OCR Text |
Show DBAINAGE SYSTEMâ€"MIOCENE EEOSION. 17 can be clearer than the fact that the structural deformations (unless older than Tertiary time) never determined the present courses of the drainage. The rivers are where they are in spite of faults, flexures, and swells, in spite of mountains and plateaus. As these irregularities rose up the streams turned neither to the right nor to the left, but cut their way through in the same old places. It is needless to multiply instances. The whole province is a vast category of instances of river channels running where they never could have run if the structural features had in any manner influenced them. What, then, determined the present distribution of the drainage 1 The answer is that they were determined by the configuration of the old Eocene lake bottom at the time it was drained. Then, surely, the water-courses ran in conformity with the surface of the uppermost Tertiary stratum. Soon afterward that surface began to be deformed by unequal displacement, but the rivers had fastened themselves to their places and refused to be diverted. Many of the smaller streams have dried up and perished through the failure of their springs and the advent of an arid climate. These have left traces here and there in the shape of dry canons and gulches. Many more are still perishing. But the larger streams heading far up in moist Alpine highlands still meander through the desert, and have never ceased to flow from the beginning. In order to comprehend the relations of the High Plateaus to the province at large, it is necessary to advert to some of the salient features of the general erosion of the Plateau Country which followed the desiccation of the great lake, and which continued without interruption during Miocene time and dowrn to the present day. Its history during Miocene time must be spoken of only in general terms. In truth, during that great age there is no evidence of the occurrence of any critical event aside from the general processes of uplifting and erosion which affected the province as a whole. What forms and what topography were sculptured we know not-Of its climatal condition we can only suppose that it was similar to that of neighboring regions similarly situatedâ€"moist and subtropical. The vast erosion of the region has swept away so much of its mass, that most of the evidence as to details has vanished with its rocks. But the more important features of the work, its general plan in outline, have left well-marked 2 h p |