OCR Text |
Show SYSTEM OF PLATEAU FAULTS. 27 a part through shearing. In any case the effect is in its broader aspects the same. One side has been uplifted, the other side "thrown." The true monoclinal in its perfect form is much more common in the sedimentary than in the volcanic beds The latter seem to lack that flexibility or rather adaptability which enables strata to undergo differential distortion without fracture. In the sedimentaries, on the other hand, the monoclinal seems to be the favored form of displacement, though trenchant faults are common enough. In the volcanics there is a tendency to the monoclinal form, but the unyielding nature of the rocks has produced comminuted fracture in places where a monoclinal would doubtless have been produced had the strata been more compliant. Hence the volcanics seldom preserve the unbroken monoclinal, though there is one good eiample of this preservation. This comminution is a source of perplexity in resolving the displacement into its constituents, and frequently renders it necessary to stajr long and scrutinize abundantly before the extent of it and its true method can be properly ascertained. Another striking characteristic of these displacements is their systematic arrangement. Viewed in one way they approach parallelism, but there is a noticeable convergence of the lines as we trace thein from south to north. In disturbed regions the faults and flexures usually tend to parallelism, and while the tendency is as decided here as it is elsewhere, yet the converging tendency is a noticeable characteristic. These great displacements of the High Plateaus are the northward continuations of those which have been described by Powell and Gilbert in the vicinity of, and crossing, the Col-rado River at the Grand Canon. But in the Grand Canon district (where they gave origin to the Kaibabs) the belt of faulted country is wider and the intervals between the faults and flexures are greater than in the High Plateaus. This width diminishes northward, and several of the grander faults at length become merged into one vast monoclinal fleiure, forming the western flank of the Wasatch Plateau. South of the Colorado these faults have not been studied, but the indications now are that they also converge in that direction, giving the greatest expansion to thb system just where the Colorado cuts across it. It is impossible to separate the faults of the High Plateaus from their systematic association with those of the Kai- |