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LAND-FORMS IN REGIONS AFFECTED BY NORMAL FAULTS OR VERTICAL DISPLACEMENTS
NORMAL FAULTS, GENERAL FEATURES OF THEIR CONNECTION WITH FOLDS THEIR ORIGIN HOW THEY AFFECT THE SURFACE FAULTS OF THE COLORADO REGION, AND OF THE GREAT
BASIN DEPRESSION OF THE DEAD SEA AND THE JORDAN
LAKE-DEPRESSIONS OF EAST AFRICA FAULTS OF BRITISH
COAL-FIELDS—BOUNDING FAULTS OF SCOTTISH HIGHLANDS AND LOWLANDS FAULT-BOUNDED MOUNTAINS GENERAL
IN Chapter III. a short account was given of the dislocations or fractures by which rocks are frequently traversed. These, as we saw, are of two kinds—normal faults and reversed faults or overthrusts. The latter have been sufficiently referred to in connection with the appearances presented by highly flexured strata, amongst which, indeed, they are most usually encountered. Normal faults of various importance may likewise often be seen traversing areas of disturbed and contorted rocks. When such is the case, however, the larger of these faults not infrequently prove to be of later date than the flexures and thrust-planes. The latter are the result of former horizontal movements of the crust; the normal faults, on the other hand, are vertical displacements due to later movements of direct subsidence. It will be understood, therefore, that reversed faults or overthrusts are practically confined to regions of highly flexed and contorted strata, while normal faults traverse every kind of geological structure. The latter, however, are certainly best displayed in areas of horizontal and moderately inclined strata, while they often form lines of separation between these and contiguous areas of highly disturbed rock-masses.
The amount of downthrow of normal faults is very variable. Sometimes it does not exceed a few feet or yards, in other cases it may reach thousands of feet, so that strata of vastly different ages may be brought into juxtaposition. The smaller faults usually extend for very short distances, while the larger ones may continue for hundreds or even thousands of miles. The course of great faults is usually approximately straight, but not infrequently it is curved. Very often they are accompanied by a series of smaller parallel dislocations; and now and again, in place of one great fault, with accompanying minor dislocations, we may find a series of more or less closely set parallel minor faults. When the downthrow of all these minor faults is in one and the same direction, the result is practically the same as if there had been only one major dislocation with a large downthrow. Another fact may be noted: faults, especially large ones, often split up, as it were, into two or more. A major fault may begin as a mere crack or fracture, with little or no accompanying rock-displacement. But as it continues the amount of downthrow gradually increases until a maximum is reached, after which the displacement usually decreases until finally the fault dies out. In not a few cases, however, the degree of downthrow varies very irregularly.
Frequently faults are intimately connected with folds and flexures. This is shown at once by the fact that large dislocations very often trend in the same direction as the strike of the strata. Now and again, indeed, when a large fault can be followed to the end, it is found gradually to die out in a fold or flexure. In other words, what is a fault in one place is represented elsewhere by a flexure. It is not hard to see how that should be. Strain or tension must obviously be set up along the margin of a sinking area. If, for example, subsidence should take place within an area of horizontal strata, the horizontal position of the rocks along the margin of the sinking area will be interfered with. The pull or drag of the descending mass will cause the strata of the adjacent relatively stable area either to bend over or snap across. Should the movement be slow and protracted, the rocks will probably at first yield by bending; but as the movement continues they will eventually give way, and a fold will thus be replaced by a fracture. Towards either end of such a fault, therefore, we should expect it to die out into a simple flexure or monoclinal fold. Probably most normal faults are in this way preceded by folding, except in cases -where they have been more or less suddenly produced.
Although normal faults may be looked upon as the result of direct subsidence, it is obvious that in some cases they may well have resulted from movements of elevation. During the slow uplifting of a broad plateau strain and tension will come into play along the margin of the rising area. Folds will thus be formed, and these will be replaced eventually by fractures and displacements. The resulting structure a.
Fig. 60. Section Of Normal Fault.
will thus be practically the same as if the folding and faulting had been produced by a movement of subsidence. Thus in Fig. 60 the fault f might have been caused either by the direct subsidence of the strata at x or by the elevation of the strata at a.
There is reason to believe that some large faults have resulted from crustal movements continued through long periods of time. The rock-displacements may have been very slowly and gradually effected, or the movement may have been more rapid, but interrupted again and again by longer or shorter pauses. Or, again, the rate of movement may have varied from time to time, and occasionally it may even have been sudden and catastrophic. But such evidence as we have would lead us to infer that vertical displacements, whether the result of downward or of upward movements, have not been more rapidly effected than horizontal deformations. No doubt a sudden dislocation of the crust of large extent would show directly at the surface. But somewhat similar results would follow if the dislocation, without being quite sudden, were yet to be developed more rapidly than the rate of superficial erosion and denudation. Lasers of the kind are well known, and to some of these reference will presently be made. It is with faulted rocks, however, as with folded mountains: when movement has ceased the inequalities caused at the earth■s surface tend to be reduced and greatly modified. The epigene forces are untiring in their action, so that in course of time areas of direct subsidence tend to become filled up and the surrounding high-lying tracts to be worn down. To such an extent has this taken place, that in the case of certain great faults of high geological antiquity no inequality at the surface indicates their presence, and it is only by studying the geological structure that we are able to ascertain that such dislocations exist.