« PreviousContinue »
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.
Bearing in mind the activity of the denuding agents, we might expect that normal faults of geologically recent date should show most prominently at the surface. And this to a large extent is doubtless true. Nevertheless, as we shall learn by-and-by, there are certain faults of prodigious antiquity which still cause very marked inequalities at the surface. These often form the boundaries between highlands and lowlands. In such cases, however, the disparity of level is due not so much to vertical displacement, as to the fact that the lowlands are usually composed of less enduring materials than those which enter into the framework of the adjacent highlands. When a fault of great age traverses strata of much the same consistency (say sandstones and shales), the rocks on either side of the dislocation, we find, have been planed down to
Fig. 61. Normal Fault, With High Ground On Downthrow Side.
the same level. Thus in the low-lying coal-fields of Scotland the gently undulating surface gives no indication of the presence of the numerous dislocations which have been detected underground. Downthrows of hundreds of feet give rise to no superficial inequalities. It is only when one of these faults has brought relatively hard and soft rocks into juxtaposition that a marked surface-feature results. And in this case the hard rock invariably rises above the level of the soft rock, no matter on which side of the dislocation it happens to lie. Thus in Fig. 61 the hard rock a forms an eminence, although it is on the downthrow side of the fault, simply because it has withstood denudation more effectually than the soft rock (b). In Fig. 62, again, it is obvious that the high ground at x owes its origin to the presence of the relatively hard rock (^). To this matter, however, we shall return in the sequel. Meanwhile we must consider, first, the appearances presented in regions where vertical movements of the crust have taken place within relatively recent times. The Colorado Plateau affords some excellent examples of simple folds and normal faults of comparatively recent age. These have often profoundly affected the surface, lines of cliffs and bold escarpments rising along the high side of each dislocation.
The plateau, in short, has been split across by wellmarked normal faults, some of which can be followed for hundreds of miles. Yet the strata on both sides of such dislocations are of much the same character and consistency. Here, then, it might be supposed that the fracturing and displacement had been suddenly effected. There is striking evidence, however, to show that such has not been the case. Although some of the faults referred to have a downthrow of several thousand feet, yet they have had no effect in disturbing the course of the Colorado River, which traverses the faulted region. The same, as we have seen.
holds true with regard to the flexures of that area. It is obvious, in a word, that the process of flexuring and faulting has proceeded so slowly that the river has been able to saw its way across the inequalities as fast as these appeared. But while the rate of river erosion has equalled that of crustal movement, the denudation of the plateau outside of the rivercourses has not. Deformation and dislocation of the plateau have thus given rise to marked surface-features. Yet even in the case of these relatively young faults we find that the features determined by them have been very considerably modified by denudation. In the following section, for example, we see three
Fig. 63. Faults In Queantowf.f.p Vallev, Grand Cason District.
faults of 1300 feet, 300 feet, and 800 feet displacement respectively traversing the same series of strata, and yet giving rise to marked inequalities at the surface. The dotted lines, however, show to what an extent these features have been modified by denudation. There is an obvious tendency of the escarpments and cliffs to become benched back as they retreat, so that they do not show the abrupt character which they would have possessed had no superficial waste accom