation, as is seen by tracing the bed Z across the fissure. These pairs of converging normal faults are called trough faults; and this is the only way in which we can conceive of important faults as terminating at moderate depths below the surface, and not affecting the entire thickness of the earth’s crust.
Important reversed faults are believed to occur chiefly along the axes of overturned anticlines ([Fig. 24]) where the strata have been broken by the unequal strains, and those on the upper side shoved bodily over those on the lower or inverted side.
An extensive displacement of the strata is sometimes accomplished by short slips along each of a series of parallel fractures, producing a step fault.
Faults cutting inclined or folded strata are divided into two classes, according as they are approximately parallel with the direction of the dip or of the strike. The first are known as transverse or dip faults, and the second as longitudinal or strike faults. The chief interest of either class consists in their effect upon the outcrops of the faulted strata, after erosion has removed the escarpment produced by the dislocation.