Fault (geology)

“ That is what we often see when examining the geological earth strata. One stratum, instead of lying evenly with another where a crack has occurred, rests a little above it or below it. This inequality or unevenness geologists call a fault. Now we can easily see that whenever, and by whatever causes, a fault is produced, there is probably at the same time an earthquake. The fault cannot be produced without a great and shaking disturbance. ”

“ The simplest form of a fault is that shewn in the following diagram, where strata of sandstone and shale, with a coal-seam, S, have been shifted along the line f. The direction in which the fault is inclined [D] is its hade, and the degree of vertical displacement of the beds is the amount of the dislocation. Generally, the beds seem to be pulled down in the direction of the downthrow, and drawn up on the opposite side of the fault, as shewn in the diagram. ”

“ Great faults have sometimes brought ancient crystalline rocks in contact with weaker and younger sedimentary rocks, and long after erosion has destroyed all fault scarps the harder crystallines rise in an upland of rugged or mountainous country which meets the lowland along the line of faulting. ”

“ After the upthrown block has been worn down to this level, differential erosion produces fault scarps wherever weak rocks and resistant rocks are brought in contact along the fault plane; and the harder rocks, whether on the upthrow or the downthrow side, emerge in a line of cliffs. Where a fault is so old that no abrupt scarps appear, its general course is sometimes marked by the line of division between highland and lowland or hill and plain. ”

“ Where strata and normal fault surfaces lie at various angles, and also in thrust faults, those angles must be determined in addition to the data above named. In many mining regions, where valuable deposits are affected by faulting, accurate knowledge of the direction and amount of displacements of faults is of great economic importance. ”

“ What is a fault? What is hade? How are the strata affected on either side of a fault? What is the appearance called slickensides? Under what circumstances should we term a fault a downthrow? and when should we term it an upcast? How is the approximate age of a fault sometimes shewn?
51. What are metamorphic rocks, and what is their general appearance? In what districts of the British Islands are they most abundantly developed? What are some of the appearances relied upon for distinguishing metamorphic from igneous granite? ”

“ Yet it [Pg 75] often happens that there is no irregularity at the surface to betray the existence of a dislocation. The ground may be flat as a bowling-green, and yet, owing to some great fault, the rocks underneath one end of the flat may be geologically many hundred feet, or even yards, higher or lower than the strata underneath the other end of the same level space. What has become of the missing strata? ”

“ Oftentimes a great line of fault has been repeated, or the movements have been continued through successive periods, so that, newer deposits having covered the old line of displacement, the strata both newer and older have given way along the old line of fracture. ”

“ Fig. 46.—A series of parallel faults indicated by successive offsets in the course of an easily recognizable rock formation.
There are in addition many indications rather than proofs of the presence of faults, which must be taken account of in every general study of the geology of a district. Thus the outcrops of all neighboring formations may terminate abruptly upon a straight line which intersects all alike. Deep-seated fissure springs may be aligned in a striking manner, and so indicate the course of a prominent fracture, though not necessarily of a fault. ”

“ Faults afford a valuable indication of the conditions of stress to which a region has been subjected, but some caution must be exercised in their interpretation. Normal faults usually indicate an extension of the surface sufficient to permit the fault-blocks to settle down unequally. Reversed faults usually signify a compression of the surface which requires the blocks to overlap one another more than they did before the faulting. In other words, normal faulting usually implies tensional stress, and reversed faulting compressional stress. ”

“ It is true that now and again streams are found flowing in the direction of a fault, but that is simply because the dislocation is a line of weakness, along which it is easier for the denuding forces to act. ”

“ Favorable conditions for trapping the oil may be furnished by impervious clay "gouges" along fault planes, or by dikes of igneous rock. Favorable conditions may also be merely differences in porosity of beds in irregular zones, determined by differences either in original deposition or in later cementation. ”

“ When a fault takes place, and one rock mass is slidden past another to which it had been joined it is usually the case that the opposed surfaces no longer fit together as they did before the movement, and interspaces are left. ”

“ In all thrust faults one mass has been pushed over another, so as to bring the underlying and older strata upon younger beds; and when the fault planes are nearly horizontal, and especially when the rocks have been broken into many slices which have slidden far one upon another, the true succession of strata is extremely hard to decipher. ”

“ It is not uncommon to find the mass of rock, on one side of a fissure, thrown up above or down below the mass with which it was once in contact on the other side. This mode of displacement is called a shift, slip, or fault. "The miner," says Playfair, describing a fault, "is often perplexed, in his subterraneous journey, by a derangement in the strata, which changes at once all those lines and bearings which had hitherto directed his course. When his mine reaches a certain plane, which is sometimes perpendicular, as in A B, fig. ”

“ At a very remote period of the past, the rocks comprising these various strata were subjected to some powerful natural force, which resulted in their fracture, dislocation, and the gradual upheaval of a large portion of them, producing at the point of disruption what is known to geologists as a fault. ”

“ We must inquire to what regions the ruins of the white chalk, greensand, Wealden, and other strata which have disappeared were transported. We are then led at once to the examination of all the deposits newer than the chalk, and first to the oldest of these, the Lower Eocene, and its sand, shingle, and clay. In them, so largely developed in the immediate neighbourhood of the denuded area, we discover the wreck we are in search of, regularly stratified, and inclosing, in some of its layers, organic remains of a littoral, and sometimes fluviatile character. What more can we desire? ”

“ An original horizontal rock layer may be upfolded into anticlines, downfolded into synclines, and broken by either reverse or normal faults. A fault is a fracture or a zone of fractures within the earth’s crust along which movement has taken place. ”

“ From these causes, the surface and ornaments of castles and other ancient edifices, and of boulders, and all insulated rocks, are found to be decayed, and often to a considerable depth. It is from these causes that a soil is produced on every surface of rock which is not so exposed to the action of currents that the debris is removed as fast as it is formed. ”

“ It may be safely affirmed that the quantity of igneous and aqueous action,—of volcanic eruption and denudation,—of subterranean movement and sedimentary deposition,—not only of past ages, but of one geological epoch, or even the fraction of an epoch, has exceeded immeasurably all the fluctuations of the inorganic world which have been witnessed by man. ”

“ An unconformity between two marine formations records the fact that between the periods when they were deposited in the sea the area emerged as land and suffered erosion (p. 227) . The attitude and structure of the strata tell also of the foldings and fractures, the deformation and the metamorphism, which they have suffered; and the igneous rocks associated with them as lava flows and igneous intrusions add other details to the story. Each formation is thus a separate local chapter in the geological history of the [292] earth, and its strata are its leaves. ”

“ Such a chain may be the result of one colossal disturbance; but those of high geological antiquity usually furnish proofs of successive uplifts with more or less intervening denudation. Formed along lines of continental displacement in the crust, they have again and again given relief from the strain of compression by fresh crumpling, fracture and uprise. The chief guide in tracing these successive stages of growth is supplied by unconformability. ”

“ Such I conceive to have been the origin of the sand, gravel, and imbedded plants of our boulder. It may be well, however, in going into the details of the subject, to take a wider view of this interesting branch of geology, and look for a little at the forms and modes of the decomposition of rocks, and the varied manner in which new sedimentary accumulations are formed.
All over the world, wherever a land surface spreads out beneath the sky, there goes on a process of degradation and decay. ”

“ All materials, both sedimentary and igneous, composing the earth’s crust, are liable to be involved in movements of deformation, becoming subject to pressures and chemical agencies, which are often intense in character and productive of far-reaching internal changes. The result is the formation of a series of rocks, which may be of divers origins, but which having lost their original characteristics, may be grouped as metamorphic. ”

“ The phenomena presented by faults, therefore, are just as eloquent of denudation as is the truncated appearance — 43 — of our strata; and only after we have carefully examined the present extension and mutual relations of our rock-masses, their varied inclination, and the size of the dislocations by which they are traversed, can we properly appreciate the degree of erosion which they have sustained. Before we are entitled to express any opinion as to the origin of the surface-features of a country, we must first know its geological structure. ”

“ How vast, then, must be the spaces which this abstraction of matter has left vacant! how far exceeding in dimensions all the valleys, however numerous, and the hollows, however vast, which we can prove to have been cleared out by aqueous erosion! The evidences of the work of denudation are defective, because it is the nature of every destroying cause to obliterate the signs of its own agency; but the amount of reproduction in the form of sedimentary strata must always afford a true measure of the minimum of denudation which the earth's surface has undergone. ”

“ It can be proved that strata miles in thickness have been removed bodily from the surface of our own country by the seemingly feeble agents of denudation. All the denuded material—mud, sand, and gravel—carried down into the sea has been re-arranged into new beds, and these have ever and anon been pushed up to the light of day, and scarped and channelled by the denuding forces, the resulting detritus being swept down as before into the sea, to form fresh deposits, and so on. ”

“ If now a region thus underlaid by what we may call incipient lavas is subjected to the peculiar compressive actions which lead to mountain-building, we should naturally expect that such soft material would be poured forth, possibly in vast quantities through fault fissures, which are so readily formed in all kinds of rock when subject to irregular and powerful strains, such as are necessarily brought about when rocks are moved in mountain-making. ”

“ We have observed the same processes at work, the same results produced, the same methods utilized, in the world about us, and the clearest analogy compels us to accept a theory which ascribes the morainic débris piled up in hills and islands, the engraved rocks, the excavated basins, and the rounded slopes, to an identical though vastly-magnified cause in times only within the ken of Geology in its retrospective glance of ages. ”

“ When we reflect, therefore, on the fractional state of the annals which are handed down to us, and how little even these have as yet been studied, we may wonder that so many geologists should attribute every break in the series of strata and every gap in the past history of the organic world to catastrophes and convulsions of the earth's crust or to leaps made by the creational force from species to species, or from class to class. ”

“ The preceding discussion, and the illustrations which accompany it, give some idea of the topography which characterizes an area in various stages of its erosion history. Whether the valleys are deep or shallow, and the intervening ridges high or low, depends on the original height of the land and its distance from the sea. The higher the land, and the nearer it is to the sea, the greater the relief developed by erosion. A plateau near the sea may become mountainous in the mature stage of its erosion history, while a plain in the same situation would only become hilly. ”

“ A third fault, the effect of which is not so evident at first sight, throws the strata down roughly 50 ft. to the south. These are only small faults, but sometimes faults occur, in which the strata have been moved on opposite sides of the fault thousands of feet away from one another. We might think we should see a wall of rock rising up on the surface of the ground where a fault occurs; but the faults have mostly taken place ages ago; and, when they do happen, the rocks are generally moved only a little way at a time. ”