In faults of the preceding groups there and these are the pivotal faults. The result of the movement is that the hanging wall drops on one side of the axis of rotation, producing a fault of the normal type, and rises on the other side, forming a reversed fault. Thus, one and the same fault is "normal" in one part of its course and "reversed'" in the other.

Fig. 179. - Model illustrating horizontal faulting, with hanging wall moved against the dip and producing an apparently normal fault. Upper figure (modified from Ransome) block after dislocation. Lower figure, cross-section on plane S.S.S.S': BB, B'B', stratum of reference. FFFF, fault-plane.

Fig. 180. - Model illustrating horizontal faulting, with hanging wall moved In direction of dip and producing an apparently reversed fault. Upper figure (modified from Ransome) block after dislocation. Lower figure, cross-section on plane SSSS. Lettering as in Fig. 179 is apt to be more or less rotation, because of unequal friction and resistance of the walls, but in certain cases this movement of rotation is the principal one, exceeding any movement of translation.

Systems of faults of different dates frequently traverse the same region, intersecting and crossing one another at all angles. An older fault crossed by a newer one is itself faulted and offset. The intersecting faults divide the rocks into large and small fault-blocks, which are generally tilted in different directions, but, as a rule, their beds are not strongly folded. As was pointed out in the discussion of earthquakes (Chapter I), this mosaic of fault-blocks is an important element in the production of seismic disturbances. Though faults often occur in regions of strata that are not folded, there is, nevertheless, frequently a close connection between faults and folds, especially monoclinal flexures, which so often pass into faults, the strata bending along part of their course, fracturing and dislocating in another part.

Fig. 181. - Horizontal slickensides, Oklahoma. (U. S. G. S).

Fig. 182. - Model illustrating pivotal faulting. Upper figure, before dislocation. Lower figure, after dislocation. (J. A. Reid).