| Fig. 14.—Plan of strata traversed by a diminishing strike fault. |
| Fig. 15.—Plan of an anticline (A) and syncline (S), dislocated by a fault. |
| Fig. 16.—Section along the upcast side of the fault in fig. 15. |
| Fig. 17.—Section along the downcast side of same fault. |
The effect of normal faults upon the outcrop may be thus briefly summarized:—a strike fault that hades with the direction of the dip may cause beds to be cut out at the surface on the upthrow side; if it hades against the dip direction it may repeat some of the beds on the upthrow side (figs. 11 and 12). With dip faults the crop is carried forward (down the dip) on the upthrow side. The perpendicular distance between the crop of the bed (dike or vein) on opposite sides of the fault is the “offset.” The offset decreases with increasing angle of dip and increases with increase in the throw of the fault (fig. 13). Faults which run obliquely across the direction of dip, if they hade with the dip of the strata, will produce offset with “gap” between the outcrops; if they hade in the opposite direction to the dip, offset with “overlap” is caused: in the latter case the crop moves forward (down dip) on the denuded upthrow side, in the former it moves backward. The effect of a strike fault of diminishing throw is seen in fig. 14. Faults crossing folded strata cause the outcrops to approach on the upthrow side of a syncline and tend to separate the outcrops of an anticline (figs. 15, 16, 17).
In the majority of cases the upthrown side of a fault has been so reduced by denudation as to leave no sharp upstanding ridge; but examples are known where the upthrown side still exists as a prominent cliff-like face of rock, a “fault-scarp”; familiar instances occur in the Basin ranges of Utah, Nevada, &c., and many smaller examples have been observed in the areas affected by recent earthquakes in Japan, San Francisco and other places. But although there may be no sharp cliff, the effect of faulting upon topographic forms is abundantly evident wherever a harder series of strata has been brought in juxtaposition to softer rocks. By certain French writers, the upstanding side of a faulted piece of ground is said to have a regard, thus the faults of the Jura Mountains have a “regard français,” and in the same region it has been observed that in curved faults the convexity is directed the same way as the regard. Occasionally one or more parallel faults have let down an intervening strip of rock, thereby forming “fault valleys” or Graben (Grabensenken); the Great Rift Valley is a striking example. On the other hand, a large area of rock is sometimes lifted up, or surrounded by a system of faults, which have let down the encircling ground; such a fault-block is known also as a horst; a considerable area of Greenland stands up in this manner.
Faults have often an important influence upon water-supply by bringing impervious beds up against pervious ones or vice versa, thus forming underground dams or reservoirs, or allowing water to flow away that would otherwise be conserved. Springs often rise along the outcrop of a fault. In coal and metal mining it is evident from what has already been said that faults must act sometimes beneficially, sometimes the reverse. It is a common occurrence for fault-fissures and fault-rock to appear as valuable mineral lodes through the infilling or impregnation of the spaces and broken ground with mineral ores.
In certain regions which have been subjected to very great crustal disturbance a type of fault is found which possesses a very low hade—sometimes only a few degrees from the horizontal—and, like a reversed fault, hades beneath the upthrown mass; these are termed thrusts, overthrusts, or overthrust faults (Fr. recouvrements, failles de chevauchement, charriages; Ger. Überschiebungen, Übersprünge, Wechsel, Fallenverwerfungen). Thrusts should not be confused with reversed faults, which have a strong hade. Thrusts play a very important part in the N.W. highlands of Scotland, the Scandinavian highlands, the western Alps, the Appalachians, the Belgian coal region, &c. By the action of thrusts enormous masses of rock have been pushed almost horizontally over underlying rocks, in some cases for several miles. One of the largest of the Scandinavian thrust masses is 1120 m. long, 80 m. broad, and 5000 ft. thick. In Scotland three grades of thrusts are recognized, maximum, major, and minor thrusts; the last have very generally been truncated by those of greater magnitude. Some of these great thrusts have received distinguishing names, e.g. the Moine thrust (fig. 19) and the Ben More thrust; similarly in the coal basin of Mons and Valenciennes we find the faille de Boussu and the Grande faille du midi. Overturned folds are frequently seen passing into thrusts. Bayley Willis has classified thrusts as (1) Shear thrusts, (2) Break thrusts, (3) Stretch thrusts, and (4) Erosion thrusts.
| Fig. 18.—Diagram to illustrate the terminology of faults and thrusts. |
| Fig. 19.—Section of a very large thrust in the Durness Eriboll district, Scotland. |
Dr J.E. Marr (“Notes on the Geology of the English Lake District,” Proc. Geol. Assoc., 1900) has described a type of fault which may be regarded as the converse of a thrust fault. If we consider a series of rock masses A, B, C—of which A is the oldest and undermost—undergoing thrusting, say from south to north, should the mass C be prevented from moving forward as rapidly as B, a low-hading fault may form between C and B and the mass C may lag behind; similarly the mass B may lag behind A. Such faults Dr Marr calls “lag faults.” A mass of rock suffering thrusting or lagging may yield unequally in its several parts, and those portions tending to travel more rapidly than the adjoining masses in the same sheet may be cut off by fractures. Thus the faster-moving blocks will be separated from the slower ones by faults approximately normal to the plane of movement: these are described as “tear faults.”
Faults may occur in rocks of all ages; small local dislocations are observable even in glacial deposits, alluvium and loess. A region of faulting may continue to be so through more than one geological period. Little is known of the mechanism of faulting or of the causes that produce it; the majority of the text-book explanations will not bear scrutiny, and there is room for extended observation and research. The sudden yielding of the strata along a plane of faulting is a familiar cause of earthquakes.