The rocks are often unable to accommodate themselves by bending or plastic flow to the stresses to which they are subjected, and therefore break, usually with more or less dislocation. A simple fracture, not accompanied by dislocation, is called a fissure., and the strata on the two sides of the fracture are the same at corresponding levels, so that the crack was evidently made through continuous beds.
When the strata on one side of a fissure have been shifted in any. direction relatively to the beds on the other side, so that the strata, which were once continuous across the fracture, are now separated by a vertical interval and lie at different levels, the structure is called a fault. We have learned that faulting is an accompaniment of many great earthquakes (see Chapter I), and these modern faults show that the movement may be in any direction, vertical, horizontal, oblique, or rotational. "Whenever the rocks of the earth's crust are subjected to strain, fractures take place in them as in any other body under similar conditions, and the different parts of the rock tend to move past one another along the fracture-planes, seeking to obtain relief from the strain and to accommodate themselves to new conditions. In this movement one part of the fractured rock-mass may move upon the other in any direction, up, down, sidewise or obliquely, according to the conditions, which are different in each instance." (Spurr).
It is obvious that faulting displays highly complex phenomena, which cannot be adequately presented by diagrams, since three dimensions are involved, and the simple cross-section may be altogether misleading. To add to the difficulty, movements frequently occur at intervals along the same fracture-planes, but, it may be, in entirely different directions, so a vertical movement may be succeeded by a horizontal one, or vice versa, and the final outcome may be the resultant of, many different movements. It is very unfortunate that several of the terms used in the description of faults, adopted from the miners by English geologists, should in American practice have acquired meanings quite different from those originally given to them, so that the student finds in different books the same terms employed in different senses. The following definitions are those commonly to be found in the text books: -
Fig. 167. - Normal fault, fault-plane hading against dip of beds; b' c, throw; be, heave; bb', stratigraphic throw, which in this case is measured along the fault-plane, because the latter happens to be at right angles to the bedding-planes. The angle bb'c is the angle of hade; b'bc, the angle of dip. Foot-wall to the right of the fault, and hanging wall to the left. - N.B. The line b' c should, have been drawn from the top of the obliquely lined bed, slightly increasing both throw and heave.
Faults are usually inclined, and the angle of inclination; measured from a vertical plane, is called the hade, or slope, of the fault, while the dip of the fault, like that of a stratum, is measured from a horizontal plane, and is thus the complement of the hade. For example, if the fault is vertical, the hade = o, and the dip = 900; if the fault is horizontal, the hade = 900, and the dip = 0, while a hade of 450 gives a dip of the same amount. The side on which the beds lie at a higher level than their continuations on the other side of the fault-plane is called the upthrow side and the other is the downthrow side, without reference to the actual direction of the movement. Owing to the inclination of the fault, the rocks on one side project over those on the other, and are hence called the hanging wall, and the. side which projects undeneath the other is called the foot-wall. Either the hanging or the foot-wall may be on the upthrow or the downthrow side, according to the nature of the fault.
The vertical displacement between the fractured ends of a given stratum is called the throw (b'c, Fig. 167) and the heave, or horizontal throw, is the horizontal distance through which one end of a faulted bed has been carried past the corresponding end on the other side of the fault-plane (be, Fig. 167). When the movement has been vertical, the heave is due to the obliquity of the fault and therefore increases, in proportion to the throw, as the hade increases. A fault with plane perpendicular to the surface has no heave, for it has no hade. Offset is the distance between the two corresponding ends of a faulted bed, measured on a horizontal plane and usually applied to the outcrop (see Fig. 177, III). The stratigraphic throw is the thickness of beds which is included between the two fractured ends of a faulted stratum and is taken at right angles to the bedding-planes. (DB, Fig. 168).
Fig. 168. - Normal fault hading with dip of beds. DB, stratigraphic throw; AC, throw; CB, heave.
The throw of faults varies greatly in different cases, from a fraction of an inch up to thousands of feet. In those of small throw the plane of fracture is frequently a clean, sharp break; but in the greater faults the rocks in the neighbourhood of the fault are often bent, crushed, and broken, forming a confused mass of fragments, large and small, which may be cemented into a breccia, which is then called fault-breccia or fault-rock. In soft rocks the fault is always closed by the immense weights and pressures involved, but in rigid rocks it may remain partly open, especially if the break be not a plane, but of curved, warped, and irregular course, as is usually the case. The term fault-plane is thus rarely accurate, though it is constantly employed as a matter of convenience. In faults of considerable throw the ends of adjoining strata are apt to be bent more or less sharply upward or downward, in accordance with the direction of movement. This is drag (Fig. 178).
Fig. 169. - Fault-breccia of limestone.
Fig. 170. - Vertical slickensides; Rondout, N.Y. (Photograph by van Ingen).
In the more rigid rocks the friction of the masses grinding against one another on the fault-plane grooves and polishes them, which produces the characteristic appearance known as slickensides. The grooves or striae indicate the direction of the last movement along the fault-plane, for ordinarily this last movement obliterates the earlier striae, but does not always do so, for we sometimes find cases in which two, or even three, sets of striae are preserved, each demonstrating motion in a different direction.
Fig. 171. - Limestone faulted on bedding-planes, with vertical slickensides; Rondout, N.Y. (Photograph by van Ingen).
In stratified rocks faults usually break across the strata, separating each bed into two or more parts, according to the number of dislocations, yet sometimes the fault-planes coincide with the bedding planes, which are slickensided, pushing each stratum upon those above and below it, but without fracture.
The preceding discussion of faults deals only with those of stratified rocks, but this is merely because such displacements are the easiest to observe. As a matter of fact, dislocations may and do traverse rocks of all kinds, but it may be quite impossible to detect a fault, even one of great throw, in a thick, homogeneous, crystalline mass, for lack of any definite points of reference on the two sides of the fault-plane. On the other hand, in thinly laminated rocks with well-defined colour lines the most minute displacements are strikingly apparent.
Fig. 172. - Minute vertical fault, of recent date, interrupting glacial striae.
(G. F. Matthew).