Fig. 423.—Diagram showing a fault grading into a monocline horizontally.
Fig. 424.—Slickenside surface. (Prestwich.)
The rock on either side of a fault-plane is often smoothed as the result of the friction of movement. Such surfaces are slickensides ([Fig. 424]). A slickenside surface has some resemblance to a glaciated surface, but generally gives evidence of greater rigidity between the moving surfaces.
Faults are of two general classes, normal and reversed. In the normal fault ([Fig. 418]) the overhanging side is the downthrow side, i.e., the downthrow is on the side towards which the fault-plane inclines, as though the overhanging beds had slidden down the slope. Normal faults, as a rule, indicate an extension of strata, this being necessary to permit the dissevered blocks to settle downwards. In the reversed fault, the overhanging beds appear to have moved up the slope of the fault-plane, as though the displacement took place under lateral pressure. This is clearly shown to be the case where an overfold passes into a reversed fault ([Fig. 420]). Reversed faults are further illustrated by Figs. [425], [426], and [427]. Where the plane of the reversed fault approaches horizontality, the fault is often called a thrust-fault, or an overthrust. In such cases the throw is to be distinguished from the stratigraphic throw (see [Fig. 426]). In thrust-faults, the heave is often great. The eastern face of the Rocky Mountains near the boundary-line between the United States and Canada has been pushed over the strata of the bordering plains to a distance of at least eight miles.[219] Overthrusts of like gigantic displacement have been detected in British Columbia,[220] Scotland,[221] and elsewhere.
Fig. 425.—Perspective view and vertical section of a thrust-fault. (Willis, U. S. Geol. Surv.)
Fig. 426.—Diagram of a thrust-fault illustrating the several terms used in describing faults. The distinctions between heave and displacement, and between throw and stratigraphic throw, are to be especially noted. (Willis, U. S. Geol. Surv.)
