Fig. 42.—A fault in previously disturbed strata. AB, displacement; AC, throw; BD, stratigraphic throw; BC, heave; angle CAB, hade.
Methods of detecting faults.—The first effect of a fault is usually to produce a crack at the surface of the earth; and, provided there is a vertical displacement or throw, an escarpment which rises upon the upthrown side of the fault. In general it may be said that escarpments which appear at the earth’s surface as plane surfaces probably represent planes of fracture, though not necessarily planes of faulting. In many cases the actual displacements lie buried under loose rock débris near to and paralleling the escarpment, and in some cases as a result of the erosional processes working upon alternately hard and soft layers of rock, the escarpment may later appear upon the downthrown side or limb of the fault ([Fig. 43]). As an illustration of a fault escarpment, the façade of El Capitan and many other rock faces of the Yosemite valley may be instanced.
Fig. 43.—Diagrams to show how an escarpment originally on the upthrown side of the fault may, through erosion, appear upon the downthrown side.
Fig. 44.—A fault plane exhibiting “drag.” The opening is artificial (after Scott).
When we have further studied the erosional processes at the earth’s surface, it will be appreciated that faults tend to quickly bury themselves from sight, whereas fold structures will long remain in evidence. Many faults will thus be overlooked, and too great weight is likely to be ascribed to the folds in accounting for the existing attitudes and positions of the rock masses. Faults must therefore be sought out if mistakes of interpretation are to be avoided.
The most satisfactory evidence of a fault is the discovery of a rock bed which may be easily identified, and which is actually seen displaced on a plane of fracture which intersects it ([Fig. 42], [p. 59]). When such an easily recognizable layer is not to be found, the plane of displacement may perhaps be discovered as a narrow zone composed of angular fragments of the rock cemented together by minerals which form out of solution in water. Such a fractured rock zone which follows a plane of faulting is a fault breccia. If the fault breccia, or vein rock, is much stronger than the rock on either side, it may eventually stand in relief at the surface like a dike or wall. At other times the displacement produces little fracture of the walls, but they slide over each other in such a manner as to yield either a smoothly corrugated or an evenly polished surface which is described as “slickensides.” It may be, however, that during the movement either one or both of the walls have “dragged”, and so are curled back in the immediate neighborhood of the fault plane ([Fig. 44]).