Fig. 32.—Plan of a dip fault.
Dip faults cause a lateral shift or displacement of the outcrops, as shown in [Fig. 32], which represents a plan or map-view of the strata traversed by the fault b b, the down throw being on the right and the up throw on the left. The dip of the strata is indicated by the small arrows and the accompanying figures; and it will be observed on tracing the outcrop of any stratum, a a, across the fault that it is shifted to the right. If the throw of the fault were reversed, the displacement of the outcrop would be reversed, also. Strike faults are of two kinds, according as they incline in the same direction as the strata, or in the contrary direction. The effect of the first kind is to conceal some of the beds, as shown in [Fig. 33], in which beds 5 and 6 do not outcrop, but we pass on the surface abruptly from 4 to 7. The apparent thickness of the section is thus less than the real thickness. When the fault inclines against the strata, on the other hand ([Fig. 34]), the outcrops of certain strata are repeated on the surface; and a number of parallel faults of this kind, a step fault, will, like a series of closed folds ([Fig. 25]), cause the apparent thickness of the section to greatly exceed the real thickness. Repetition of the strata by faulting is distinguished from repetition by folding by being in the same instead of the reverse order.
Fig. 33.—Strike fault, concealing strata.
Fig. 34.—Strike fault, repeating strata.
Folds and faults are really closely related. In the former the strata are disturbed and displaced by bending; in the latter by breaking and slipping; and the displacement which is accomplished by a fold may gradually change to a fracture and slip. This relation is especially noticeable with monoclinal folds ([Fig. 23]), in which the tendency to shear or break the beds is often very marked.
Important faults are rarely simple, well-defined fractures; but, in consequence of the enormous friction, the rocks are usually more or less broken and crushed, sometimes for a breadth of many feet or yards. The fragments of the various beds are then strung along the fault in the direction of the slipping, and this circumstance has been made use of in tracing the continuation of faulted beds of coal. In other cases the direction of the slip is plainly indicated by the bending of the broken ends of the strata ([Fig. 35]), and the beds are sometimes turned up at a high angle or even overturned in this way.
