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Show 38 GEOLOGY OF THE HIGH PLATEAUS. their general outlines or even their larger details along the structure lines The same is true of the heart of the province. The evidence is clear and irrefragable that at a comparatively recent epoch there has been a widespread uplifting coming upon the country suddenly as it were after an immense period of repose. Before its advent the streams had long remained at the limiting levels where they could sink no more, and the slower processes of decay, the recession of cliffs, the widening of valleys, the shrinkage of mesas, the lateral expansion of canons, had been in progress long enough to have produced very extensive results. As this uplifting came upon the land the rivers were at once disturbed and resumed their occupation of deepening their channels, and sank them almost as fast as the country rose. But they remain to-day with walls but little affected by lateral waste. Every indication points to the conclusion that they are freshly cut and are still cutting. Thus the study of the effect of erosion upon the uplifted sides of the great displacements of the High Plateaus everywhere indicates relative recency. The time during which these displaced edges have been subject to the action of the elements is trifling when compared with the interval which separates us from the Eocene. It is represented only by a work which is relatively small and easy of accomplishment and performed under circumstances most favorable to rapidity and efficiency. But the general denudation which dates back to the Eocene is incomparably greater in amount, considering only equal areas; and represents in chief part the kind of degradation which is relatively slow, performed under circumstances not always favorable to rapidity. There is another point of view from which we arrive at the same conclusion, that the great displacements are very young. The volcanism of the country has a history which we are able to unravel as to its broader features. It began after the disappearance of the Eocene lake which covered the Plateau Province. How long after the desiccation we cannot say even relatively. The lake had withdrawn apparently from the High Plateau District soon after the close of the Upper Green River epoch, which represents a period in the latter part (but before the close) of the local Eocene Resting unconformably upon the Upper Grreen River beds is a |
