OCR Text |
Show 70 GEOLOGY OF THE HIGH PLATEAUS. district, though a few important ones are found in the northern part of the field. The great western wall of the Awapa, the central and southern mass of the Sevier Plateau, the southern Tushar and northern Markagunt, are composed chiefly of such formations. The grand escarpments which wall the imposing fronts of these plateaus are conglomerates, sometimes capped with lava, sometimes intercalated, and more frequently without them. Near the center of Grass Valley we have, on the east, bounding the western verge of the Awapa, a wall of conglomerate which is more than 2,500 feet thick; and directly opposite, to the west, forming the eastern front of the Sevier Plateau, is an exposure of very nearly equal magnitude, both stretching southward for 25 miles without interruption, save where erosion has opened great gorges and ravines, though diminishing in thickness. From a point a few miles southeast of Marysvale the western front of the Sevier Plateau exhibits a wall of similar nature, extending south a distance of more than 40 miles to the terminus of the plateau, with only two brief interruptions. The southward expansion of the Sevier Plateau is made up chiefly of such masses, and they reappear in the western flank of the Aquarius beneath its monstrous lava cap. Their thickness will average here much more than a thousand feet. In the northern part of the Markagunt they appear to constitute the principal bulk of the area, though no deep exposures are found and their thickness cannot even be conjectured. The southern part of the Tushar rears a wall of similar nature, revealing nearly or quite 2,000 feet of conglomerate, covering an area of at least 150 square mil(»s, and probably very much more. The East Fork Canon is cut transversely through the narrowest part of the Sevier Plateau, and exhibits on either side a series of terraces rising J,500 to 4,000 feet above the bed of the stream. The lower 600 to 800 feet consist of " tufaceous " sandstones, and above them are more than 2,500 feet of coarse conglomerate, with a few massive sheets of intercalary lava. These clastic beds are everywhere seen throughout the central and southern portions of the district and are built upon a giant scale. Equally striking is the remarkable variety presented in their mechanical texture and structure, whether we consider it in the hand specimen or in the palisade and canon wall. We may consider them under two classes, |