OCR Text |
Show BOKDEB LINES OF PBOVINCES. 7 the fact that before these " mountains were brought forth " the platform of the country from which they arose had been plicated, and the plications planed down again by erosion. The Basin area is the oldest of the West,* its final emergence being of older date than the Jurassic, and most probably as ancient as the close of the Carboniferous. Between the Plateau and Park Provinces there is no definite boundary. Gradually as we proceed westward from the easternmost ranges of the Eocky system the valleys widen out, and the country gradually expands into a medley of terraces bounded by lofty cliffs, which stretch their tortuous courses across the land in every direction, yet not without system.. The boundary separating the Plateau Province from the Basin is, on the contrary, tolerably definite, and in some portions of its extent remarkably so. It lies along the eastern flank of the Wasatch, south of the Uintas, as far as Nebo ; thence along the Juab Valley, in the Pavant Range, as far as the Tushar Mountains. Here for a time it is concealed by immense floods of old lavas, and is not seen for a distance of 50 miles. It reappears near the southern end of that range, continuing south-southwest along the western base of the Markagunt Plateau, near a string of Mormon settlements scattered along the route from Beaver to Saint George, and follows the great fault which makes the Hurricane Ledge to the Arizona boundary. Here an offset carries it to the westward to another fault which walls the Grand Wash, and it then extends southward to the mouth of the Grand Canon of the Colorado and crosses the river. Here is the maximum westing of the Plateau Province. A. few miles south of the crossing it swings back to the southeastward, and continues beyond the explorations of this survey. This boundary is frequently very sharp and distinct, and throughout the greater portion of its extent the breadth of the doubtful or transitional zone lies wholly within the limits of a narrow valley or a narrow mountain range. The Pavant is a range of which the eastern side presents conspicuously the features of the Plateau type, while the western side presents those of the Basin type The Tushar Range shows a distinct plateau form in its southern half, while the northern half is masked by floods of volcanic rock From Toquerville to Parowan the Markagunt Plateau faces * I refer only to large areas. * There may be, and probably are,- small areas of equal or greater antiquity. |