Fig. 427.—Step-fold showing (in 1) break in the massive limestone bed which determines the plane of the break-thrust along which the displacement shown in 2 takes place. (Willis, U. S. Geol. Surv.)
Sometimes a fault branches ([Fig. 428]) and sometimes the faulting is distributed among a series of parallel planes at short distances from one another,[222] instead of being concentrated along a single plane, thus giving rise to a distributive fault ([Fig. 429]). This is perhaps more common in normal than in reversed faulting.
Fig. 428.
Fig. 429.
Fig. 428.—Branching-fault. (Powell.)
Fig. 429.—Diagram showing a series of small faults—distributive faulting.
Fig. 430.—Fault in Gering series. Near Rutland Siding, near Crawford, Neb. (Darton, U. S. Geol. Surv.)
The amount of throw occasionally reaches several thousand feet. Occasionally faults of incredible dimensions are reported, but these are perhaps misinterpretations. Faults are observed to die out gradually when traced horizontally, sometimes by passing into monoclinal folds, and sometimes without connection with folding. In depth they probably die out in similar ways in most cases. Where the throw is great, they probably give place to folds below ([Fig. 421]). Other phenomena of faulting are illustrated by Figs. [430–435]. A fault of thousands, or even hundreds of feet is probably the sum of numerous smaller slippings distributed through long intervals of time. Faulting is probably one of the common causes of earthquakes.