| OCR Text |
Show 36 GEOLOGY OF THE HIGH PLATEAUS. whether the period is closed or not, goes, I believe, only back into the late Pliocene. These faults are so important not only to the history of the High Plateaus, but also to the general history of the Plateau Province at large, that it seems proper to enter at some length upon the considerations which have led to this opinion concerning their age. Recognizing the great magnitude of the results accomplished in this region by erosion since the Eocene, we are naturally led to inquire whether we may not here and there gain some conception of the relative ages of certain events by ascertaining the amount of erosion which has been effected since their occurrence. The laws of erosion, both generally and in their somewhat abnormal application to this strange region, are sufficiently understood to enable us to decide where erosion ought to be most rapid and where most sluggish. Of all portions of the Plateau Province the best watered is the District of the High Plateaus. It is also the loftiest, and gives, therefore, to its water-courses the swiftest descents and the greatest transporting power. On the other hand, its rocks are the hardest and most durable. Thus the altitude and copious rainfall favor a rapid rate of erosion, while the greater durability of the rocks retards it. Not all of the rocks, however, are of this adamantine character. Indeed, some of the most voluminous formations are conglomerates, some well consolidated, but most of them only moderately so. Around the borders of the district are the sedimentaries, differing litliologically in no material respect from those of the province at large. By comparing the effects of erosion in rocks of different classes similarly situated we find great irregularities, but so far as can be seen these irregularities are due chiefly to the relative durability of the rocks. The sedimentaries are most powerfully eroded, and clearly disintegrate far more rapidly than the volcanics, and considerably more so than the conglomerates. There is seldom difficulty in distinguishing the erosion which has occurred during or since the faulting from that which may have occurred before it; and when we first separate this erosion from the earlier we find that in the sedimentaries it is very considerable. Vast ravines have been scored and deep canons cut into the risen blocks. The fronts have been battered and scoured by the storms of unknown millenniums and pared off until they stand back of the fault-planes which mark the rifts where they |
